Experimental Compression Tests on the Stability of Structural Steel Tubular Props

Ibrahim Al-Jumaili, Samer Barakat and Zaid A. Al-Sadoon

#### Abstract

The stability of structural steel props was investigated experimentally. Specimens were of steel tubular props with a total length of 4.5 m and had two hollow steel tubes with the ability to slide into each other. The effect of inserted length between the two steel tubes on the prop buckling capacity was investigated. Four full-scale specimens were tested using a 100-kN hydraulic actuator to determine the buckling strengths exhibited by the props. The steel specimens were of 2 mm and 3 mm thicknesses, while the inserted lengths were of 250 and 1000 mm. The effects of the different parameters on the buckling capacity and modes of failure were discussed. It was concluded that the buckling strengths were sensitive to the inserted length and wall thickness of the inner tube. The prop buckling capacity increased with the increase of the inserted length, and this increase is more pronounced if combined with the increase in the wall thickness. Such increase in the prop buckling capacity that leads to reduction in the required number of steel props and lateral supports for the same working area also contributes to the sustainable solutions in terms of reducing the material use, the construction time, and the costs while taking into consideration the safety requirements.

Keywords: structural steel, prop, scaffold, stability, experimental research, sustainable material

#### 1. Introduction

Nowadays, the design philosophies and trends focus on sustainability that is decreasing the resource use while reducing the impacts on human health and the environment during the building's lifecycle [1] and as meeting the human needs without compromising needs of the future generations [2]. Steel is considered more efficient in reducing the use of natural resources accompanied with less emissions and energy usage due to its high recyclability that results in less wastes and emissions and its durability that results in many sustainability favors. Many ways could be adopted to limit the environmental-related issues associated with the production of steel, starting from the design optimization, material recyclability, and reuse [3].

The construction methods being used in the construction industry contain shoring systems used to avoid slab deflection and buckling to limit deflections and slab failures. These systems consist of steel props which are evenly distributed under the slab whereby each slab shares the formwork weight as well as the weight of the dead loads. In order to avoid buckling, lateral supports are introduced in freshly placed floors, but they exhibit a disadvantage of free movements to the workforce on-site. Besides, steel props may fail due to flexural collapse, prop base failures, and the buckling of tubes.

Many studies have been conducted to check the efficiency and safety of the scaffold shoring system. Yanlong et al. [4] analytically investigated the influence of the connection stiffness by the concept of the effective length. The study showed that load capacity increased linearly with the increase of the effective number of steel balls, inserted at the intersectional area between the two tubes of the prop. The stable load capacity increases with the gap of the inner and outer tubes decreasing, under a fixed radius of balls. Compared to the hydraulic prop, the stable load capacity can be achieved. The maximum plastic zone depth increases with the increase of the embedded depth of steel ball in the inner tube. The maximum support height of the prop is a power function and decreases with the increase of designed stable load capacity.

Barakat [5] experimentally studied the buckling capacity of a 6 m length steel prop consisted of two hollow steel tubes. Each specimen consisted of two pipes, one slides inside the other for a certain length, tightened together by two bolts. Different (outer, inner, and inserted) length combinations are tried in order to maximize the elastic buckling capacity of the whole prop. Based on the experimental results, it was concluded that the typical failure mode of the studied steel props was the elastic global buckling. The elastic buckling strength of the prop was found to be sensitive to the inserted length of the smaller tube, and it increases with increasing inserted length.

Hongbo et al. [6] conducted an experimental and analytical study to obtain the strength and failure mode of twelve full-scale structural steel tubes and coupler scaffolds (STCs). The study concluded that the typical failure mode was global buckling about the weak longitudinal axis. The strength was sensitive to the rotational stiffness of the couplers and the story height but insensitive to the post spacing, the U-head height, and the sweeping staff height. The study developed a simplified model for the STCS post design that was verified by the full-scale test and analytical results.

A number of other studies [7, 8] examined the stability and the non-linear behavior of the steel scaffolding in full-scale testing and computer simulations. The studies showed that structural failure of props was due to a number of reasons, among them insufficient design, weak installation, and overloading. This failure does not only lead to project delays but causes more serious injuries to the construction workers. These strong scaffold tubes should be used when heavy loads are needed to be carried, and multiple platforms must reach several stories.

Various analytical and experimental studies had been carried out to examine the structural behavior of high-clearance scaffold in the efforts of determining their safety when used in construction sites. Most of the research focuses on the two-wall formed steel, door-type steel scaffolds, and the coupler scaffolds used in residential and commercial buildings with limited research on structural coupler scaffolds and steel tubes [9].

