TrusSteel - Cold Formed Steel Trusses

1 2 3 4 5 6 7 8 3.12 SEISMIC LOADING Diaphragms In most instances when buildings with trusses require seismic or wind analysis, the lateral forces on the building are resisted by a system of diaphragms. Roofs and floor planes covered with wood sheathing (plywood or OSB) or metal decks can be designed to create "horizontal" diaphragms that can resist lateral loads. Vertical members such as exterior walls and interior shear walls are connected to the horizontal diaphragms and to the building foundation to tie the entire structure together. Specific trusses may be designed to be located directly over the shear walls to transfer the horizontal load from a portion of the roof to the shear wall. These trusses are called "collectors," or "drag trusses," because they collect the forces from the diaphragms and transmit them to the shear walls. Determination of the required location, loading and connections for these drag trusses is the responsibility of the building design professional. The model codes publish tables of shear values for plywood panel systems and the metal deck manufacturers publish their own proprietary values. Typically, shear panel systems designed using the code tables specify nail or screw patterns for the perimeter of the diaphragm and for the interior edges of the individual structural panels within the diaphragm. CFS Trusses and Seismic Resistance Buildings in earthquake-prone regions should be designed to protect occupants during a reasonably probable seismic event. Damaging earthquakes have large motions but are usually short in duration, lasting only a few seconds. This is fortunate because the longer an earthquake lasts the more damage it can cause. All types of structural members and connections can fail during long load cycles, as material fatigue occurs or connections slip apart. CFS (Cold-Formed Steel) trusses are well suited for use in seismic applications. They are light in weight so the forces are low. They are quite stiff for their weight, so lateral displacements are minimized. They are also ductile which means that trussed systems are more likely to deform under overload than to fail suddenly. In some structures trusses must be designed to resist horizontal loads generated by the sideways acceleration of their own mass during an earthquake. This requirement is usually ignored because the connections designed for gravity loads and wind uplift loads are judged sufficient to withstand any lateral loads that might occur. If the roofing materials assembly is sufficiently heavy and the seismic event severe enough the building designer may require the inclusion of additional loads during analysis or the use of special connections. Another common horizontal load on trusses occurs when wind or seismic motion are imposed perpendicular to a wall that supports the trusses. In this case a concentration of load is induced into the heel of the truss that must be transferred up to the roof diaphragm. This is the opposite of a drag truss load, where the load along the roof must be transferred to the wall below. In either case the connections between the horizontal diaphragm and the vertical support are critical to the safe design of the structure. ENGINEERED BY ALPINE S P E C I F Y I N G / D E S I G N I N G

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