Failure Mode of Orthotropic Two-way Composite Slab under Concentrated Load

Ordinary reinforced concrete two-way slabs generally suffer from punching failure under concentrated loads, which is a brittle failure and unfavorable to the structures. The composite slabs with steel sheets and PBL (perfobond rib) shear connectors have been applied for reducing the damage to bridge decks from vehicles. Although it is widely used on bridges, the failure mode of the orthotropic two-way composite slabs has rarely been studied when subject to concentrated loads. Accordingly, static loading tests with concentrated forward central load was carried out on 4 two-way composite slabs, in which the effects of the thickness of steel bottom sheets and concrete plates and spacing of PBL shear connectors on mechanical properties were investigated. The test results show that about 12% of the ultimate bearing capacity decreases when the perfobond ribs are spaced farther apart from 180 mm to 240 mm or the thickness of steel bottom sheets is reduced from 10 mm to 8 mm. And the bearing capacity drops by 38.2% when the thickness of concrete plates decreases by 30 mm. The steel-concrete composite slabs exhibit clear bidirectional force characteristics during the loading process, with the main forced direction being along the perfobond ribs, which is related to the stiffness ratio of the two directions. The two-way composite slabs have high bearing capacity and ductility, with obvious signs before failure. The specimens continue to bear the load after local punching failure of concrete near the loading point, which shows the features of both bending and punching failure. Based on the test results, a bending punching failure mode is proposed, which provides a new approach for the calculation of ultimate bearing capacity. An analytical calculation method for ultimate bearing capacity of orthotropic two-way composite slabs based on the failure mode above is derived with yield line theory and plastic theory, which is in excellent agreement with the tested results.