1. Introduction

The prevailing system used for the construction of single-family homes in the USA is platform framing using light wooden dimensional lumber. Structural assemblies such as the roof, floors, and walls are generally constructed with nominal 50.8 mm (2 inch) lumber members ranging in nominal depths from 101.6 to 304.8 mm (4–12 inches) and sheathed with structural wood panels for stability and security, such as oriented strand board (OSB) or plywood.

Wood structural materials are preferred by US homebuilders largely because (1) the US home building industry is mostly familiar with wood framing method, (2) the units of construction (i.e., studs, joists, panels, etc.) are small and easily transportable, and (3) wood-framed structures can be erected without the need for specialized tools or large equipment.

In this chapter, the complete process of designing a typical US residential dwelling using wood-frame systems will be illustrated. The typical US design methodology and basis will be used to accomplish the designs. The International Residential Code (IRC) [1] is the design basis used by most authorities to regulate the design

and construction of single-family residences. The following major aspects are discussed in this chapter:

veneer lumber (LVL) components, or roof trusses are used in design, a structural

This is typically the extent of a structural engineer's involvement in residential design other than specialized situations not covered by the IRC and occasionally foundation design. If engineered design is necessary in conjunction with the prescriptive standards, then compliance with the 2015 International Building Code (IBC) [3] requirements for those portions of the design is required. Engineers will conduct their analysis based on requirement set forth in the IRC, IBC if necessary, and ASCE 7-10 minimum design loads for buildings and other structures (ASCE 7) [4] [ASCE stands for American Society of Civil Engineers]. The IRC and IBC also permit designers to refer to the 2015 AWC Wood Frame Construction Manual (WFCM) [5] for an alternative prescriptive or engineered approach [AWC stands

engineer is required to review their specification and application.

Structural Design of a Typical American Wood-Framed Single-Family Home

3. External load determination and serviceability requirements

this design, only live load deflection limits will be considered.

(LL), roof live load (RL), and snow load (SL).

ASCE 7 Table C3-1. Dead loads are listed in Tables 1–3.

listed in Table 4 will be used for this study.

This study will focus on the appropriate residential structural building loads for the State College, PA area, for an example design case. The designs will include only the effects of dead loading, floor live loading, roof live loading, snow loading, and wind loading. Residential structures in ordinary situations are designed to resist both gravity loads and lateral loads. External loading for homes is prescribed in either Chapter 3 of the 2015 IRC or in ASCE 7. ASCE 7 is the standard referenced in the 2015 IRC, and therefore this version will be referenced in this study. Both the IRC and the ASCE 7 will be used to develop the external loads for this study. In addition to the external loads, the serviceability criteria must also be considered. For

The gravity loads are those loads that act in the direction of gravity. The gravity loads of importance for residential structures are dead load (DL), floor live load

Dead load is the load that is permanently and continuously applied to a structure. Typically, dead load refers to the self-weight of the material used in construction or a load that is applied in a permanent nature such as a known location of a piece of heavy equipment or a large island in the kitchen. Unless noted otherwise, the S&A Homes dead load criteria will be used for the wood-framed design of this home. These loads are typical for residential design and were largely derived from

Live loading is a gravity loading that is temporary or intermittent in nature. The three live loads considered for the design of this home are floor live (LL), roof live (RL), and snow load (SL). The IRC prescribes the minimum uniformly distributed loads that must be used by designers for residential structures. Such minimum loads

for American Wood Council].

DOI: http://dx.doi.org/10.5772/intechopen.85929

3.1 Gravity loads

3.2 Dead load (DL)

3.3 Live load

29


The scope is limited to the structural design and performance of one singlefamily residential dwelling. The load-bearing wall systems are the primary components of the building enclosure, and the structural properties of the wall system are only one of many considerations that must be taken into account. While cladding compatibility, thermal performance or the hygrothermal characteristics of a wall system are very important, such aspects are not the focus of this study and will not be discussed.

The home design considered in this study is a two-story regular-shaped home with a basement and attached two-car garage. The floor plan was provided by S&A Homes, which is a midsized homebuilder that builds homes and provides architectural design services to customers in Pennsylvania and West Virginia. The floor plans and drawings for one of their standard home packages are provided in the Appendix. Clients of S&A Homes can select this floor plan from an array of floor plans and make slight variations to it if desired. S&A Homes will then design, detail, and construct the home for the client on the chosen lot, typically one of S&A's own residential developments.

The home plan/style shown in the Appendix is a popular model in S&A's territory and is representative of the size and style of homes desired by the average homebuyer of this decade. The home consists of nearly 214 m<sup>2</sup> (2300 ft2 ) of finished floor area with the basement available for finishing if so desired by the prospective homeowner. The floor plan has features typically seen in modern homes. The first floor contains a large kitchen open to the family room with access to both the dining room and the attached two-car garage. The second floor has four bedrooms with the master suite containing its own large bathroom as well as a sitting area and walk-in closet (WIC).

#### 2. Applicable codes and standards

The IRC is the prevailing design code used for the construction of one- or twofamily dwellings in the USA. The 2015 IRC [1] is the current adopted code in the State College, PA area, and will be used as the governing design code for this study. In order to construct a single-family dwelling, the homebuilder must first apply to the local code office for a building permit. It is necessary to provide a complete architectural plan set detailing how the builder intends to comply with the requirements of the IRC, along with several other items such as the manual J [2] heat lossgain calculations for the structure and selection of energy compliance path. The IRC largely provides a prescriptive basis for home design and in many instances is adequate for single-family home design. The envelope and structural components are typically selected by the architect, builder, or homeowner from design tables within the code. If prefabricated engineered components such as I-joists, laminated

#### Structural Design of a Typical American Wood-Framed Single-Family Home DOI: http://dx.doi.org/10.5772/intechopen.85929

veneer lumber (LVL) components, or roof trusses are used in design, a structural engineer is required to review their specification and application.

This is typically the extent of a structural engineer's involvement in residential design other than specialized situations not covered by the IRC and occasionally foundation design. If engineered design is necessary in conjunction with the prescriptive standards, then compliance with the 2015 International Building Code (IBC) [3] requirements for those portions of the design is required. Engineers will conduct their analysis based on requirement set forth in the IRC, IBC if necessary, and ASCE 7-10 minimum design loads for buildings and other structures (ASCE 7) [4] [ASCE stands for American Society of Civil Engineers]. The IRC and IBC also permit designers to refer to the 2015 AWC Wood Frame Construction Manual (WFCM) [5] for an alternative prescriptive or engineered approach [AWC stands for American Wood Council].

#### 3. External load determination and serviceability requirements

This study will focus on the appropriate residential structural building loads for the State College, PA area, for an example design case. The designs will include only the effects of dead loading, floor live loading, roof live loading, snow loading, and wind loading. Residential structures in ordinary situations are designed to resist both gravity loads and lateral loads. External loading for homes is prescribed in either Chapter 3 of the 2015 IRC or in ASCE 7. ASCE 7 is the standard referenced in the 2015 IRC, and therefore this version will be referenced in this study. Both the IRC and the ASCE 7 will be used to develop the external loads for this study. In addition to the external loads, the serviceability criteria must also be considered. For this design, only live load deflection limits will be considered.