Freitas et al. [10] investigated the performance of five steel prop samples subjected to gradually applied load and developed an analytical study using the finite element method. Better results were found after using the computational study in the comparison of the analytical and experimental studies. It was found out that the critical load was 4% less than the value arrived at using the experimental study.Gearhart [11] carried out studies to examine the effect of end conditions and eccentric load profiles on props. It was established that the stability of props is affected by parameters such as slenderness ratio, boundary conditions, the orientation of the load application, and the material properties of the specimens.

Experimental Compression Tests on the Stability of Structural Steel Tubular Props DOI: http://dx.doi.org/10.5772/intechopen.87836

In this study, the buckling capacity of steel tubular props with different geometric combinations subjected to an axial load is investigated. Axial compression tests up to failure are carried out on eight specimens under static loads. These specimens consisted of two hollow steel tubes in which one slides inside the other for a certain length and comprise two thicknesses of 2 and 3 mm and two inserted lengths of 250 mm and 1000 mm. The research draws conclusions on the effects of the inserted length and the tube wall thickness on the axial load capacity of the tested steel props.

#### 2. Theoretical formulation and AISC procedure

Steel tubular props contain numerous uses in the construction sector. In an ideal situation, they are viewed as the column under axial load. However, columns fail in events the stress in the column surpasses the yield stress of the material. For example, large compression load makes the column unstable causing a sudden lateral defection that leads to buckling. The factors which dictate the load required to make the column buckle include column length, the elastic modulus of the material member, configuration and dimensions of the column cross section, and the column supports. The assumptions taken in the Euler buckling load are that the column is perfectly straight before loading, column material is homogenous, the load is applied through the centroid of the cross section of the column, and the material stresses remain in the linear elastic region of the stress-strain curve. The axial critical load Pcr and critical stress Fcr of which buckling occur by Euler are

$$P\_{cr} = \frac{\pi^2 EI}{\left(kL\right)^2} \tag{1}$$

$$F\_{cr} = \frac{\pi^2 E}{\left(\frac{kL}{r}\right)^2} \tag{2}$$

where E is Young's modulus, kL isthe effective length, kL/risthe slendernessratio, pffiffiffiffiffiffiffiffi <sup>r</sup> <sup>¼</sup> <sup>I</sup>=<sup>A</sup> isthe radius of gyration, <sup>I</sup> isthe leastsecond moment of inertia, and <sup>A</sup> isthe cross-sectional area. The (k) is dependent on the constraints of the boundaries.

It should be noted that laboratory conditions are ideal and do not apply in actual column tests. Besides, even when all the experiments are performed in the same laboratory, different results will be achieved due to difficulties met when centering the loads, the absence of material uniformity, varying dimensions of the sections provided, residual stresses, and end restraint variations, among other factors. In this case, formulas will be developed to yield results which act as an approximate value of the expected results. Inelastic buckling refers to a buckling phenomenon whereby the proportional limit of a material is exceeded within the cross section before the occurrence of buckling. Columns with such specifications undergo buckling where permanent deformations occur after the critical buckling load is reached. Columns are sometimes classed as being long, short, or intermediate. For long columns where the axial buckling stress remains below the proportional limit, such columns will buckle elastically, and the Euler formula (Eq. (1)) predicts very well the strength of these columns. For short columns, no buckling will occur and the failure stress will equal the yield stress. Intermediate columns will fail inelastically by both yielding and buckling. Formulas with which the AISC-LRFD estimates the strength of columns in these differences may be determined as follows:

$$P\_n = \phi \, F\_{cr} \, A\_{\text{g}} \tag{3}$$

$$\text{if } \frac{K \text{ L}}{r} \le 4.71 \sqrt{\frac{E}{F\_{\text{y}}}} \text{ or } \left(\frac{F\_{\text{y}}}{F\_{\text{e}}} \le 2.25\right) F\_{cr} = \left[0.658^{\frac{F\_{\text{y}}}{F\_{\text{e}}}}\right] F\_{\text{y}} \tag{4}$$

$$\text{if } \frac{K \, L}{r} > 4.71 \sqrt{\frac{E}{F\_{\text{y}}}} \text{ or } \left(\frac{F\_{\text{y}}}{F\_{\text{e}}} \le 2.25\right) F\_{cr} = 0.877 \, F\_{\text{e}} \tag{5}$$

$$F\_{\varepsilon} = \frac{\pi^2 E}{\left(\frac{K \cdot L}{r}\right)^2} \tag{6}$$

#### 3. Experimental program

The experimental program involves testing four 4.5 m long steel tubular props using 100-kN hydraulic actuator. The two hollow steel tubes have the ability to slide into each other and are tightened together with one 10 mm pin. The duplicate of the four combinations (two thicknesses, 2 and 3 mm; and two inserted lengths, 250 mm and 1000 mm) is considered and composed of the eight test specimens of the experimental program. The details of the steel props are illustrated in Table 1 as well as in Figure 1.

