Software Knowledge Base

Note ID: 000064 – Applications: SLABS4

Title: SLABS4 manual – Results - Slab Reinforcement: General

Description: General notes on the theory employed in calculating the reinforcement results and detailed notes about extracting results and how certain parameters affect the final output

See Also:

Date Created: 25/03/2008

Date Modified: 09/05/2008

The reinforcement of the slab is evaluated at each nodal point, in both orthogonal directions separately (a nodal point is a corner of a finite element triangle)

At each nodal point, the slab section is designed as a rectangular single reinforced section with a width of 1.0m and thickness defined by the slab panel. The reinforcement is designed for the envelope bending moment.

Note: The envelope bending moment is the maximum value of the bending moment (either positive or negative) assembled from one or several load combinations selected by the user. The load combinations included in the moment envelope are selected on the ‘Load Combinations’ input panel on the main menu.

The reinforcement design procedure assumes an under-reinforced section, and is based on the following expression:

where:

M - design (factored) bending moment (M*)

f - strength reduction factor, equal to 0.8

b - section width, equal to 1 m for the slab

d - effective section depth for the x and y directions

As - reinforcement cross section

fsy - steel strength

fc' - concrete strength

Note: The above expression is reproduced from the book:

Reinforced Concrete by R.F. Warner, B.V. Rangon and A.S. Hall, Third Edition, Longman Cheshire, 1989, (equation 4.24)

For the slab, the effective depth ‘d’ of the concrete cross section is calculated by subtracting the value of the parameter (Concrete Cover + One Bar) from the slab thickness (D).

d = D - (concrete cover + one bar)

(The value of [Concrete Cover + One Bar] parameter is entered on the [ Material Properties ] input panel).

At each nodal point, the effective section depth ‘d’ is equal in both directions and the software assumes an identical value for the effective depth ‘d’ for the top and bottom of the slab for both orthogonal directions. This will underestimate the effective depth for the primary direction which will result in a slightly larger reinforcement requirement.

In the secondary direction, the value of ‘d’ is overestimated and the program will provide slightly smaller values for the reinforcement. In both cases the error in the calculation of the effective depth ‘d’ is equal to half a bar diameter. The effect of this small error is insignificant, considering that the adopted reinforcement will always exceed the reinforcement required by the design. If a more precise evaluation of the reinforcement is required, it is recommended to perform a spot check of the reinforcement using INDUCTA’s BEAM software.

When a nodal point is at a location where two or more slab panels with different thickness are connected, the program will take the minimum slab thickness to design the reinforcement.

The reinforcement in the slab is always designed and presented parallel to the global X and Y axes. If there are areas of the slab where the reinforcement is to be placed at an angle to the global X and Y axes, the model needs to be rotated so the reinforcement in this area is parallel to the global X and Y axes and the model reanalysed.

Sample Screen shot – Reinforcement results in X-X ,Top

Sample Screen shot – Reinforcement results in Y-,Bottom

If the value of the basic reinforcement entered on the [ Material Properties] input panel is larger than zero, that value will be used as minimum reinforcement (top and bottom) in all of the slab sections. The software will design the steel in the slab using the above expressions, and then it will check if it is smaller than the basic reinforcement. If that is the case the basic reinforcement will be adopted, however the software will always check the minimum strength requirements according to AS 3600 - 2001, clause 8.1.4.1 and 9.1.1. The minimum strength requirement will be always satisfactory regardless of the basic Reinforcement value.

Sample Screen shot – Input for the basic slab reinforcement values

When there is a value entered for either (or both) the Bottom and Top Basic Reinforcement, SLABS4 will display an additional option in the DTB that allows the user to extract the reinforcement that is required in addition to the value entered for the Bottom or Top Basic Reinforcement. For the example shown in the screen shot below, a basic reinforcement value of 377mm²/m was entered for both the Bottom and Top, and the values displayed are in addition to this amount, marked as ‘extra’.

Sample Screen shot – Reinforcement results for X-X bottom, total reinforcement required

Sample Screen shot – Reinforcement required (X-X bottom) in addition to the Basic value of 377mm²/m

Note: Please review the Input Data chapter for a detailed explanation of all input variables used in the reinforcement design.