#### 3.1 Gravity loads

and construction of single-family residences. The following major aspects are

1. Provide introductory material such as the description of the home to be designed, applicable design codes, and external loading assessment for

2. Design the home using a wood-framed platform system. The load path will be discussed as well as specific design codes relating to wood-framed structures. The result of specifying and detailing typical structural elements of the home

The scope is limited to the structural design and performance of one singlefamily residential dwelling. The load-bearing wall systems are the primary components of the building enclosure, and the structural properties of the wall system are only one of many considerations that must be taken into account. While cladding compatibility, thermal performance or the hygrothermal characteristics of a wall system are very important, such aspects are not the focus of this study and will not

The home design considered in this study is a two-story regular-shaped home with a basement and attached two-car garage. The floor plan was provided by S&A Homes, which is a midsized homebuilder that builds homes and provides architectural design services to customers in Pennsylvania and West Virginia. The floor plans and drawings for one of their standard home packages are provided in the Appendix. Clients of S&A Homes can select this floor plan from an array of floor plans and make slight variations to it if desired. S&A Homes will then design, detail, and construct the home for the client on the chosen lot, typically one of S&A's own

The home plan/style shown in the Appendix is a popular model in S&A's territory and is representative of the size and style of homes desired by the average homebuyer of this decade. The home consists of nearly 214 m<sup>2</sup> (2300 ft2

floor area with the basement available for finishing if so desired by the prospective homeowner. The floor plan has features typically seen in modern homes. The first floor contains a large kitchen open to the family room with access to both the dining room and the attached two-car garage. The second floor has four bedrooms with the master suite containing its own large bathroom as well as a sitting area and walk-in

The IRC is the prevailing design code used for the construction of one- or twofamily dwellings in the USA. The 2015 IRC [1] is the current adopted code in the State College, PA area, and will be used as the governing design code for this study. In order to construct a single-family dwelling, the homebuilder must first apply to the local code office for a building permit. It is necessary to provide a complete architectural plan set detailing how the builder intends to comply with the requirements of the IRC, along with several other items such as the manual J [2] heat lossgain calculations for the structure and selection of energy compliance path. The IRC largely provides a prescriptive basis for home design and in many instances is adequate for single-family home design. The envelope and structural components are typically selected by the architect, builder, or homeowner from design tables within the code. If prefabricated engineered components such as I-joists, laminated

) of finished

discussed in this chapter:

be discussed.

closet (WIC).

28

residential developments.

2. Applicable codes and standards

residential structures.

Timber Buildings and Sustainability

will be specified and details provided.

The gravity loads are those loads that act in the direction of gravity. The gravity loads of importance for residential structures are dead load (DL), floor live load (LL), roof live load (RL), and snow load (SL).

## 3.2 Dead load (DL)

Dead load is the load that is permanently and continuously applied to a structure. Typically, dead load refers to the self-weight of the material used in construction or a load that is applied in a permanent nature such as a known location of a piece of heavy equipment or a large island in the kitchen. Unless noted otherwise, the S&A Homes dead load criteria will be used for the wood-framed design of this home. These loads are typical for residential design and were largely derived from ASCE 7 Table C3-1. Dead loads are listed in Tables 1–3.

#### 3.3 Live load

Live loading is a gravity loading that is temporary or intermittent in nature. The three live loads considered for the design of this home are floor live (LL), roof live (RL), and snow load (SL). The IRC prescribes the minimum uniformly distributed loads that must be used by designers for residential structures. Such minimum loads listed in Table 4 will be used for this study.


Weight is derived from Weyerhaeuser publication #TJ-4000 for 230 or 360 series joists.

#### Table 1.

Floor/ceiling assembly weight.


#### Table 2.

Roof assembly weight.<sup>a</sup>


Zones Case 1 Case 2 Minimum A 1.13 (23.6) 1.13 (23.6) 0.77 (16) B 0.77 (16.1) 0.77 (16.1) 0.38 (8) C 0.90 (18.8) 0.90 (18.8) 0.77 (16) D 0.62 (12.9) 0.62 (12.9) 0.38 (8) E 0.09 (1.8) 0.44 (9.1) 0 F 0.68 (14.3) 0.34 (7.1) 0 G 0.03 (0.6) 0.38 (7.9) 0 H 0.59 (12.3) 0.24 (5.0) 0 EOH 0.40 (8.3) 0.40 (8.3) 0 GOH 0.45 (9.5) 0.45 (9.5) 0

Parameter Description Risk category II Basic wind speed (V) 51 m/s 115 mph Exposure category B Topographic factor (Kzt) 1.0 Mean roof height 7.0 m (23 ft) Adjustment factor (λ) 1.0 Roof pitch 30 degrees

Based on State College area prescriptive requirements. Applied on the horizontal projection rather than along the

Load description Weight kN/m<sup>2</sup> (lbf/ft2

LL (sleeping rooms) 1.44 (30.0) LL (other) 1.92 (40.0) LL (habitable attics) 1.44 (30.0) LL (attics w/limited storage)a,b 0.96 (20.0) LL (Attics w/o limited storage)c 0.48 (10.0) Roof live load 0.77 (16.0) Design roof snow load<sup>d</sup> 1.44 (30.0)

Structural Design of a Typical American Wood-Framed Single-Family Home

Attics defined as the unfinished area between the roof and the ceiling of the floor below.

Add to attic space less than 1.07 m (42 inch).

DOI: http://dx.doi.org/10.5772/intechopen.85929

Minimum uniformly distributed live loads.

Limited storage refers to non-habitable attic space greater than or equal to 1.07 m (42 inch).

)

Values in kN/m<sup>2</sup> (lbf/ft<sup>2</sup>

Table 6.

31

Table 5.

a

b

c

d

slope.

Table 4.

Wind load parameters.

).

Simplified design wind pressure (Ps) case A θ = 30.

a <sup>2</sup> 6 wood studs at 406.4 mm (16 inch) O.C. with 12.7 mm (½ inch) gypsum wallboard and vinyl siding. <sup>b</sup> Wood or steel studs with 12.7 mm (½ inch) gypsum wallboard on each side.

#### Table 3.

Miscellaneous materials.

#### 3.4 Lateral loading

The only lateral load being considered for this study is the wind loading. In the State College area, seismic loading does not typically control the design of structural components. The procedures in ASCE 7 will be used to determine wind loading, e.g., Chapter 28 Envelope Procedure Part 2 can be used for this structure. Chapter 28 requires that the structure meets the definition of a low-rise, enclosed simple diaphragm building that is regular-shaped in accordance with Section 26.2.

## Structural Design of a Typical American Wood-Framed Single-Family Home DOI: http://dx.doi.org/10.5772/intechopen.85929


a Attics defined as the unfinished area between the roof and the ceiling of the floor below.

b Limited storage refers to non-habitable attic space greater than or equal to 1.07 m (42 inch).

c Add to attic space less than 1.07 m (42 inch).

d Based on State College area prescriptive requirements. Applied on the horizontal projection rather than along the slope.

#### Table 4.

Minimum uniformly distributed live loads.


#### Table 5.

Wind load parameters.


#### Table 6.

3.4 Lateral loading

Miscellaneous materials.

a

30

a

Table 2.

Roof assembly weight.<sup>a</sup>

Table 3.

a

Table 1.

Floor/ceiling assembly weight.

Timber Buildings and Sustainability

Engineered wood truss roof system.

The only lateral load being considered for this study is the wind loading. In the State College area, seismic loading does not typically control the design of structural components. The procedures in ASCE 7 will be used to determine wind loading, e.g., Chapter 28 Envelope Procedure Part 2 can be used for this structure. Chapter 28 requires that the structure meets the definition of a low-rise, enclosed simple diaphragm building that is regular-shaped in accordance with Section 26.2.

Sub-component Weight N/m<sup>2</sup> (lbf/ft2

Carpet/vinyl 47.9 (1.0)<sup>a</sup> 19.1 mm (¾ in) plywood 114.9 (2.4) 301.6 mm (11 7/8 in) I-joists<sup>b</sup> 91.0 (1.9) Mechanical allowance 95.8 (2.0) 12.7 mm (½ in) gypsum ceiling 105.3 (2.2) Total ≈454.9 (10)

For floor areas known to have ceramic tile floor covering, increase load to 0.96 kN/m<sup>2</sup> (20 lbf/ft<sup>2</sup> ). <sup>b</sup> Weight is derived from Weyerhaeuser publication #TJ-4000 for 230 or 360 series joists.

