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Clark-Nielsen concrete forces [Fc(T) and Fc(B)]

The basis of the Clark-Nielsen calculation is ultimate limit state yield criterion. Accordingly the tensile strength of concrete is ignored and the calculations primarily relate to reinforcement in tension. It is assumed that the concrete cracks and the principal stress in the concrete is zero perpendicular to those cracks. Stress in the concrete parallel to the cracks (Fc) may be calculated in order to enable a check against crushing strength of concrete. It is not normally necessary to derive the direction of this stress.

In LUSAS Modeller, top and bottom stresses are calculated [Fc(T) and Fc(B)] assuming a doubly reinforced slab. The equivalent sandwich analogy is used to determine Fc(T) and Fc(B) for a stress field which includes both moments and in-plane forces. However the calculation of Fc(T) and Fc(B) is not available for angles of reinforcement other than orthogonal.

Where the concrete is uncracked (in compression in the reinforcement directions), Fc has no literal meaning, being "the  concrete stress parallel to the cracks".  In this instance the second principle stress (most compressive) is reported.  This is as described in the procedure outlined on the Clark-Nielsen calculation page.

In the notes under the Results Table (pg 402 etc) it is stated "If a non-zero K-factor is specified, Fc(T) and Fc(B) are not available". This should read "non-unity". In any case the parameter is unavailable for alteration in the Modeller interface when Fc(T) or Fc(B) are selected.

Other Wood-Armer related topics.

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