Vertical Load Analysis

 

SLABS and R/C BUILDING can both evaluate reactions in the walls and columns, but each program will produce different values for the reactions because of the following reasons:

- SLABS software considers one floor independently, ignoring the rest of the building and neglecting the interaction between the floors in the vertical direction.  The floor is laterally restrained, which may include the reactions for non-symmetrical floors.   In SLABS software all the floors from above are treated as load only.

- R/C BUILDING software considers the entire building and all the structural elements on each floor are “working” together.  The vertical elements, such as walls and columns, interact in the vertical direction, which will influence the reaction values.

 

Both programs, SLABS and R/C BUILDING, have two alternative methods to evaluate the reactions:

-Static Analysis

-Tributary Area 

The Static Analysis method is more appropriate for concrete buildings, where there is a clear “frame action”.  In this case some of the walls or columns may be in tension.

The Tributary Area method is more appropriate for masonry buildings where the walls cannot take any tension.  This method is more commonly used because of its suitability for hand calculation checks, and will also produce larger reactions in the walls and columns that terminate on a transfer level.

Tributary Area method will spread the load to the surrounding supports, in a similar manner as it would be done manually by hand calculations. 

We recommend using R/C BUILDNG for the vertical load analysis, evaluation of the reactions in the walls and columns, and for the design of all columns and walls rather than SLABS.  Since R/C BUILDNG is used to model the entire building including all the loading, it will provide the most accurate values for the reactions. 

 

R/C BUILDNG has three alternative methods to evaluate the reactions:

-Static Analysis (1st or 2nd order)
-Static Analysis - Construction Stages
-Tributary Area Method  (more suitable for masonry buildings)

 

The Static Analysis method will provide much smaller reactions in the walls and columns terminating on transfer slabs in comparison to the Tributary Area method.  In some cases the difference can be more than two times.

Static Analysis – Construction Stages will provide the most accurate values for the reactions in a concrete building, as this method will simulate the gradual increase of the loading as the building is constructed. 

An alternative approach (which provides very reliable results) is to use the Static Analysis but to set the modulus of elasticity of the transfer slab 4 times larger than the “real” value.  This will provide larger reaction values in the walls and columns terminating on the transfer slab, providing results that are closer to the Tributary Area approach. 

This is our recommended approach for evaluating reactions that are to be used in the design of concrete walls and columns.