Verification of Design Criteria for Bridge Approach Slabs

Approach slabs are used in bridges to reduce the differential settlement between the bridge abutment and the approaching road pavement. In ideal conditions the approach slab is simply supported at the abutment and cast on a uniform, well-compacted surface. In reality compaction specifications under an approach slab are not fully achieved due to the difficulty of compacting in a restricted area. A very conservative method of designing an approach slab would be to assume that the fill underneath provides no elastic support to the approach slab, and to design the slab as simply supported over the entire length. Most codes and design guides take into account that the fill underneath the slab provides a varying degree of support to the approach slab and require the designer to use an effective span factor. South African (TMH7 and SANRAL) design codes and guidelines give very general design criteria for the design of an approach slab with no consideration for site-specific conditions. SANRAL recommends an effective span factor of 0.67 in the "code of procedure for the planning and design of highways and road structures in South Africa." This paper considers the influence of varying elastic support conditions on the effective span of an approach slab. Different support conditions are investigated, varying from well-compacted to poorly compacted and the recommended "rule of thumb" given in the SANRAL design guide is assessed.