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Division of column and middle strips in Continuous Beam

Division of panels

PROKON’s Continuous Beam module designs for a flat slab by dividing the flat slab into column and middle strips. When generating reinforcement, the program considers the column and middle strips separately.

Division of moments between column and middle strips

The design moments obtained from the analysis of the continuous frame is divided between column and middle strips in the proportions as follows:

Bending Moment Column strip Middle strip
Moment over columns (Negative) 75% 25%
Moment in-between columns (Positive) 55% 45%

Column and middle strips

As stated by BS8110-1997:

3.7.3.1 The column and middles trips should be designed to withstand the design moments obtained from 3.7.2. In general, two-thirds of the amount of reinforcement required to resist the negative design moment in the column strip should be placed in a width equal to half that of the column strip and central with the column.

As stated by SANS0100:

4.6.5.4 In general, two-thirds of reinforcement required to resist the negative design moment in the column strip should be placed in a width equal to half that of the column strip and central to the column.

The column strip is effectively reduced to half the width of the column strip (narrowed to a quarter of the panel width), and the two thirds of the reinforcement resisting the negative moment in the column strip is placed inside the new column strip width. The remainder third of the reinforcement is added to the middle strip.

The positive moments aren’t governed by the clause, however, for simplicity during detailing, the new column strip width is used in the calculations.

The middle strip is widened by fifty percent from half the panel width to three-quarters of the panel width. Reinforcement added to the widened middle strip is taken from the column strip. The column strip reinforcement is divided by two and one half is placed in the width of the new column strip width. The remainder half is added to the middle strip required reinforcement.

The remaining reinforcement is checked, and additional reinforcement added where necessary. This is done to ensure that the amount of reinforcement resisting hogging moment is greater than or equal to two-thirds of the reinforcement required for the original column strip.