Sub-component Weight N/m<sup>2</sup> (lbf/ft2

Truss framing 95.8 (2.0) 11.1 mm (7/16 in) sheathing 81.4 (1.7) Asphalt shingles 114.9 (2.4) 228.6 mm 9 in insulation 86.2 (1.8) 12.7 mm (½ in) gypsum board 105.3 (2.2) Miscellaneous 95.8 (2.0) Total ≈ 579.4 (12)

Sub-component Weight Exterior wall assembly<sup>a</sup> 526.7 N/m<sup>2</sup> (11.0 lbf/ft<sup>2</sup>

Interior wall assembly<sup>b</sup> 383.0 N/m<sup>2</sup> (8.0 lbf/ft<sup>2</sup>

Plain concrete 22.8 kN/m<sup>3</sup> (145 lbf/ft<sup>3</sup>

Reinforced concrete 23.6 kN/m<sup>3</sup> (150 lbf/ft<sup>3</sup>

Wood or steel studs with 12.7 mm (½ inch) gypsum wallboard on each side.

<sup>2</sup> 6 wood studs at 406.4 mm (16 inch) O.C. with 12.7 mm (½ inch) gypsum wallboard and vinyl siding. <sup>b</sup>

)

)

)

)

)

)

Simplified design wind pressure (Ps) case A θ = 30.

The wind loads calculated in Table 6 are based on the parameters listed in Table 5 and in accordance with Figure 1. The simplified design wind pressure magnitudes in Tables 6 and 7 include both windward and leeward pressures. The combined pressure will be applied to only the windward side of the structure. For this design, two load cases must be evaluated because the roof pitch is between 25 and 30 degrees. Additionally, these two cases must be compared to the minimum load case described in ASCE 7 Section 28.6.4. The case that produces the larger load effect will be used for design of structural members.

## 3.5 Serviceability criteria

The main serviceability criterion considered in the design of residential homes is deflection. The IRC prescribes the maximum allowable deflection of structural

members and assemblies. Excessive deflections can cause problems for the occupants and potentially damage to nonstructural components such as cladding or fenestration. Excessive interior floor deflections are generally noticed in the form of floor vibration or "spongy" floors. Excessive deflection of roof members can lead to ponding and ultimately moisture issues or overloading of structural members. A portion of Table R301.7 from the IRC that prescribes residential deflection limits is

Sub-component Span ratio Interior walls and partitions Height/180 Floors and plaster ceilingsa,b Length/360 All other structural members Length/240 Exterior walls—brittle finish Length/240

Structural Design of a Typical American Wood-Framed Single-Family Home

Both allowable stress design (ASD) and load resistance and factor design (LRFD) load combinations will be utilized for different aspects of the home structural design. For example, the ASD approach will be used for wood design, whereas the LRFD approach will be used for concrete foundation design. Approaches for the designs will be discussed as appropriate. The load combinations that will be used for

design are listed below and are reproduced from ASCE 7.

reproduced below in Table 8.

Limit floor beam deflection to 12.7 mm (½ inch).

DOI: http://dx.doi.org/10.5772/intechopen.85929

Limit I-joist deflection ratio to length/480.

Live load maximum deflection limits.

3.6 Combination of loads

3.6.1 ASD load combinations

3. D + (Lr or S or R)

5. D + (0.6W or 0.7E)

3.6.2 LRFD load combinations

2. 1.2D + 1.6L + 0.5(Lr or S or R)

3. 1.2D + 1.6(Lr or S or R) + (L or 0.5W)

7. 0.6D + 0.6W

1. 1.4D

33

4.D + 0.75L + 0.75(Lr or S or R)

6.D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R)

1. D

a

b

Table 8.

2. D + L


#### Table 7. Simplified design wind pressure (Ps) case B θ = 0.

Structural Design of a Typical American Wood-Framed Single-Family Home DOI: http://dx.doi.org/10.5772/intechopen.85929


#### Table 8.

The wind loads calculated in Table 6 are based on the parameters listed in Table 5 and in accordance with Figure 1. The simplified design wind pressure magnitudes in Tables 6 and 7 include both windward and leeward pressures. The combined pressure will be applied to only the windward side of the structure. For this design, two load cases must be evaluated because the roof pitch is between 25 and 30 degrees. Additionally, these two cases must be compared to the minimum load case described in ASCE 7 Section 28.6.4. The case that produces the larger load

The main serviceability criterion considered in the design of residential homes is

deflection. The IRC prescribes the maximum allowable deflection of structural

ASCE 7-10 Chapter 28 wind loading designation (with permission from the ASCE).

Zones Case 1 Minimum A 1.01 (21.0) 0.77 (16) C 0.67 (13.9) 0.77 (16) E 1.21 (25.2) 0 F 0.68 (14.3) 0 G 0.68 (14.3) 0 H 0.53 (11.1) 0

effect will be used for design of structural members.

3.5 Serviceability criteria

Timber Buildings and Sustainability

Figure 1.

Values in kN/m<sup>2</sup> (lbf/ft<sup>2</sup>

Table 7.

32

).

Simplified design wind pressure (Ps) case B θ = 0.

Live load maximum deflection limits.

members and assemblies. Excessive deflections can cause problems for the occupants and potentially damage to nonstructural components such as cladding or fenestration. Excessive interior floor deflections are generally noticed in the form of floor vibration or "spongy" floors. Excessive deflection of roof members can lead to ponding and ultimately moisture issues or overloading of structural members. A portion of Table R301.7 from the IRC that prescribes residential deflection limits is reproduced below in Table 8.

## 3.6 Combination of loads

Both allowable stress design (ASD) and load resistance and factor design (LRFD) load combinations will be utilized for different aspects of the home structural design. For example, the ASD approach will be used for wood design, whereas the LRFD approach will be used for concrete foundation design. Approaches for the designs will be discussed as appropriate. The load combinations that will be used for design are listed below and are reproduced from ASCE 7.

3.6.1 ASD load combinations

1. D 2. D + L 3. D + (Lr or S or R) 4.D + 0.75L + 0.75(Lr or S or R) 5. D + (0.6W or 0.7E) 6.D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R) 7. 0.6D + 0.6W 3.6.2 LRFD load combinations 1. 1.4D 2. 1.2D + 1.6L + 0.5(Lr or S or R) 3. 1.2D + 1.6(Lr or S or R) + (L or 0.5W)

4.1.2D + 1.0W + L + 0.5(Lr or S or R)

5. 0.9D + 1.0W

In the above load combination, the notation is defined as follows: D for dead load, L for live load, Lr for roof live load, S for snow load, R for rain load, and W for wind load.

specialized knowledge or tools. Lastly, wood-framed construction has been well

As noted before, much of the wood-framed structural design can be accomplished using design aids. The design professional will typically use these design aids to the greatest extent possible and then perform structural analysis and design for any item that is beyond the scope of the design aids. This is the approach that will be used for this study. The design drawings are shown in the Appendix. The associated detailed calculation is not provided due to space limitation; only the necessary

External loads must be transmitted to ground through the structural system of the building. Two main systems are needed to accomplish this transfer properly: gravity system and the main wind force resisting system (MWFRS). The gravity system transmits the vertical loads through a system of trusses, joists, and beams to foundation, which in turn transmits the load to ground, while the MWFRS transfers lateral wind load to foundation through a system of shear walls and flexible diaphragms. It is important to recognize that the ground must be properly prepared and evaluated to ensure good load transfer. Typically, foundations are placed on virgin soil or engineered (compacted) fill. All organic materials should be removed

