Analysis of Deflected Waffle Slab a Case Study of Station officer Mess and Overview of Retrofitting Teachniques for Strengthening of Slabs

Abstract

Reinforced concrete floor/roof systems spanning more than permissible limits are more susceptible to deflection problems. In addition to strength requirements they are also required to satisfy the deformation criteria in terms of low deflection and crack width. Serviceability refers to the conditions under which a building is still considered useful. This thesis details physical measurement, manual calculation and software computation of deflection in a waffle slab which experienced excessive deflection very after the removal of formwork. It was an 85x55 ft dining hall. Retrofitting techniques for strengthening of slabs are also discussed. The objective of this study was to compare the three deflections and find out the reasons of large immediate deflection. Deflection in the slab was measured physically with the help of surveying equipment i.e. a level and a staff. Inverted staff was put at the bottom of the slab at 4 feet interval along the long dimension and elevation reading was taken by the level. A graph was drawn between the staff reading and horizontal distance which gave the deflection curve. Slab loads and section properties were calculated manually and the deflection was computed by direct method. Software “SAFE” (slab analysis by finite element method) was used to compute the slab deflection. The building was modeled on SAFE with the help of all structural drawings of the building. Software and manual results agreed well with the actual measured deflection. It was concluded that thickness of waffle slab provided for 85 ft span was much less than minimum thickness imposed by ACI code’s span to depth ratios thus providing very small moment of inertia and increasing the deflection. A new design was proposed at the end and verified on the same software. The slab deflection reduced from 5.5 inches to 1.1 inches.