When is it ok to average moments for slab patch designs?
Environment
Not environment-specific
Question
When is it ok to average moments for slab patch designs?
Answer
Within the Properties window for a patch there is the option to average the moment over the width of the patch or patch strip.
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This option exists to counteract localized and unrealistically high forces (from the analysis of the 2D (Finite Element) mesh of the slab) within the slab. Such forces can occur at locations commonly termed singularities, which are a product of elastic analysis using the Finite Element Method (FEM) and thus will occur in all software which implement this.
Singularities will tend to occur at the following locations:
- Walls ending within the slab
- Openings
- Columns
- Point supports
- Concentrated loads
The image below is taken from "Finite Element Design of Concrete Structures" by G A Rombach, and details potential locations where singularities may occur within a slab panel.
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It is common for engineers to want to design for the maximum moment found at these locations, however this urge should be resisted as these (very localized) areas of unrealistically high moment would never occur in reality. If they were to do so, the concrete would crack, reinforcement would yield and the moment would dissipate.
Tekla Structural Designer therefore employs a method to "flatten out" these high moment 'peaks' by allowing the engineer to average the slab moments over the finite width of patch strips.
For further guidance on sizing patches please see the Related article to the right entitled When designing slabs, what size patches should I use?
Suggested Reading:
Concrete Center Guide: How to design reinforced concrete flat slabs using Finite Element Analysis, O Brooker
Finite Element Design of concrete structures, G A Rombach.