Max Truss Span; Internal Bearing Locations

[spieng89]

I've been looking through TRUSTEEL manual by Alpine to get some guidance on typical max spans by cold formed trusses. In their material, they have trusses that can comfortably span over 80 feet with no mention of internal bearing locations. I've always had the mindset, due to past mentors, to place internal bearing points for trusses at about max spacing of ~60 feet. I'm trying to see if that is just an outdated rule of thumb with the advancement of truss design or if there is solid justification for that internal bearing recommendation.

Does anyone have any experience of designing trusses with longer spans (+60 feet) and no internal bearing? Any concerns of special inspections I may be skimming over? I have a technical request into a manufacturer, but figured I'd ask here as well. I place their response when it comes in

 

[r13]

I think you need to tie the length and depth of the truss, in order for the "rule of thumb" makes sense.

 

[Lomarandil]

I recently designed and set CFS trusses spanning 65ft with my little 2mm thick gage tubes available here. I'm sure that with additional thickness and custom engineering, 80ft would be feasible (with the right depth and lateral bracing).

----
just call me Lo.

 

[phamENG]

You can definitely go longer. The key is bracing - particularly during construction. If you do it, make sure you discuss the bracing requirements with the contractor during whatever pre-construction conference you may have. If they don't have experience with long span trusses, they may not realize the importance. Shortly before I started working for my last firm, there was an incident with a 70 or 80ft light gauge truss failure. If I remember correctly, the contractor had never seen the odd bracing details before so he figured they were just "over engineered" and not worthwhile. He got the trusses up, but as soon as the first loads of deck came up the entire roof system buckled and came down.

 

[r13]

Joists are separated by type and designated by letters: K, KCS, LH, DLH and SLH. The K and KCS joists are used for shorter span conditions where lighter loads are required. These joists are typically used in roof structures. KCS joists are a version of the K-series joists, but designed for a constant shear and moment, which allows for greater flexibility for locating extra loads such as those from rooftop units. LH- and DLH-series joists are for longer span conditions and are capable of supporting larger loads. These joists allow for special profiles to accommodate a variety of conditions. Some of the typical special profiles include gable joists and arched joists. SLH-series joists are used to support loads over very long spans, typically 120' and greater.

 

[JAE]

retired13, the OP was talking about cold formed steel trusses, not SJI joists.

 

[r13]

Just an example to demonstrate how joist manufacture handle the "perceived" long span truss by different span-depth ratio and member sizes. Several decades back, joist span beyond 60' was considered long span, only half of that they can do today. I think every industry followed the same trend, renders the old rule of thumb outdated, or at least needs to be scrutinized.

 

[JAE]

spieng89 - The limit of 60 ft. may have had more to do with:
1. Ability to ship long sections >60 ft. over public roads (permitting costs, etc.)
2. Cost of taking >60 ft. spans and splicing them in the field in response to shipping long sections on roads.
3. With the longer spans, the deflection limits of L/360, for example, could be met but the actual total deflection might not work with the walls below. (L/360 for an 80 ft. span = 2.67"!!!!)
4. With long spans come deep sections which, again, would be wider than 12 ft. - the typical maximum width for truck shipping - requiring splices on top of the truss as well as along the span.
5. Per phamENG - stability during erection is much more difficult, more bracing, temporary stiffeners, etc.

I'm not aware of any unique IBC/code special inspections that would be contingent on the span length.

 

[spieng89]

Received information back from a manufacturer/designer. Many of you were correct, +60' span can easily be achieved. But with a few extra costs. The complications come, as JAE mentioned, with shipping. Often those trusses will need to be designed as a "piggy back" system with the top of the truss being split along a horizontal plane to allow for fitment on a truck. Not a huge issue, but something to consider. In my particular case, the addition of steel, connections, and erection time for main frame may not justify adding additional bearing points given the relatively small roof sqft.