Design of Test Specimens

The dimensions and material properties of the specimens were selected so that they could comply with the scope of this study, satisfy the constraints imposed in the experiment and allow the columns to reach a plastic phase while maintaining stable response (i.e., preventing complete collapse) under combined axial load and lateral impact load. Firstly, the constraints imposed on the experiments were identified. Secondly, the dimensions and material properties were determined to fulfil the scope of the study and satisfy the constraints. Subsequently, the

46 Chapter 3: Experimental Set-up and Procedure preliminary numerical model of the CFDST column was developed to determine the most

suitable dimensions and material properties of the columns which allow the columns to reach a plastic phase under specified axial and impact loads while preventing unstable response. In the development of the numerical model, use was made of the verified techniques, namely knowledge obtained from numerical simulation of CFST columns in comparison with experiments available in literature. Chapter 5 and Chapter 7 describe the numerical model of CFDST column and CFST column, respectively.

3.2.1 Identified constraints

The following constraints were identified and considered in selecting the specimens’

dimensions and properties:

The study scope required that the selected columns be slender. To satisfy this, the dimensions of the specimens were selected in such a way that the unbraced column slenderness ratio, λ, was greater than 22 (AS3600, 2009). The value of slenderness ratio was calculated using Equation 2-1.

To satisfy the study scope, the tubes should be the fabricated from mild steel and classified as compact according AS4100 (1998) and AS5100 (2004). Therefore, in selection of wall thickness of mild steel tubes attention was made to ensure that the plate element slenderness is less than the yield slenderness limit. Additionally, according to the research scope, the concrete strength should be of the standard grades (i.e., 25, 32, 40, 50, or 65 MPa) (AS5100, 2004).

Features of the laboratory facilities imposed some constraints in selection of the specimens’ characteristics. The constraints included:

o The specimens’ length should be selected so that the distance between the supporting frames of horizontal impact testing system was equal to the multiple of the distance between the tie down points on the strong floor (i.e., 1120mm) as these frames are bolted to the floor at the tie down locations. The details about these frames are provided in Section 3.7.3.2. The tie down points’ location matrix on the strong floor is shown in Figure I-1 in Appendix I.

Chapter 3: Experimental Set-up and Procedure 47 o To accommodate the existing specimen end caps, specimens’ width or diameter

should not exceed 310 mm. The details about these end caps are provided in Section 3.7.2.1.

o The horizontal impact testing system does not allow the striker’s mass to exceed 325 kg.

o The horizontal impact testing system does not allow the striker’s velocity to be greater than 8 m/sec.

The spacing between the outer and inner steel tubes should be sufficient to accommodate the maximum aggregate size. Thus, the dimensions of the inner and outer tubes were selected so that Equation 3-1 is satisfied.

(𝐷𝑜−2𝑡_𝑜)−𝐷_𝑖

2 > 𝑀𝐴𝑆 Equation 3-1 where Do, Di, to and MAS are the outside diameter of the outer tube, outside diameter of inner tube, wall thickness of outer tube, and maximum aggregate size, respectively.

Commercially-available steel hollow section sizes in Australia was the final factor to consider in selecting the dimensions of the specimens. These data was found in (Australian Tube Mills, 2013).

3.2.2 Nominated dimensions and material properties of specimens

Following a preliminary numerical analysis and incorporating all the experimental constraints, it was decided to conduct the impact tests on CFDST specimens with circular outer and inner steel tubes with dimensions and properties as presented in Table 3-1 and Table 3-2, respectively. The typical CFDST section is shown in Figure 3-1.

Table 3-1: Nominated dimensions of the specimens

Parameter Value

Specimen’s length-L (m) 3

Outside diameter of outer tube-Do (mm) 165.1 Wall thickness of outer tube-to (mm) 5.4 Outside diameter of inner tube-Di (mm) ^33.7 Wall thickness of inner tube-ti (mm) ⁴

48 Chapter 3: Experimental Set-up and Procedure

Specimen’s slenderness ratio-λ ^71.8

Outer tube’s plate element slenderness- λeo 31.5 Inner tube’s plate element slenderness- λei 8.7

((Do-2to )-Di)/2 60.3

Table 3-2: Nominated material properties of the specimens

Figure 3-1: Typical profile of the CFDST specimens 3.3 Test Matrix

In this impact testing program, only the axial pre-loading and impact location were varied.

Upon validation of numerical model using the experimental results (presented in Chapter 5), the model was used to further investigate the influence of various structural and load-related parameters on the impact response of CFDST columns (presented in Chapter 6).

Four series of tests, with a total of eight tests, involving different combinations of axial load and impact location were considered as contained in Table 3-3. To ensure repeatability, more than one specimen was tested for most series. Axial pre-loadings were 0 kN, 200 kN and 400 kN, which were within a range of 0%, 15% and 30% of the CFDST specimen axial capacity.

Parameter Value

Unconfined compressive strength of concrete-f’c (MPa) 25

Maximum aggregate size-MAS (mm) 10

Yield strength of outer steel tube -fyo (MPa) 250 Ultimate strength of outer steel tube- fuo (MPa) 320 Yield strength of inner steel tube -fyi (MPa) 250 Ultimate strength of inner steel tube- fui (MPa) 320

Chapter 3: Experimental Set-up and Procedure 49 The impact locations were mid-span and two-third of column length away from one of the

supports (i.e., off-centre). The striker mass and initial impact velocity were 262 kg and 7.8 m/sec, respectively. Simply supported boundary conditions were considered for all test series.

To identify specimens, each was labelled, where the last letter refers to the first test in the particular series (A) or the repeated test in the same series (B or C), the second to last letter refers to the test series number (1, 2, 3, 4) and the rest of the letters refer to the column type (i.e., CFDST).

Table 3-3: Test matrix

Test Series Specimen Axial Load (kN) Impact Location

Series 1 CFDST1A 0 Mid-span

The inner and outer tubes were supplied by One Steel Mill Tube (Australia). These circular tubes were cold-formed structural steel hollow sections with grade C250L0 manufactured to (AS1163, 2009). Roll forming with electric resistance welding (ERW) technique was used to form these circular sections. In total, there were four inner tubes and four outer tubes with “as delivered” lengths of 6.5 m. Two 3-m long tubes were cut from each 6.5 m long tube to obtain eight inner tubes and eight outer tubes. The average length of the specimens, which was measured by a steel measure tape, was 3000 mm. Table 3-4 summarises the nominal and measured values of outside diameter and wall thickness of outer and inner tubes. Whilst an