I would say to start with F22 and F11 as 0.25 and if it still fails you can even reduce it to like 0.10 (tension will be carried only by steel) plus some portion by tensile capacity of concrete.
But remember for tension, walls will be cracked and to account for cracking you have to apply a very low value of axial stiffness. Yes walls will fail in F11, F22 (axial force, tension mostly). Because temperature is applied as axial load not as gradient like in SAP. Here is some more food for thought- Several good references:īelow is an interesting thread where a semi-rigid diaphragm might want to be used.You should not get moment from temperature load in ETABS. However finding the tension based on simple span will result in a conservative yet practical design. The actual behavior may more closely resemble a continuous beam with intermediate supports. It should be noted that the flexible diaphragm presented in the example is a conservative approach as it assumes two simple spans. See the attached examples and video to really help better understand general diaphragm design better. nail splice of 2x top plate for wood framed construction). nail slip in wood panel to joist connection) or in chords (i.e. The slip may occur in the diaphragm panel connection to the substrate (i.e. Also because the diaphragm/beam is deep relative to its span it will also have shear deflection. The deck will have deflection similar to typical beam deflection. To analyze the deflection of diaphragms bending, shear and slip must be accounted for. 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. The vertical supporting elements (shearwalls, etc) can be thought of as applying opposing point loads.
When drawing the shear and moment diagrams the applied lateral load can be uniform or triangularly varying to represent accidental torsion. When analyzing the diaphragm it is assumed to be perfectly rigid. Rigid – Assumes the diaphragm is rigid and distributes in-plane forces to supporting members based on stiffness of the supporting 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. The sides of the diaphragm transmit shear to the shearwalls and the top and bottom of the diaphragms are commonly supported by chord members. The shear walls act as supports and simple span or continuous beam and shear moment diagrams are used.
(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.įlexible – A horizontal simple span or continuous beam analogy is typically used. Flexible diaphragms are more flexible when compared to the LFRS supporting members. 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). Rigid assumes that the diaphragm is infinitely rigid. 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. Generally there are two different types of diaphragms. 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. It is usually constructed of wood sheathing, steel deck or concrete. The diaphragm can be thought of as a horizontal beam or as a plate element. Related links (design per specific material) So here is a some basics just to get started and I will cover more details for different materials later.
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.