Uplift reactions are the result of a set of conditions that generally includes load types, load combinations, bearing locations, and web patterns. In most cases, especially those involving wind load combinations, uplift reactions are relatively small and can easily be resolved through the specification of standard Seismic and Hurricane Ties, such as Simpson’s H2.5A. In other cases, such as drag load combinations, uplift reactions can be relatively or extremely high (> 10,000 lbs). Furthermore, gravity load combinations in conjunction with interior bearings that are relatively close to an exterior support can create high uplift reactions that require additional design considerations: the allowable loads for Simpson’s Seismic and Hurricane Ties, as well as Girder Tie-downs, are based on a 1.60 Load Duration Factor, and specifying tie-downs for a gravity uplift condition requires that the duration of load associated with the load combination creating the uplift condition be taken into account.
Alerting the truss designer that a high uplift condition exists during the design phase of the component may have benefits. In some cases, the design can be altered to alleviate the high uplift condition. And, if the component design cannot be changed to alleviate the high uplift condition, the truss designer can alert the Building Designer or EOR that the condition exists and that additional design considerations may need to be accounted for when specifying a connection; this is especially true if a high uplift reaction occurs at a hanger support.
Options to control uplift may be set in EnvData and applied to individual bearings in the Bearings dialog. This functionality allows designers to adjust truss designs to alleviate uplift reactions at bearings.
The sample truss shows the EnvData option set to True. Uplift reactions are reduced at the interior wall bearings. [For image, See PDF or View in Full Issue.]
Additional benefits of uplift functionality include:
- Choose where required connections for uplift will exist:
- Adjust uplift for selected bearings
- Adjust uplift for continuous bearing trusses
- Allows the truss to deflect upward, instead of being anchored to the support, for any load combination where uplift would otherwise occur.
Simpson Strong-Tie’s Component Solutions® CS Truss Studio™ software has the ability to manage uplift reactions. Contact your Simpson Strong-Tie Representative or email CSHelp@strongtie.com for more information.
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