The gravity system in this home starts at the roof and ends in the soil. Vertical loads must have a continuous path to the ground. Generally, the gravity system in this example consists of OSB sheathing, engineered roof trusses, load-bearing stud walls, dimensional lumber headers, engineered I-joist floor system, engineered

The OSB roof sheathing, as illustrated in Figure 3, serves to transfer gravity load (i.e., dead, live, and snow loads) and wind suction to roof framing members. The roof sheathing also transfers the lateral wind loading through diaphragm action to the structure. Attachment requirements of the sheathing to roof trusses are

governed by the greater of the wind uplift force or the shear transfer requirement of

roof dead load. It may be possible to use 9.5 mm (3/8 inch) sheathing, but 11.1 mm (7/16 inch) thickness is more readily available and common in the locale. In this example, the sheathing will be specified with panel edge clip support. According to IRC Table R602.3(1), the sheathing is required to be attached to the truss framing with 63.5 mm (2½ inch) 8D common nails spaced at 152.4 mm (6 inch) on center (O.C.) around the edges of the panel and 304.8 mm (12 inch) O.C. at intermediate supports (field). Note that the gable end sheathing connections must be spaced at

span rating) is acceptable for this example. The sheathing can be used with or without edge support at 609.6 mm (24 inch) spans with an allowable live load of

152.4 mm (6 inch) O.C. at both the perimeter and intermediate locations.

According to IRC Table R503.2.1.1(1), 11.1 mm (7/16 inch) roof sheathing (24/16

), and a total allowable load of 2.39 kN/m<sup>2</sup> (50 lbf/ft<sup>2</sup>

) snow loading plus 0.57 kN/m2 (12 lbf/ft<sup>2</sup>

), which

)

wood beams, structural steel girders, and a concrete foundation.

documented in the USA, and many design aids are available.

Structural Design of a Typical American Wood-Framed Single-Family Home

results will be mentioned.

4.1 External load transfer (load path)

DOI: http://dx.doi.org/10.5772/intechopen.85929

along with excessive amounts of water.

4.2 Gravity system design

4.3 Roof sheathing

the connection.

35

1.92 kN/m<sup>2</sup> (40 lbf/ft<sup>2</sup>

is less than the 1.44 kN/m<sup>2</sup> (30 lbf/ft<sup>2</sup>

## 4. Design of residence

Wood is the most popular material used in the USA for the construction of single-family dwellings. An example of residential framing can be seen below in Figure 2 [6]. Framing lumber is easily obtained in most locations. The units of construction can be easily transported by contractors or homeowners without the need for specialized equipment. Additionally, the erection of a wood-framed structural system is familiar to most and does not require excessive amounts of

#### Figure 2.

Section view of typical residential wood-framed home. Note: in this figure, a small rectangle with x inside indicates the cross section of wood member, and DBL stands for double.

#### Structural Design of a Typical American Wood-Framed Single-Family Home DOI: http://dx.doi.org/10.5772/intechopen.85929

specialized knowledge or tools. Lastly, wood-framed construction has been well documented in the USA, and many design aids are available.

As noted before, much of the wood-framed structural design can be accomplished using design aids. The design professional will typically use these design aids to the greatest extent possible and then perform structural analysis and design for any item that is beyond the scope of the design aids. This is the approach that will be used for this study. The design drawings are shown in the Appendix. The associated detailed calculation is not provided due to space limitation; only the necessary results will be mentioned.

### 4.1 External load transfer (load path)

4.1.2D + 1.0W + L + 0.5(Lr or S or R)

Timber Buildings and Sustainability

In the above load combination, the notation is defined as follows: D for dead load, L for live load, Lr for roof live load, S for snow load, R for rain load, and W for

Wood is the most popular material used in the USA for the construction of single-family dwellings. An example of residential framing can be seen below in Figure 2 [6]. Framing lumber is easily obtained in most locations. The units of construction can be easily transported by contractors or homeowners without the need for specialized equipment. Additionally, the erection of a wood-framed struc-

tural system is familiar to most and does not require excessive amounts of

Section view of typical residential wood-framed home. Note: in this figure, a small rectangle with x inside

indicates the cross section of wood member, and DBL stands for double.

5. 0.9D + 1.0W

4. Design of residence

wind load.

Figure 2.

34

External loads must be transmitted to ground through the structural system of the building. Two main systems are needed to accomplish this transfer properly: gravity system and the main wind force resisting system (MWFRS). The gravity system transmits the vertical loads through a system of trusses, joists, and beams to foundation, which in turn transmits the load to ground, while the MWFRS transfers lateral wind load to foundation through a system of shear walls and flexible diaphragms. It is important to recognize that the ground must be properly prepared and evaluated to ensure good load transfer. Typically, foundations are placed on virgin soil or engineered (compacted) fill. All organic materials should be removed along with excessive amounts of water.

#### 4.2 Gravity system design

The gravity system in this home starts at the roof and ends in the soil. Vertical loads must have a continuous path to the ground. Generally, the gravity system in this example consists of OSB sheathing, engineered roof trusses, load-bearing stud walls, dimensional lumber headers, engineered I-joist floor system, engineered wood beams, structural steel girders, and a concrete foundation.

#### 4.3 Roof sheathing

The OSB roof sheathing, as illustrated in Figure 3, serves to transfer gravity load (i.e., dead, live, and snow loads) and wind suction to roof framing members. The roof sheathing also transfers the lateral wind loading through diaphragm action to the structure. Attachment requirements of the sheathing to roof trusses are governed by the greater of the wind uplift force or the shear transfer requirement of the connection.

According to IRC Table R503.2.1.1(1), 11.1 mm (7/16 inch) roof sheathing (24/16 span rating) is acceptable for this example. The sheathing can be used with or without edge support at 609.6 mm (24 inch) spans with an allowable live load of 1.92 kN/m<sup>2</sup> (40 lbf/ft<sup>2</sup> ), and a total allowable load of 2.39 kN/m<sup>2</sup> (50 lbf/ft<sup>2</sup> ), which is less than the 1.44 kN/m<sup>2</sup> (30 lbf/ft<sup>2</sup> ) snow loading plus 0.57 kN/m2 (12 lbf/ft<sup>2</sup> ) roof dead load. It may be possible to use 9.5 mm (3/8 inch) sheathing, but 11.1 mm (7/16 inch) thickness is more readily available and common in the locale. In this example, the sheathing will be specified with panel edge clip support. According to IRC Table R602.3(1), the sheathing is required to be attached to the truss framing with 63.5 mm (2½ inch) 8D common nails spaced at 152.4 mm (6 inch) on center (O.C.) around the edges of the panel and 304.8 mm (12 inch) O.C. at intermediate supports (field). Note that the gable end sheathing connections must be spaced at 152.4 mm (6 inch) O.C. at both the perimeter and intermediate locations.

