TrusSteel - Cold Formed Steel Trusses

1 2 3 4 5 6 7 8 3.08 For example, when designing a single truss, a gravity load is a downward-acting load while a wind load is typically an uplift or upward-acting load. It is extremely important that each truss be analyzed for a stress reversal situation, so that each truss is designed to support every kind of load that it may encounter. Attachment to Supports A wide variety of TrusSteel connection hardware, with associated application details, is available for anchoring trusses to the supporting structure. These rated hardware connectors can be installed to resist wind (uplift) loads, in-plane lateral loads and out-of-plane lateral loads - in any combination of these loads. It is imperative that the building designer clearly define the loads that a truss, and the truss connections, must resist. Demise of the Allowable Stress Increase The standards associated with the design of CFS members, designers are not able to increase allowable stresses by 1/3 when the loads are from wind or seismic events. In the past, it was common practice to allow such increases. This practice was supported by design professionals, design specifications, loading standards, and building codes for a century and had deep roots in the design community. This increase was allowed for seismic loads because these loads were not considered until recently. The rationale for the increase was that seismic loads were intermittent and of short duration. Research since that time has shown that steel strength does not increase with load durations typical of wind and seismic events, has improved our accuracy in determining wind and seismic design loads, and has resulted in changes in design loads to account for the intermittent nature and variability of such loads. One such change permits a 25% reduction in live load when two or more types of live load exist, provided the 1/3 stress increase is not also taken. This 25% reduction in load is identical to a 1/3 increase in allowable stress, insofar as 3/4 is the inverse of 4/3, and has been confused as being equal to the existing 1.33 increase factor. However, this 25% reduction cannot be applied to a load case consisting solely of dead plus wind loads, which may govern the design of roof trusses in high wind regions. For this reason, the loss of the 1/3 stress increase factor may increase the amount of steel in a member by as much as 1/3. While such an increase is extreme and not typical, it is likely that trusses in high wind regions will show some greater material thicknesses (gauges) of component sections on occasion due to the removal of this factor. The above change was first published in the 1970s and used by some designers instead of the old 1/3 stress increase factor, but the old factor remained available (and in use) until recently. The IBC no longer permits the increase factor for a load case of solely dead plus wind (or seismic) load. While it can be difficult to accept building code changes that may cause increases in material costs, this change is needed to assure that CFS continues to show safe and consistent engineering performance under severe loadings like hurricanes and earthquakes. The IBC no longer permits the increase factor for a load case of solely dead plus wind (or seismic) load. WIND LOADING 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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