When designing a slab system comprising DELTABEAM® composite beams and prestressed hollow-core slabs (HCS) in fire situations, these products’ current European design codes and product approvals do not cover the special issues “indirect support” and “flexible support” arising from their combination.
At ambient temperature, the slabs are directly supported on the steel bottom flange of steel or composite beams. In a fire, the direct support beneath the ends of the slab elements may not be maintained when the unprotected steel material loses most of its strength and stiffness. As the slab elements are not supported directly anymore, they will rest indirectly on the inclined web of DELTABEAM® by means of a concrete strut.
When HCS are supported on flexible supports (beams bending due to vertical loads from the slab), their vertical shear resistance at ambient temperature is reduced due to additional stress in the HCS webs. It was not known if this effect, called flexible shear, persists in fire situations.
To assess these issues, Peikko has carried out a test program of single HCS unit shear tests and system fire tests.
Five different single HCS units were tested for their behavior under combined loading with a vertical shear force and a horizontal forced distortion at high temperatures. The tests aimed to assess the ductility, the influence of longitudinal cracking on vertical shear resistance, and to calibrate finite element analysis calculations.
Peikko undertook full-scale, loaded system fire tests with a height of 200 mm and 320 mm to verify design models for indirect and flexible support. The slabs were loaded with a vertical shear force of 100% of vertical shear resistance in the case of fire according to standard EN 1168, and ISO (International Organization for Standardization) fire exposure was applied for up to 90 minutes and 120 minutes without a failure at beam deflections of up to L/32.
The DELTABEAM® with hollow-core slabs tests suggest there is no flexible shear issue in the event of fire, and that the indirect support model with straight tie reinforcement is safe.