O.C. The advantage of this is that when using a double 2 6 top plate, the joists or trusses that bear on the wall do not have to bear directly on the stud. If using a single top plate or studs spaced at 609.6 mm (24 inch) O.C., then the joists or trusses must either be directly above the stud or within 25.4 mm (1 inch) of the stud

Structural header members are used to create openings in a load-bearing wall assembly for fenestration (windows and doors) as shown in Figure 2. Dimensional lumber headers are preferred by designers when loading is low. Often times when point loading is present on a header or spans are large, an engineered lumber header, such as an LVL, may become cost-effective. An example of a typical LVL is shown in Figure 4. LVLs are also often used in wall systems when smaller depth

When specifying headers, the designer may choose to specify larger headers in some locations for consistency sake. By minimizing the amount of different beam sizes on the plan, the designer reduces the risk of misplacement of headers. As in the case of the roof sheathing, it may also turn out that some beam sizes may be more readily available, and therefore larger sections may be more economical. For example, a two-ply 2 8 beam, with a demand capacity ratio of 0.944 controlled by bearing, is adequate for BM3, but because the entire back wall on the first floor is composed of two-ply 2 10 headers and all the other headers in the building are 2 6's, it makes sense just to specify a two-ply 2 10 beam for this location as well. This eliminates the need to have another beam size on site and provides for the opportunity to use trim pieces from a different header cut to make up this short

In this home design, an engineered floor system will be used. As shown in Figure 4, I-joists have become popular and cost-effective in the residential home construction market. I-joists have several advantages over dimensional lumber

according to IRC Section R602.3.2. It is possible to use 2 4 studs spaced at 406.4 mm (16 inch) O.C., but this is not common because of the popularity of using fiberglass batts to meet the International Energy Conservation Code (IECC) [7] envelope insulation requirements. The connections between the studs and the plates are according to IRC Table 603.2(1). The connections are typically nails, and the

Structural Design of a Typical American Wood-Framed Single-Family Home

nail sizes vary between 8D and 16D based on the detail.

members are required due to space constraints.

4.6 Headers within wall system

DOI: http://dx.doi.org/10.5772/intechopen.85929

4.7 Above-grade floor system

Typical I-joist and LVL (courtesy Timber Rock Homes).

beam.

Figure 4.

37

Figure 3. Roof sheathing illustration.

#### 4.4 Engineered roof trusses

Prefabricated trusses are intended to be used on this residence and required engineering design by the manufacturer. Wood roof trusses must be designed in accordance with IRC Section R802.10. A designer or architect will typically draw the shape of the roof system, and then the truss designer will design the truss system to fit the concept. Typically, it is the responsibility of the home designer to ensure that the gravity and lateral loads from the trusses are properly transferred to the wall below. This involves specifying the connection to wall system below. When the truss drawings are received by the home designer, the loads to the structure, based on the analysis conducted by the truss designer, are typically listed on the engineered truss plans. The designer would use these loads for design. For the example case presented here, however, a set of detailed truss drawings are not available. The assumed loadings described earlier will be used for design. This is typical of an initial home design. A designer will use their assumptions and then verify such assumptions when the final truss plans are received.

#### 4.5 Exterior walls

The gravity load-bearing elements of the wall system presented here are the 2 6 dimensional lumber studs and the top and bottom plates (or sole plate). See Figure 2 for the location of the top and bottom plates. The 2 6 designation refers to a wood framing member with a nominal 50.8 mm (2 inch) width and a 152.4 mm (6 inch) depth. The actual measurements of the member are approximately 38.1 mm (1½ inch) wide and 139.7 mm (5½ inch) deep. The top and bottom plates serve to transfer both gravity and lateral loads between floors. The top plate serves three purposes: (1) a chord for the MWFRS, (2) a strut between shear panels in a wall line, and (3) a means to transfer gravity loads to the stud from the joists and trusses.

According to IRC Table 602.3(5), 2 6 studs can be used at 609.6 mm (24 inch) O.C.; however, it is more typical for the studs to be spaced at 406.4 mm (16 inch)

Structural Design of a Typical American Wood-Framed Single-Family Home DOI: http://dx.doi.org/10.5772/intechopen.85929

O.C. The advantage of this is that when using a double 2 6 top plate, the joists or trusses that bear on the wall do not have to bear directly on the stud. If using a single top plate or studs spaced at 609.6 mm (24 inch) O.C., then the joists or trusses must either be directly above the stud or within 25.4 mm (1 inch) of the stud according to IRC Section R602.3.2. It is possible to use 2 4 studs spaced at 406.4 mm (16 inch) O.C., but this is not common because of the popularity of using fiberglass batts to meet the International Energy Conservation Code (IECC) [7] envelope insulation requirements. The connections between the studs and the plates are according to IRC Table 603.2(1). The connections are typically nails, and the nail sizes vary between 8D and 16D based on the detail.

#### 4.6 Headers within wall system

Structural header members are used to create openings in a load-bearing wall assembly for fenestration (windows and doors) as shown in Figure 2. Dimensional lumber headers are preferred by designers when loading is low. Often times when point loading is present on a header or spans are large, an engineered lumber header, such as an LVL, may become cost-effective. An example of a typical LVL is shown in Figure 4. LVLs are also often used in wall systems when smaller depth members are required due to space constraints.

When specifying headers, the designer may choose to specify larger headers in some locations for consistency sake. By minimizing the amount of different beam sizes on the plan, the designer reduces the risk of misplacement of headers. As in the case of the roof sheathing, it may also turn out that some beam sizes may be more readily available, and therefore larger sections may be more economical. For example, a two-ply 2 8 beam, with a demand capacity ratio of 0.944 controlled by bearing, is adequate for BM3, but because the entire back wall on the first floor is composed of two-ply 2 10 headers and all the other headers in the building are 2 6's, it makes sense just to specify a two-ply 2 10 beam for this location as well. This eliminates the need to have another beam size on site and provides for the opportunity to use trim pieces from a different header cut to make up this short beam.

#### 4.7 Above-grade floor system

In this home design, an engineered floor system will be used. As shown in Figure 4, I-joists have become popular and cost-effective in the residential home construction market. I-joists have several advantages over dimensional lumber

Figure 4. Typical I-joist and LVL (courtesy Timber Rock Homes).

4.4 Engineered roof trusses

Timber Buildings and Sustainability

Roof sheathing illustration.

Figure 3.

4.5 Exterior walls

trusses.

36

Prefabricated trusses are intended to be used on this residence and required engineering design by the manufacturer. Wood roof trusses must be designed in accordance with IRC Section R802.10. A designer or architect will typically draw the shape of the roof system, and then the truss designer will design the truss system to fit the concept. Typically, it is the responsibility of the home designer to ensure that the gravity and lateral loads from the trusses are properly transferred to the wall below. This involves specifying the connection to wall system below. When the truss drawings are received by the home designer, the loads to the structure, based on the analysis conducted by the truss designer, are typically listed on the engineered truss plans. The designer would use these loads for design. For the example case presented here, however, a set of detailed truss drawings are not available. The assumed loadings described earlier will be used for design. This is typical of an initial home design. A designer will use their assumptions and then

The gravity load-bearing elements of the wall system presented here are the 2 6 dimensional lumber studs and the top and bottom plates (or sole plate). See Figure 2 for the location of the top and bottom plates. The 2 6 designation refers to a wood framing member with a nominal 50.8 mm (2 inch) width and a 152.4 mm

According to IRC Table 602.3(5), 2 6 studs can be used at 609.6 mm (24 inch) O.C.; however, it is more typical for the studs to be spaced at 406.4 mm (16 inch)

(6 inch) depth. The actual measurements of the member are approximately 38.1 mm (1½ inch) wide and 139.7 mm (5½ inch) deep. The top and bottom plates serve to transfer both gravity and lateral loads between floors. The top plate serves three purposes: (1) a chord for the MWFRS, (2) a strut between shear panels in a wall line, and (3) a means to transfer gravity loads to the stud from the joists and

verify such assumptions when the final truss plans are received.

joists, one of which is a greater span-to-depth ratio. This allows for shallower floor assemblies, longer spans, and higher ceilings. I-joists are generally more stable than dimensional lumber. This almost eliminates the need for bridging in a floor system and ensures consistency of engineering properties.

made by USP [10] or Simpson Strong Tie [11] is used to make any flush beam-tobeam or joist-to-beam connections within the floor system. An example would be

Structural Design of a Typical American Wood-Framed Single-Family Home

A double joist or LVL product can be used to function as stair trimmers in an engineered floor system. When loads are low, double joists are economical, but as loading and span increase, an LVL is sometimes needed. LVLs are sometimes used because the installation is cleaner looking and easier to finish than double joists. Double joists often require padding at connections and sometimes bearing, which is usually OSB, to compensate for the space between the web and flanges. LVLs are conveniently made in the same depths as I-joists, which makes it easy to use within

