General Diaphragm Design

Ryan Freund — Thu, 26 Apr 2012 02:04:50 +0000

UPDATED 4/26/12

Maybe I missed that day of class when diaphragm design was covered but for me I do not recall having done any diaphragm design in school. So here is a some basics just to get started and I will cover more details for different materials later.

The diaphragm can be thought of as a horizontal beam or as a plate element. It is usually constructed of wood sheathing, steel deck or concrete. Just as the floor (or roof) is checked for vertical load capacity, it is considered a diaphragm in the plane of the floor and check for shear when designing the Lateral Force Resisting System. Generally there are two different types of diaphragms. Rigid and Flexible. There is also semi-rigid which acounts for the stiffness of the diaphragm and the shearwalls and is similiar to a continuous beam supported on springs. We will cover this later. Rigid assumes that the diaphragm is infinitely rigid. Rigid diaphragms are generally concrete diaphragms which are very stiff compared to the LFRS supporting elements (supporting elements might be – Moment Frame, Braced Frame, Shear Walls, etc). Flexible diaphragms are more flexible when compared to the LFRS supporting members. (Side note – technically speaking the diaphragm is part of the LFRS.) So, how to know which to use? Well generally if the diaphragm deflects twice as much or more than the supporting vertical elements (shearwalls, moment frame, braced frame, etc.) of the LFRS than a flexible diaphragm may be assumed.

Summary :

Flexible – A horizontal simple span or continuous beam analogy is typically used. The shear walls act as supports and simple span or continuous beam and shear moment diagrams are used. The sides of the diaphragm transmit shear to the shearwalls and the top and bottom of the diaphragms are commonly supported by chord members. These members are subject to tension and compression forces and are usually designed by taking the moment of the diagram and dividing by the depth of the diaphragm/beam.

Rigid – Assumes the diaphragm is rigid and distributes in-plane forces to supporting members based on stiffness of the supporting members. When analyzing the diaphragm it is assumed to be perfectly rigid. When drawing the shear and moment diagrams the applied lateral load can be uniform or triangularly varying to represent accidental torsion. The vertical supporting elements (shearwalls, etc) can be thought of as applying opposing point loads. Therefore for the case of stiff end walls relative to the interior shear walls, the moment diaphragm is comparable to a simply supported beam spanning between the end walls.

To analyze the deflection of diaphragms bending, shear and slip must be accounted for. The deck will have deflection similar to typical beam deflection. Also because the diaphragm/beam is deep relative to its span it will also have shear deflection. There will also be slip. There will also be slip. The slip may occur in the diaphragm panel connection to the substrate (i.e. nail slip in wood panel to joist connection) or in chords (i.e. nail splice of 2x top plate for wood framed construction).

See the attached examples and video to really help better understand general diaphragm design better. It should be noted that the flexible diaphragm presented in the example is a conservative approach as it assumes two simple spans. The actual behavior may more closely resemble a continuous beam with intermediate supports. However finding the tension based on simple span will result in a conservative yet practical design.
General Diaphragm Design Video Tutorial
General Flexible Diaphragm Tutorial PDF

Here is some more food for thought- Several good references:
http://www.nehrp-consultants.org/publications/download/nistgcr10-917-4.pdf
http://www.hnd.usace.army.mil/techinfo/ti/809-04/ch7d.pdf

Below is an interesting thread where a semi-rigid diaphragm might want to be used. Basically it is a situation were there are different diaphragm spans and diaphragm/shearwall stiffness on the same floor:

http://www.eng-tips.com/viewthread.cfm?qid=318931

I will try to work up and example for this situation in the future in the mean time here is a decent referance:

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/ram-ss-semirigid-diaphragms-tn.aspx

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Flexible diaphragm – How To Engineer

General Diaphragm Design