During testing, the axial load and stroke, the lateral vertical and horizontal displacements, and the axial and transverse strains are recorded. The displacements are measured by a linear variable differential transformer (LVDT), while the strains are measured by strain gauges. Figures 2 and 3 present the test setup and instrumentations.


PT, prop thickness; O, outer tube; I, inner tube; D, diameter

#### Table 1.

Test specimen designation and geometric dimensions.

Figure 1. Prop jack test configurations.

Experimental Compression Tests on the Stability of Structural Steel Tubular Props DOI: http://dx.doi.org/10.5772/intechopen.87836

Figure 2. Experimental configuration of the prop jacks with dynamic actuator.

#### Figure 3.

Instrumentation of the testing props. (a) Prop jack test setup, (b) (LVDTs) location, and (c) strain gauges locations.

#### 4. Results and discussion

The geometric dimensions and the corresponding test results are summarized and presented in Table 2. Four combinations of the tube thickness and the inserted length were considered.

Figure 4 illustrates the load versus the stroke of an individual sample of the four combinations. Due to the increase in the inserted length, 750 mm, there is an 8.1% gain in the prop axial capacity for the prop with thickness 2 mm and 14.3% gain in terms of lateral displacement. For the props with thickness 3 mm, a 20.3% gain in the prop axial capacity and a 17.2% gain in terms of lateral displacement were observed. In terms of the increased thickness, there is an average 60% increase in the buckling load capacity for the props with the same inserted length. It was


Table 2. Experimental test results.

Figure 4. Axial load—stroke displacement curves for props configurations.

observed that the failure mode exhibited by the eight full-scale specimens was flexural buckling. Besides, the eight steel props did not return to the original position after unloading and exceeding inelastic buckling. Figure 5 shows the deflected shape for the specimen PT3.0-O2.0-I2.75 at maximum buckling load (Figure 5a) and after unloading (Figure 5b).

Figure 5. Deflection for props PT3.0-O2.0-I2.75 #3. (a) At maximum buckling load and (b) after unloading.

Experimental Compression Tests on the Stability of Structural Steel Tubular Props DOI: http://dx.doi.org/10.5772/intechopen.87836

#### Figure 6.

The state of the pins after the testing. (a) Prop with thickness 3 mm and (b) prop with thickness 2 mm.


#### Table 3.

Comparison of thermotical calculations and experimental results of specimens.

Figure 6 shows the pins and holes of specimens after testing. The specimen with thickness 3 mm shows a failure in the pins, while the prop with thickness 2 mm does not. Also, props with higher thickness showed an extra elongation in the pinhole area which causes a change in shape from a circle to an elliptical.

The prop buckling capacity is calculated using the AISC equations (Eqs. (3) and (6) above) considering fixed-fixed ends with k = 0.5. Table 3 summarizes the calculated buckling load and the k-value needed to produce the experimental buckling load for each specimen. It is clear from the calculated k values that the end conditions are in between the fixed-fixed (k = 0.5) and the fixed-pinned ends (k = 0.7). A value of k = 0.63 is considered a conservative one.

#### 5. Conclusions

This work aims to experimentally study the buckling capacity of a 4.5 m length steel prop consisting of two hollow steel tubes in which one slides inside the other for a certain length. In order to obtain the elastic strength of the props, four

full-scale prop specimens in duplicate with various geometric properties were constructed and tested statically and elastically in the lab. The total unsupported length is 4.5 m for the eight props. Each specimen consists of two pipes, one inserted in the other for a certain length, tightened together by a 10 mm pin. Two different tube thicknesses and two (outer, inner, and inserted) length combinations are tried in order to maximize the elastic buckling capacity of the whole prop. All props are loaded up to elastic buckling using 100-kN capacity hydraulic actuator. Based on the experimental results, the following conclusions can be drawn:

	- For the same inserted length increase, 3 mm thick props can carry about 2.5 times the axial load capacity of the 2 mm thick props.
	- For the same inserted length and increasing the prop thickness from 2 mm to 3 mm, there is an average of 60% increase in the prop buckling load capacity.

#### Acknowledgements

This work was supported by the Deanship of Graduate Studies and Research and the Sustainable Construction Materials and Structural Systems (SCMASS) research group, University of Sharjah. This support is highly acknowledged.

#### Conflict of interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Experimental Compression Tests on the Stability of Structural Steel Tubular Props DOI: http://dx.doi.org/10.5772/intechopen.87836

#### Author details

Ibrahim Al-Jumaili\*, Samer Barakat and Zaid A. Al-Sadoon Department of Civil and Environmental Engineering, University of Sharjah, Sharjah, UAE

\*Address all correspondence to: u17105679@sharjah.ac.ae

© 2019 The Author(s). Licensee IntechOpen. This chapteris distributed underthe terms oftheCreative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 18**
