# Concrete Slab Properties : Calculation

A number of parameters which control the calculation can be adjusted.

## Definition

**Solution Method**

The information below presents a practical overview of the two methods. More detail on the RC Slab analysis procedure is available in the RC Slab theory section.

Two solution methods are available; a ‘fast’ method and a ‘rigorous’ method.

*Fast method*

The ‘fast’ method selects a faster non-iterative solution method wherever the loading is considered to be either predominately in-plane or predominately out-of-plane. For each element position where this be a reasonable assumption, this method will yield faster analysis times. When the loading is not considered to be predominately in-plane or out-of-plane, the method will fall-back to the slower iterative technique. The decision is made independently for each element position, on a position by position basis.

The user may also restrict use of the non-iterative technique to any case where the total reinforcement area is below a certain percentage of the total cross-sectional area. Should the total reinforcement calculated by the ‘fast’ method be above this threshold, the iterative technique is used.

Any use of the non-iterative technique will typically lead to slightly more conservative results. Note that by design, its use is limited to those cases that represent the most favourable trade-offs between analysis time and approximation accuracy.

More detail on the non-iterative technique is available in the RC Slab theory.

*Rigorous method*

The ‘rigorous’ method selects the iterative solution technique in all cases. This approach will always yield the most accurate result available, regardless of any opportunity for a performance/approximation trade-off.

A common work-flow scenario may be one that adopts the ‘fast’ method during the early stages of design, before later switching exclusively to the ‘rigorous’ approach.

Note that the ‘rigorous’ method is equivalent to the (sole) solution technique prior to GSA version 8.7.

**Design Thickness**

By default the thickness used is the thickness defined in 2D element property for the analysis. If this proves difficult to reinforce the slab, the thickness can be overridden as a trial thickness to see if a solution can be found without the need to change 2D element property thickness and redo the analysis. Once a suitable thickness is worked out, it can be used as the slab thickness in 2D element property, in this case, a re-analysis should be carried out after modifying the analysis (2D element property) thickness.

**Eccentricity**

The minimum eccentricity to be used in the design, it is normally taken from the relevant design code, but it can be overridden and defined explicitly.

Additional eccentricity is used to consider the second order effect (increase) on the applied moment due to the slenderness of the slab. It is an user defined parameter to incorporate the second order moment if the slab is slender.

Two-way spanning modifier, normally 0 should be used for for one-way spanning slab and 50 to 100% for two-way spanning slab, see details for how to work out the two-way spanning modifier on RC slab theory manual.

**Stress/strain limits**

The value of $x/d_{max}$ will normally be that specified in the design code but can be overridden if required. The design strain limit ($ε_{su}$) limits the allowable strain in the section. This may be a function of both the concrete and reinforcement material. The crack rotation limit is specified in some of the codes, but is normally 90˚.

**Punching shear**

Note that punching shear is not considered: you must make separate design checks and provision for this as necessary.