Abstract:
With increasing population and limited land resources, optimum use of
available space in buildings is becoming important. Furthermore, architects
demand to hide columns inside walls. This leads to flat slabs supported on
non-rectangular column grid. Despite the long history of flat slab construction,
no design or assessment method for flat slabs supported on non-rectangular
column grid is available yet with sound experimental validation. To fill
this vacuum, flexural behaviour of such slabs is discussed in this dissertation.
To select a design method, available design methods for flexural reinforcement
are reviewed. Comparisons are made between Hillerborg’s strip method and
Saether’s structural membrane approach. In both methods twisting moment is
assumed zero. The validity of this assumption, and implications of any violation
are checked by extending Gurley’s bimoment concepts. Furthermore, proposals
are made to extend the application of Saether’s structural membrane approach
to edge and corner columns.
Using Saether’s method nine flat slab panels were designed and tested in
the laboratory. Instead of testing multipanel slabs, single panel specimens
with edge shear applied in a proper manner to simulate the adjacent panels
were considered. To apply uniform load, a novel vacuum rig was constructed.
Column layouts in the specimens were varied from parallelogram, triangle
to a completely irregular layout. Further in the parallelogram column layout,
direction of the reinforcement also varied exploiting Wood-Armer equations.
A corner panel of a skew column grid, designed according to the proposed
extension to Saether’s method, was also tested. Based on the experimental
results comparisons are made with the design values to suggest improvements
to the design procedure.
Punching shear failure is a common failure mode in flat slabs. Design codes
don’t allow provision of shear reinforcement in the form of links or bent
up bars in thin slabs. Also the method adopted to avoid punching shear failure
must not interfere with flexural capacity; especially in an experiment performed
with the purpose of validating a flexural design method. In the present work,
spirals were introduced as punching shear resistors.
The experiments pave the way to propose some improvements to the design method
and provide some valuable data to validate analytical methods. Based on the
experimental observations a series of yield line patterns for flat slabs
supported on parallelogram layout were identified and used to assess the
experimental slabs. Further confirmation of the failure modes was obtained
from observed change in deflection at the final stages of loading, crack
patterns, and readings from strain gauges stuck on reinforcement bars. Two
flat slabs supported on completely irregular column layout were also assessed
using yield line analysis.
In flat slabs controlling deflections at service load is also a key issue
in the design. Using a commercially available finite element
package (DIANA), experimental slabs were analysed and predictions were compared
with experimental observations. Agreement between predicted failure loads
by analytical methods (yield line analysis and finite element analysis) and
experimental observations is satisfactory.
[Cambridge University | CUED | Structures Group | Geotechnical Group]