A benefit of using I-joists over dimensional lumber is that it is easier to put holes

predetermined locations or precut holes in the joists where mechanical penetrations are anticipated. Some guidance is typically specified in the manufacturer literature. Holes in dimensional lumber typically require structural analysis and stress evalua-

For this example home design, a central steel girder will be used to collect the floor loads and transfer to pad footings in the center of the basement. It is common for designers to use either steel girders or manufactured lumber girders in homes today. These types of girders are much stronger than dimensional lumber beams and are necessary in many instances because of the longer allowable engineered I-

Steel girders are often chosen over manufactured lumber girders when girder spans are long, head room in the basement is a premium, or steel is readily available. For this particular builder, the head room in the basement is important because they like to advertise their homes with basements that can be finished in the future.

through the joists for mechanical runs. Most I-joist manufacturers will have

tion as they become large relative to the depth of the joist or beam.

joist spans and homeowner request for open basement floor plans. Both manufactured lumber girders and steel girders must be either specified or the

the stair trimmer detail shown in Figure 5.

DOI: http://dx.doi.org/10.5772/intechopen.85929

design reviewed by a professional engineer.

the floor systems.

4.8 Girder sizing

Figure 5. Stair trimmer detail.

39

An I-joist floor system is an engineered product. Typically, a designer will send their floor plan along with preliminary input from the designer to the I-joist manufacturer. The manufacturer will then design the floor system according to the requests of the homeowner and designer. Live load deflections are often limited to L/480 (beam span/480). Because longer spans can be achieved by using an I-joist product, the chances of floor vibration occurring increase, but can be controlled, as designers will often restrict deflection to L/480.

It is common for designers to use span tables to select an initial floor joist size. This will provide a fairly accurate estimate and allow the designer to select a floor assembly depth. The improved stability and increased stiffness of I-joists allow designers to consider larger spacing for the floor joists. It is common to specify I-joists at 487.7 mm (19.2 inch) O.C., whereas it was generally common in the past to specify dimensional lumber joists at 406.4 mm (16 inch). Additionally, lumber joists are only available in certain lengths. This made the need for a splice at an internal bearing wall or beam a very common occurrence. The length of I-joists is generally only limited by transportation and site restrictions. An I-joist package will typically arrive at the site precut and ready to be installed with minimal modification.

As in the case of roof sheathing, floor sheathing serves two purposes. First, it acts in the gravity system to distribute floor loads to the joists. Secondly, it is the primary shear resisting component in the floor diaphragm, which will be discussed subsequently. Typically, the gravity loads govern the thickness choice of subflooring, and the shear requirements dictate connection to joists [8].

Once again IRC Table R503.2.1.1(1) will be used to size the sheathing. In this case, the sheathing will serve as both the underlayment and the subflooring. From the table, either 18.3 mm (23/32 inch) or 19.1 mm (3/4 inch) tongue and groove oriented strand board (OSB) sheathing would be appropriate, whichever is more cost-effective and readily available. It is possible that 15.1 mm (19/32 inch) or 15.9 mm (5/8 inch) sheathing could be used, but spans are restricted to 508 mm (20 inch). Although the joists will be specified at 487.7 mm (19.2 inch), which is less than the limit, it is likely that at least a few joists within the floor system will need to be spaced greater than 508 mm (20 inch). An example is when joist bays are used for heating, ventilating, and air conditioning (HVAC) ductwork, the joists are often spread in those locations to 609.6 mm (24 inch). In this instance, the thinner sheathing would be inadequate. IRC Table 602.3(1) specifies attachment of the sheathing to joists with a 50.8 mm (2 inch) 6D deformed nail or a 63.5 mm (2½ inch) 8D common nails spaced at 152.4 mm (6 inch) O.C. around sheathing edges and 304.8 mm (12 inch) O.C. for intermediate field spacing.

Joists for this project are selected from the Trus Joist #TJ4000 specifier's guide [9]. From the span tables within the guide, TJI110 301.6 mm (11 7/8 inch) joists are adequate for both the first and second floors of this residence. The maximum span in the home is approximately 4.70 m (15 foot–5 inch). The TJI110 301.6 mm (11 7/8 inch) joist can span a maximum of 4.90 m (16 foot–1 inch) considering L/480 deflection limit, 1.92 kN/m<sup>2</sup> (40 lbf/ft<sup>2</sup> ) live load, and a 0.96 kN/m<sup>2</sup> (20 lbf/ft<sup>2</sup> ) dead load. The TJI 28.6 mm (1 1/8 inch) engineered rim board will be used for the perimeter of the floor system. The rim board serves to transfer compressive and shear loads from the exterior walls above to foundation below. It also acts to enclose the perimeter of the floor system. Typically, joists are toenailed to sill plates at ends and nailing plates at intermediate points. Metal hardware such as that Structural Design of a Typical American Wood-Framed Single-Family Home DOI: http://dx.doi.org/10.5772/intechopen.85929

made by USP [10] or Simpson Strong Tie [11] is used to make any flush beam-tobeam or joist-to-beam connections within the floor system. An example would be the stair trimmer detail shown in Figure 5.

A double joist or LVL product can be used to function as stair trimmers in an engineered floor system. When loads are low, double joists are economical, but as loading and span increase, an LVL is sometimes needed. LVLs are sometimes used because the installation is cleaner looking and easier to finish than double joists. Double joists often require padding at connections and sometimes bearing, which is usually OSB, to compensate for the space between the web and flanges. LVLs are conveniently made in the same depths as I-joists, which makes it easy to use within the floor systems.

A benefit of using I-joists over dimensional lumber is that it is easier to put holes through the joists for mechanical runs. Most I-joist manufacturers will have predetermined locations or precut holes in the joists where mechanical penetrations are anticipated. Some guidance is typically specified in the manufacturer literature. Holes in dimensional lumber typically require structural analysis and stress evaluation as they become large relative to the depth of the joist or beam.

#### 4.8 Girder sizing

joists, one of which is a greater span-to-depth ratio. This allows for shallower floor assemblies, longer spans, and higher ceilings. I-joists are generally more stable than dimensional lumber. This almost eliminates the need for bridging in a floor system

An I-joist floor system is an engineered product. Typically, a designer will send their floor plan along with preliminary input from the designer to the I-joist manufacturer. The manufacturer will then design the floor system according to the requests of the homeowner and designer. Live load deflections are often limited to L/480 (beam span/480). Because longer spans can be achieved by using an I-joist product, the chances of floor vibration occurring increase, but can be controlled, as

It is common for designers to use span tables to select an initial floor joist size. This will provide a fairly accurate estimate and allow the designer to select a floor assembly depth. The improved stability and increased stiffness of I-joists allow designers to consider larger spacing for the floor joists. It is common to specify I-joists at 487.7 mm (19.2 inch) O.C., whereas it was generally common in the past to specify dimensional lumber joists at 406.4 mm (16 inch). Additionally, lumber joists are only available in certain lengths. This made the need for a splice at an internal bearing wall or beam a very common occurrence. The length of I-joists is generally only limited by transportation and site restrictions. An I-joist package will typically arrive at the site precut and ready to be installed with minimal

As in the case of roof sheathing, floor sheathing serves two purposes. First, it acts in the gravity system to distribute floor loads to the joists. Secondly, it is the primary shear resisting component in the floor diaphragm, which will be discussed

