Frequently Asked Questions
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FAQs
Loads that are lateral to the joists or joist girders should be resisted with structural bracing that is specifically designed for that purpose. Joist bridging should not be expected or relied upon to absorb these loads. Bridging provides lateral restraint for the joists under construction loads and for wind uplift conditions only. It should not be relied upon or considered as bracing for lateral loads.
It is acceptable for the Engineer of Record not to show the bridging on the structural drawings or, if it is shown, a note should be included stating that the bridging shown on the structural drawings is for reference purposes only and the final bridging shall be by the joist manufacturer. The bridging will be determined by the joist manufacturer based on the final joist top and bottom chords, the requirements of the SJI specifications, and the need for uplift bridging. The required bridging will be shown on the joist placement plans. This allows for any structural bracing to be clearly indicated on the structural drawings and avoid confusion with joist bridging.
Diagonal bridging should be used when the joists fall in the red shaded zones of the SJI Load Capacity Tables. When the joist span is 60 feet or greater all rows of bridging must be bolted cross bridging and they must be installed prior to releasing the hoisting cables.
It is possible but an analysis is required. The manufacturer can provide assistance with an alternate bridging layout.
No. The design by the manufacturer may not require that the top and bottom chord bridging be aligned. Special attention should be addressed to bridging anchorage / termination when lines are not aligned.
All horizontal bridging lines need to be anchored at the ends either by attaching to a beam, wall anchors or with X bridging termination. Anchorage of the bridging is critical to its performance. We are often asked after the joists have been installed if the bridging can be removed to clear the way for mechanical ducts. In most all cases it can, especially if the bridging is X welded or X bolted and as long as the bridging in the adjacent spaces remains to supply the lateral stability to the joist. In the case of horizontal, removing the bridging and terminating the discontinuity with X welded is acceptable.
Uplift bridging is required whenever the bottom chord goes into compression. This will be true if there are net concentrated up loads.
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The bridging that is required only for erection stability can be removed. However the bridging needed for lateral stability during the life of the joist cannot be removed without an analysis. Generally the deck provided the lateral stability of the top chord so it might be able to be removed. The bridging generally provides the lateral stability of the BC. The removal of this bridging will require an analysis of the loading as well as consultation with the manufacturer.
Bridging is only designed for the lateral stability forces in the joists. In general those values are in the SJI bridging tables in the specification. If bridging is intended to brace other components of the structure the size and connections must be indicated on the contract documents. Normally these systems are independent of the joist system.
Yes, Erection bridging must be installed prior to the hoisting cable being released, consequently if welding is being placed on erection bridging, the welding must be done prior to the hoisting cables being released.
For any joist where the center of gravity is above the support point, all rows of bridging should be bolted cross bridging. The number of rows will vary with length, pitch and weight of joist. A joist manufacturer can give direction on this.