Effect of diaphragm stiffness
Not version-specific
Tekla Structural Designer
Environment
Not environment-specific
Question:
What is the appropriate stiffness for a semi rigid diaphragm?
Answer:
Semi-rigid diaphragms have an option allowing the engineer to control their in-plane stiffness to be used in analysis. Their initial stiffness is determined by; the material of the slab/ panels that form the diaphragm (this sets their elastic material properties of E and G) and their thickness. The initial stiffness can be further controlled by the "Divide stiffness by" parameter, the default for which is 1.0 (i.e. no adjustment). The stiffness to be used, and hence the value for this parameter, is a matter of engineering judgement and so for you to decide. In this article we consider a number of options to see their effect.
Example
Once a slab has had its properties set to Semi-rigid, you can edit the Divide stiffness by value to adjust its stiffness.
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In the picture below, the diaphragm stiffness divisor has been set as 1. Looking at the force in the beams in the braced bay, we can see that results are similar to a rigid diaphragm; whilst there is some axial force in the connecting beam, it is relatively low. It is cetainly not what one would expect if assuming that the beam takes all the brace force and the diaphgram none (though actually this is both an analytical contradiction and physical impossibility)
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Increasing the divisor to 500 produces a significant increase in the axial load being taken by the connecting beams. The results for a divisor of 1000 are quite similar. Logically, the axial force in the beam is a function of the relative stiffnesses of the beam (its axial stiffness) and the diaphragm. When these are similar they will share the load more or less equally. As the diaphgram stiffness is reduced, more axial force will develop in the beam and so on. As the relative stiffness of the diaphgram is reduced, beyond a certain point further reductions become insignificant and the force in the beam is relatively unchanged.
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In this example, we see that increasing the divisor value further has no significant impact on the axial forces shown in the beam e.g. using a value of 10,000 as illustrated below.
Ultimately, the value for the divisor and hence stiffness of the semi-rigid diaphragm is a matter for the engineer's judgement and should be set to give results they consider realistic for the diaphgram/ bracing system used.
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