Once again IRC Table R503.2.1.1(1) will be used to size the sheathing. In this case, the sheathing will serve as both the underlayment and the subflooring. From the table, either 18.3 mm (23/32 inch) or 19.1 mm (3/4 inch) tongue and groove oriented strand board (OSB) sheathing would be appropriate, whichever is more cost-effective and readily available. It is possible that 15.1 mm (19/32 inch) or 15.9 mm (5/8 inch) sheathing could be used, but spans are restricted to 508 mm (20 inch). Although the joists will be specified at 487.7 mm (19.2 inch), which is less than the limit, it is likely that at least a few joists within the floor system will need to be spaced greater than 508 mm (20 inch). An example is when joist bays are used for heating, ventilating, and air conditioning (HVAC) ductwork, the joists are often spread in those locations to 609.6 mm (24 inch). In this instance, the thinner sheathing would be inadequate. IRC Table 602.3(1) specifies attachment of the sheathing to joists with a 50.8 mm (2 inch) 6D deformed nail or a 63.5 mm (2½ inch) 8D common nails spaced at 152.4 mm (6 inch) O.C. around sheathing

Joists for this project are selected from the Trus Joist #TJ4000 specifier's guide [9]. From the span tables within the guide, TJI110 301.6 mm (11 7/8 inch) joists are adequate for both the first and second floors of this residence. The maximum span in the home is approximately 4.70 m (15 foot–5 inch). The TJI110 301.6 mm (11 7/8 inch) joist can span a maximum of 4.90 m (16 foot–1 inch) considering

) dead load. The TJI 28.6 mm (1 1/8 inch) engineered rim board will be

used for the perimeter of the floor system. The rim board serves to transfer compressive and shear loads from the exterior walls above to foundation below. It also acts to enclose the perimeter of the floor system. Typically, joists are toenailed to sill plates at ends and nailing plates at intermediate points. Metal hardware such as that

) live load, and a 0.96 kN/m<sup>2</sup>

subsequently. Typically, the gravity loads govern the thickness choice of subflooring, and the shear requirements dictate connection to joists [8].

edges and 304.8 mm (12 inch) O.C. for intermediate field spacing.

L/480 deflection limit, 1.92 kN/m<sup>2</sup> (40 lbf/ft<sup>2</sup>

and ensures consistency of engineering properties.

Timber Buildings and Sustainability

designers will often restrict deflection to L/480.

modification.

(20 lbf/ft<sup>2</sup>

38

For this example home design, a central steel girder will be used to collect the floor loads and transfer to pad footings in the center of the basement. It is common for designers to use either steel girders or manufactured lumber girders in homes today. These types of girders are much stronger than dimensional lumber beams and are necessary in many instances because of the longer allowable engineered Ijoist spans and homeowner request for open basement floor plans. Both manufactured lumber girders and steel girders must be either specified or the design reviewed by a professional engineer.

Steel girders are often chosen over manufactured lumber girders when girder spans are long, head room in the basement is a premium, or steel is readily available. For this particular builder, the head room in the basement is important because they like to advertise their homes with basements that can be finished in the future.

Figure 5. Stair trimmer detail.

A W8x18 girder works well for them because it's a shallow beam and the flange width is small enough that the beam can fit in a 2 � 6 wall making the girder unnoticeable if the basement is ever finished.

A W8x18 steel girder, with a design moment capacity of 86.5 kN-m (63.8 kip-ft), is more than adequate to resist the internal moment of 31.5 kN-m (23.2 kip-ft) for the controlling load case. It is possible that a smaller girder could have been used, but W8x18 is the minimum size the builder will use. Small sizes tend to have stability issues and can be susceptible to local buckling problems caused by larger point loads. In addition, this is a readily available steel section from the builder's steel supplier.

The design of residential girders involves assumptions regarding the bracing of the beam. The American Institute of Steel Construction (AISC) Steel Construction Manual 14th Ed.(SCM) in Chapter B3.6, F1 (2) [12] and Appendix 6.3 all require that girders are restrained against rotation about their longitudinal axis at the points of support unless it can be shown that the restraint is not required. The amount of restraint provided by the adjustable column, which is typically four bolts through the bottom flange, may need a detailed analysis because of the slenderness of the columns.

Steel girders in most residential cases are usually ordered in a single length if possible to avoid splices and therefore are continuous over their intermediate supports. Negative moment occurs at the intermediate supports, which puts the bottom flanges in compression in those regions.

If it is assumed that the columns do not provide adequate bottom flange support, then these negative moment regions would be destabilizing, and since inflection points are not typically recognized as a brace point (SCM Appendix 6.3), the unbraced length would have to be taken as the entire beam length of 11.0 m (36 feet), which would require a very large section. Additionally, if no compression flange bracing is assumed at the supports, then the beam fails the concentrated load check in SCM J10.4 for web sidesway buckling. Section J10.4 requires the supports to be adequately braced under these circumstances.

If it is assumed that the column is braced against rotation at the supports by either assuming the column connection is adequate or providing additional bottom flange support, then the unbraced length reduces to the distance between the columns, which in this case is 9<sup>0</sup> -0″ and the beam passes both strength and concentrated load checks.

made using Computers and Structures, Inc. (CSI) SAP2000 finite element modeling software to verify the results of Enercalc and determine the controlling permutation. Results were within 1% of each other between the two analysis packages.

Pattern loading was significant in this example. If only the full intensity live load

Adjustable columns are generally used in residential construction as intermedi-

ate supports for basement girders. Adjustable columns are readily available at almost any hardware stores and can be adjusted in height to match site conditions by the contractor. Figure 8 shows an example of typical adjustable columns. The maximum loading, as reported by the manufacturer, is a factored allowable ASD load capacity (Ra). Reactions determined by ASD load combination can be used to directly size the column from the manufacturers testing data. For this particular home design case, the maximum ASD girder reaction is 80.5 kN (18.1 kip). According to the manufacturers data, an 88.9-mm (3½ inch) and 2.31-mm-thick (11 gauge) column with a height between 2.21 m (7 foot–3 inch) and 2.31 m (7 foot–7 inch) has an allowable load of 95.6 kN (21.5 kip), which is greater than the maximum column axial demand of 80.5 kN (18.1 kip). All three columns

application was to be considered, then the design moment would have been underestimated by approximately 5%, and the support reactions would have been underestimated by approximately 5% at supports 2, 4, and 12% at support 3. If ignored, this could have led to the undersizing of both adjustable column and pad

Moment diagram showing maximum internal moment over support 2.

Structural Design of a Typical American Wood-Framed Single-Family Home

DOI: http://dx.doi.org/10.5772/intechopen.85929

footing.

41

Figure 7.

Figure 6.

Steel girder beam pocket detailing.

4.9 Adjustable columns

Also restraint against rotation should be provided at the ends of the beams, which are seated in the beam pockets. Typically, beam pockets in the concrete wall are oversized to facilitate easy installation of the beams. This creates the opportunity for twisting. SCM Section J10.7 requires all unframed girder ends to have a pair of transverse stiffeners if unrestrained. In this case, a better idea would be to grout the pocket as shown in Figure 6, or provide some type of shim, after installation to restrain the end against rotation. It should be noted that the required moisture management and thermal envelope components are not shown for clarity in the figure.

Another consideration for girder sizing is live load pattern loading. Since the girder is a continuous beam having multiple spans, ASCE 7 Section 4.3.3 requires the consideration of pattern loading. In this case, it turns out that applying live loading to spans 1, 2, and 4 only produced the largest internal moment of 31.5 kN-m (23.3 kip-ft) in the beam. Figure 7 shows the moment diagram for the controlling load combination and the spans that were loaded to produce it.

