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Truss geometry and erection questions

Truss geometry and erection questions

Truss geometry and erection questions

(OP)
Hi, I'm new to this forum so I apologize if these questions have already been addressed somewhere else.

I am designing structural steel trusses for a new fire station and am looking for a little guidance on a couple of construct-ability issues. The gable-shaped roof has a clear span of 80 feet and a 4:12 pitch; so the total height of the truss is 13'-4". Looking through the project's state regulations, it looks like the max height that can be shipped is 13'-8" without special considerations like routes. I'm assuming this means the height of the load plus the truck bed height, so I would exceed this. The fire station will also be located on a somewhat narrow street (20'-0" wide). So my concerns/questions are the following:

* Should I really be looking at a piggy-back style truss here with a base truss height of 6'-8" and max piggy-back truss height of 6'-8"? What height is the "breaking point" for making a decision between a single truss and a base truss + piggy-back truss? Is there a rule of thumb for base truss height vs. piggy-back height (i.e. should I instead go with a 10'-0" base truss and 3'-4" piggy)?

* I'm going to show splice locations for the base or single truss so that it can be fabricated in two "halves" - actually ~45' long and 35' long since the splice won't be at a panel point. Does the truss need to be assembled on the ground and lifted into place? Or could the contractor erect temporary shoring to support one half of the truss and then set the other half, then attach them in-place. My reason for asking is that the site will be kind of tight once the foundation and bearing walls are in and an airport is nearby. So I'm skeptical that a crane required to lift an 80'-0" truss could be used.

Thanks for your help in advance.

RE: Truss geometry and erection questions

Well, maybe I'll chime in here......
Sounds like three issues
1. Shipping
2. Erection
3. Cost
1. Shipping. 80’ might be as much a problem as 13'4". With truck and trailer the load is looong. You can cut truss in half and max height is perpendicular to top chord at apprx 12' 7”. Max length is appx 42'2". I think a low boy trailer can buy you a couple of feet height, perhaps not enough. You might be able to lay the truss halves flat, taking two lanes on the highway with a flagged truck. Consult your steel manufacturer or a trucking company for advice in your area. Such advice is ordinarily given.
2. Erection. I'd be inclined to fully assemble the truss on the ground. I've lifted 80’ trusses with, if I remember correctly two small cranes, but mine were immediately adjacent to the trusses, so the reach was not so much as to require large cranes. Your erector might be able to advise if they could be lifted with two super forklift-style machines. Provide them with the anticipated weight.
3. Piggy backs of course would work. But you're adding extra steel to system increasing both weight and cost.

RE: Truss geometry and erection questions

Sounds like a candidate for bolt-together trusses built on site or in place. I'd be less concerned about weight than constructibility. Labor can be significant with complicated construction strategies. You say "structural steel trusses", but have you considered a Vulcraft/Canam style steel joist? They are very weight efficient and might solve the dilemma you present, and they can typically design around shipping restrictions.

In you have a number of these trusses, ideally you could have a base truss design that works for them all and a piggyback to vary height and profile. Remember also that the top chord of the lower truss must be braced both for erection and permanently.

I would never plan a two-crane lift if I can avoid it, since the truss will be relatively unstable and flexible. If you do lift it like that, try to avoid any movement requiring crawling/driving or swinging - only a clean pick and lift. The danger being that a problem with one crane can result in loss of one or both cranes and/or operators. We have seen a few major lifting failures when attempting to lift flexible assemblies. It is done all the time, but there can be significant hazard involved. (That is the safety engineer in me talking.)

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