Tests on progressive collapse response of flat slab systems

Yi et al. (2014) presented half scaled tests of two flat slab systems to assess their static collapse response. Each system comprised of two bays spanning 2.564 m in both orthogonal directions (Figure 2.2). The flat slab specimens were 90 mm thick and supported on 213mm square columns. The first test assessed flat slab system response after the static removal of an internal

column (obtained by lowering the mechanical jack on which the column is supported) and under a uniform area load of 20.5 KNm-2, which was gradually applied in six steps. After each loading step the internal support was gradually released until all the load applied was borne by the slab, without the internal support. Tests on the second specimen involved the assessment of collapse response of the flat slab system after the gradual removal of an edge and a corner support at different occasions.

Figure 2.2: Slab specimen details (Yi et al., 2014)

Concrete with compressive strength of 39.5 MPa at 28 days was used and steel reinforcement had yield and tensile strengths of 452 MPa and 589 MPa respectively. Hogging reinforcement comprised of 6.5 mm diameter bars provided at 70 mm centre to centre while the sagging reinforcement comprised of 6.5 mm diameter bars provided at 150 mm centre to centre. No punching shear reinforcement was provided.

Findings from tests by Yi et al. (2014) confirmed the contributions of compressive and tensile membrane action to the response of flat slab systems after the removal of an internal column. These mechanisms enabled the flat slab system to sustain twice the design load. Such high

residual strength results because tensile membrane action is not taken into consideration in the ultimate limit state design of structures. Tensile membrane action developed without the punching shear failure of the slab specimens though punching shear reinforcement was not provided. This may be due to the high slenderness of the slab test specimens due to scaling and low percentage of hogging reinforcement provided. Influence of these factors on the response of slab-column connections is discussed in Section 5.2.3 of this thesis.

Results also showed the flat slab system to be more vulnerable to the removal of exterior or corner columns because the slab collapse resistance in these cases were less than twice the design load that was sustained in the cases of removal of an interior column. This is due to less horizontal confinement around edge and corner connections relative to those around interior connections. Detailed description of the test set-up, material properties and report on results are provided in Yi et al. (2014).

Russell, Owen and Hajirasouliha (2015)

Russell, Owen and Hajirasouliha (2015) carried out tests on seven 1/3 scaled simplified flat slab structures (Figure 2.3) to assess dynamic load and displacement amplifications after the sudden loss of a column in a flat slab structure. The slab specimens were 80 mm thick. In the tests, the removal of a corner, penultimate edge or an internal edge column were investigated under static and dynamic modes. For dynamic tests, a temporary support was adopted which comprised of a vertical bar which rested on a bottom plated, supported on a load cell and steel rollers. This allowed for easy removal of the support.

The slab specimens were supported on 135x135x25 mm steel plates which were only restrained in the downward vertical direction. Hence, connections allowed for rotation and uplift. Concrete strength of slabs used in the tests ranged between 24.4 MPa and 37.1 MPa. Percentage of hogging reinforcement provided at the internal supports was 0.21% in both orthogonal directions. At other

areas of the slabs, the percentage of flexural reinforcement provided was 0.18%. No shear reinforcement was provided. Area loads of 3.0, 6.8 and 7.7 kNm-2 were adopted for the sudden column removal tests.

Results showed that the support removal time for the various tests ranged between 39 and 57 ms. Progressive collapse was observed to occur after the punching shear failure of some connections. Maximum strain rates in the steel reinforcement for the dynamic tests were reported to be less than 0.35 s-1_{. A maximum increase in displacement of about 50% was reported between the static}

and dynamic tests which showed that the common design recommendation of a load increase of 2.0 was conservative. Detailed description of the test set-up, material properties and report on results are provided in Russell, Owen and Hajirasouliha (2015).

Figure 2.3: Specimen details (a) corner and penultimate support removal tests (b) middle support removal test (Russell et al., 2015)

Xue et al. (2018)

Xue et al. (2018) carried out static interior column removal tests on two flat slab systems to assess the influence of area gravity load (supported by the slab) and concentrated load (transferred to interior slab-column connection from upper lying slabs) on load transfer and collapse resistance of the flat slab systems. Flat slab system assessed was extracted from the first floor of a 4-story car park designed in accordance to the Australian Concrete Standard, AS 3600 (AS, 2009). The flat slab system had two spans in each orthogonal direction, with length 2000 mm (Figure 2.4). The slab specimens were 90 mm thick and supported on columns with cross sections 150x150 mm. Specimens were cast using concrete with strength 32 MPa and 8mm reinforcement bars with yield strength 500 MPa. Percentage reinforcement provided at the column strip was 0.489% at the top and 0.237% at the bottom. Other areas of the slab specimen had percentage reinforcement of 0.237% provided. A design live load of 5 kNm-2 _{was gradually}

applied on the slab, after which the interior connection was displaced downwards to a displacement of about 500 mm.

Xue et al. (2018) identified flexural action, tensile membrane action, one-way catenary and the dowelling actions of steel rebar to be the main load resisting mechanisms. No contribution of compressive membrane action was reported. Compressive membrane action probably failed to develop due to insufficient lateral restraint at the exterior connections. Punching shear failure was observed to occur at the point of column loss due to the displacement load applied at the interior connection. Three-dimensional tensile membrane action developed in steel reinforcements around connections was found to contribute to connection capacity after punching. Detailed description of the test set-up, material properties and report on results are provided in Xue et al. (2018).

Figure 2.4: Slab specimen details (Xue et al., 2018)