HSS is often perceived to be more expensive than traditional wide flange sections. Engineers tend to stick with tried and true designs and may not delve into the many reasons why their HSS designs rack up big bills. Steel Tube Institute has explored the many causes of why HSS is perceived to be more expensive than wide flange shapes and has shared their findings to educate engineers on creating more efficient designs with HSS.
In the July 2020 SE University session, Kim Olson, PE, from Steel Tube Institute, presented What Your Fabricator Wishes You Knew About HSS. Kim offered several valuable tips during her presentation which can really have an impact on the cost of HSS fabrication.
Tip #1: HSS trusses designed to be the lightest possible do not equate to the lowest cost.
Fabrication costs tend to drive up the prices of trusses when the connections become excessive or not well planned. The goal should be to achieve a structure without stiffeners or reinforcement. Specifying stocky chords and thin branches can reduce connection costs, and using fillet welds designed to resist only the applied loads rather than developing the strength of the member should also be considered. Also, gapped K connections should always be used when possible since overlapped connections, although stronger, are also more difficult to fabricate which drives up the cost of the truss. Lastly, matched connections should also be avoided when stepped connections can be used. For instance, web members should be more narrow than the chord members so that the webs can be welded to the flat portion of the chord.
Tip #2: Round HSS should be avoided when rectangular or square shapes can be used.
Round HSS is typically more difficult and costly to store and handle in a fabrication shop. It often proves challenging to layout and find centerlines and proper orientations. When used in trusses, the ends of round HSS would need to be profile cut which is more costly, as well.
Tip #3: Avoid CJP welds. No Really… Don’t use them!
When fillet welds become too large, another option should be PJP welds. When PJP welds cannot resist the required forces, increasing the member thickness should be explored before considering CJP welds. CJP welds take much more time to complete, add costs to the owner for additional ultrasonic testing, and un-backed CJP welds require additional weld certification which make finding qualified welders more difficult.
Tip #4: Avoid through-bolted connections when possible.
Manually drilling the holes for a though-bolted connection can increase costs as well. Through bolts can end up being very long which could result in a special order bolt or it may not be available at all. Through bolts cannot be pre-tensioned unless the tube is reinforced, and reinforcing the tube can be costly. Through bolts are also difficult to install in the field, even in the best of conditions.
Being aware of these small but significant changes in your HSS designs can make an impact in the overall cost of a project. Engineers can help guide architects on which truss profiles might be more efficient, and which shapes could be changed to reduce fabrication costs. Being mindful of selecting HSS thicknesses which allow for less expensive welds can also make the design more efficient.