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.