Pattern loads are considered in the structural analysis software package Enercalc that was used for beam design. Enercalc runs all permutations of live load application and reports the worst-case scenario in envelope format. Data for individual permutations is not able to be extracted. For this example, a separate check was

Structural Design of a Typical American Wood-Framed Single-Family Home DOI: http://dx.doi.org/10.5772/intechopen.85929

#### Figure 7.

A W8x18 girder works well for them because it's a shallow beam and the flange width is small enough that the beam can fit in a 2 � 6 wall making the girder

A W8x18 steel girder, with a design moment capacity of 86.5 kN-m (63.8 kip-ft), is more than adequate to resist the internal moment of 31.5 kN-m (23.2 kip-ft) for the controlling load case. It is possible that a smaller girder could have been used, but W8x18 is the minimum size the builder will use. Small sizes tend to have stability issues and can be susceptible to local buckling problems caused by larger point loads. In addition, this is a readily available steel section from the builder's steel supplier. The design of residential girders involves assumptions regarding the bracing of the beam. The American Institute of Steel Construction (AISC) Steel Construction Manual 14th Ed.(SCM) in Chapter B3.6, F1 (2) [12] and Appendix 6.3 all require that girders are restrained against rotation about their longitudinal axis at the points of support unless it can be shown that the restraint is not required. The amount of restraint provided by the adjustable column, which is typically four bolts through the bottom flange, may need a detailed analysis because of the slenderness of the

Steel girders in most residential cases are usually ordered in a single length if possible to avoid splices and therefore are continuous over their intermediate supports. Negative moment occurs at the intermediate supports, which puts the bottom

If it is assumed that the columns do not provide adequate bottom flange support, then these negative moment regions would be destabilizing, and since inflection points are not typically recognized as a brace point (SCM Appendix 6.3), the unbraced length would have to be taken as the entire beam length of 11.0 m (36 feet), which would require a very large section. Additionally, if no compression flange bracing is assumed at the supports, then the beam fails the concentrated load check in SCM J10.4 for web sidesway buckling. Section J10.4 requires the supports

If it is assumed that the column is braced against rotation at the supports by either assuming the column connection is adequate or providing additional bottom flange support, then the unbraced length reduces to the distance between the

Also restraint against rotation should be provided at the ends of the beams, which are seated in the beam pockets. Typically, beam pockets in the concrete wall are oversized to facilitate easy installation of the beams. This creates the opportunity for twisting. SCM Section J10.7 requires all unframed girder ends to have a pair of transverse stiffeners if unrestrained. In this case, a better idea would be to grout the pocket as shown in Figure 6, or provide some type of shim, after installation to restrain the end against rotation. It should be noted that the required moisture management and thermal envelope components are not shown for clarity in the

Another consideration for girder sizing is live load pattern loading. Since the girder is a continuous beam having multiple spans, ASCE 7 Section 4.3.3 requires the consideration of pattern loading. In this case, it turns out that applying live loading to spans 1, 2, and 4 only produced the largest internal moment of 31.5 kN-m (23.3 kip-ft) in the beam. Figure 7 shows the moment diagram for the controlling

Pattern loads are considered in the structural analysis software package Enercalc that was used for beam design. Enercalc runs all permutations of live load application and reports the worst-case scenario in envelope format. Data for individual permutations is not able to be extracted. For this example, a separate check was

load combination and the spans that were loaded to produce it.


unnoticeable if the basement is ever finished.

Timber Buildings and Sustainability

flanges in compression in those regions.

columns, which in this case is 9<sup>0</sup>

trated load checks.

figure.

40

to be adequately braced under these circumstances.

columns.

Moment diagram showing maximum internal moment over support 2.

made using Computers and Structures, Inc. (CSI) SAP2000 finite element modeling software to verify the results of Enercalc and determine the controlling permutation. Results were within 1% of each other between the two analysis packages.

Pattern loading was significant in this example. If only the full intensity live load application was to be considered, then the design moment would have been underestimated by approximately 5%, and the support reactions would have been underestimated by approximately 5% at supports 2, 4, and 12% at support 3. If ignored, this could have led to the undersizing of both adjustable column and pad footing.

#### 4.9 Adjustable columns

Adjustable columns are generally used in residential construction as intermediate supports for basement girders. Adjustable columns are readily available at almost any hardware stores and can be adjusted in height to match site conditions by the contractor. Figure 8 shows an example of typical adjustable columns. The maximum loading, as reported by the manufacturer, is a factored allowable ASD load capacity (Ra). Reactions determined by ASD load combination can be used to directly size the column from the manufacturers testing data. For this particular home design case, the maximum ASD girder reaction is 80.5 kN (18.1 kip). According to the manufacturers data, an 88.9-mm (3½ inch) and 2.31-mm-thick (11 gauge) column with a height between 2.21 m (7 foot–3 inch) and 2.31 m (7 foot–7 inch) has an allowable load of 95.6 kN (21.5 kip), which is greater than the maximum column axial demand of 80.5 kN (18.1 kip). All three columns

4.11 Foundation walls

DOI: http://dx.doi.org/10.5772/intechopen.85929

information.

First floor

Second floor

Garage<sup>b</sup>

IRC Table 602.10.3.

Wall bracing. Values in meters (inches).

a

b

43

Table 9.

Based on soil categorization, the ACI provides prescriptive foundation sizing tables in Appendix A of ACI 332, which are usually appropriate for most situations. For most residential designs, geotechnical exploration and lab testing are cost prohibitive, and therefore soil pressures must be assumed. ASCE 7 provides design lateral soil load that can be used in the absence of site-specific geotechnical

Structural Design of a Typical American Wood-Framed Single-Family Home

For this design, the equivalent soil pressure will be estimated at 2.15 kn/m<sup>2</sup> per linear meter (45 lbf/ft<sup>2</sup> per linear foot). According to ASCE 7 Table 3.2.1, this is representative of a type GC soil (unified soil classification), which is described as a clayey gravel, poorly graded, gravel, and sand mix. Assuming horizontal backfill and a vertical foundation wall, this is roughly equivalent to 19.6 kN/m<sup>3</sup> (125 lbf/ft<sup>3</sup>

According to ACI 332 Table 9, 21 MPa (3000 psi) is the minimum required compressive strength for foundation walls in the severe weather probability category. Because the concrete will be exposed to weathering, it must be air entrained,

The concrete foundation wall for the main structure in this example has an unsupported height of 2.44 m (8 foot) and will be subjected to approximately 2.13 m (7 foot) of unsupported backfill when in service (Figure 9a). For this situation, considering reinforcing bars with a yield strength of 420 MPa

(60,000 psi), ACI 332 Table A.4 allows for the use of a plain concrete (no vertical reinforcing needed) 203.2 mm (8-inch)-thick foundation wall. To minimize shrinkage cracking, however, ACI 332 requires the use of three continuous horizontal bars in the wall. One must be placed within 609.6 mm (24 inch) of the top, one within 609.6 mm (24 inch) of the bottom, and the last one in between the other

Walls Length required Length provided Method

N 4.24 (167) 8.23 (324) CS-WSP S 3.40 (134) 3.66 (144) WSP E 3.20 (126) 3.66 (144)<sup>a</sup> WSP W 3.20 (126) 3.66 (144) WSP

N 1.83 (72) 2.44 (96) WSP S 1.83 (72) 2.44 (96) WSP E 1.52 (60) 3.66 (144) WSP W 1.52 (60) 3.66 (144) WSP

N 1.27 (50) 2.44 (96) WSP E 1.32 (52) 1.37 (54) WSP W 1.32 (52) 2.44 (96) WSP

For WSP methods panel lengths between 0.914 and 1.22 m (36 and 48 inches) are allowed but must be adjusted per

The required bracing for the garage/main house common wall will be added directly to the first floor north wall.

soil with an internal friction angle of 28 degrees [15].

having an air content of 6% plus or minus 1.5%.

)

#### Figure 8. Typical adjustable column.

will be specified for this maximum loading. This will decrease the chances of misplacing columns.
