×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Max Truss Span; Internal Bearing Locations

Max Truss Span; Internal Bearing Locations

Max Truss Span; Internal Bearing Locations

(OP)
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 I'm trying to see if that is just an outdated rule of thumb with the advancement of truss design or if there is solid justification for that internal bearing recommendation.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

RE: Max Truss Span; Internal Bearing Locations

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

RE: Max Truss Span; Internal Bearing Locations

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.

RE: Max Truss Span; Internal Bearing Locations

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.

RE: Max Truss Span; Internal Bearing Locations

Quote (Vulcraft)

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.

RE: Max Truss Span; Internal Bearing Locations

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

RE: Max Truss Span; Internal Bearing Locations

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.

RE: Max Truss Span; Internal Bearing Locations

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.

RE: Max Truss Span; Internal Bearing Locations

(OP)
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

White Paper - The Criticality of the E/E Architecture
Modern vehicles are highly sophisticated systems incorporating electrical, electronic, software and mechanical components. Mechanical systems are giving way to advanced software and electronic devices, driving automakers to innovate and differentiate their vehicles via the electric and electronic (E/E) architecture. As the pace of change accelerates, automotive companies need to evolve their development processes to deliver and maximize the value of these architectures. Download Now
White Paper - Model Based Engineering for Wire Harness Manufacturing
Modern cars, trucks, and other vehicles feature an ever-increasing number of sophisticated electrical and electronic features, placing a larger burden on the wiring harness that enables these new features. As complexity rises, current harness manufacturing methods are struggling to keep pace due to manual data exchanges and the inability to capture tribal knowledge. A model-based wire harness manufacturing engineering flow automates data exchange and captures tribal knowledge through design rules to help harness manufacturers improve harness quality and boost efficiency. Download Now
White Paper - Modeling and Optimizing Wire Harness Costs for Variation Complexity
This paper will focus on the quantification of the complexity related costs in harness variations in order to model them, allowing automated algorithms to optimize for these costs. A number of real world examples will be provided as well. Since no two businesses are alike, it is the aim of this paper to provide the foundational knowledge and methodology so the reader can assess their own business to model how variation complexity costs affect their business. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close