Box Truss Span Length capacity

[MRW Lights]

We’re building out a new studio in our broadcast facility and on a wall I have spec’d a 16’ span of aluminum box truss. The architect has kindly offered to save some money by bolting two pieces of 8’ truss together.

So my question was… wouldn’t that effect the weight capacity of the span? He says no it’s the same weight capacity. I said wouldn’t it still need a center support to avoid flexing and he claims a 16’ span split in half would be stronger in the middle.

Additional information- this is a key light truss similar to an FOH position, we’ll never be near the capacity or should cause any amount of flex at the center, but still these are the questions. Also, not being able to hang anything on center is a personal pet peeve, but I can deal with it.

Hoping I can learn more about span length of box truss and effective weight capacity.

 

[egilson1]

If it’s a manufactured truss, then the weakest link is the determining factor in its rated capacity. If that’s the bolt plates, then the load chart will be based on that. So 16’ of truss as a single unit, 2 segments, or 4 segments would all be the same capacity, assuming it’s put together correctly.

 

[TimMc]

What brand and model of truss? The manufacturer has loading information. And yes, you can put a load on center (if it's physically possible to attach). The truss manufacturer will have allowable deflection, point and distributed loading information. 16 foot truss will be a custom build and likely more costly to ship.

I think you're overthinking this.

 

[MRW Lights]

What brand and model of truss? The manufacturer has loading information. And yes, you can put a load on center (if it's physically possible to attach). The truss manufacturer will have allowable deflection, point and distributed loading information. 16 foot truss will be a custom build and likely more costly to ship.

I think you're overthinking this.

The manufacturer information conveniently wasn't included in this draft. Very much an architect that thinks we're just a school and we don't know better. So when the job was specified with a weight capacity of a 16' span of truss and the installation is 2 pieces of 8' truss with only a bolt plate at the center I see a point of failure. As it's a permanent install I can't put something above heads with "it's the same capacity with or without bolts" that can't actually be true? Those bolts have a failure point and the span would have a different allowable deflection with a break in the span than without. Though theoretically the deflection allowance I would expect should actually become stronger with a bolt plate in the center opposed to the full 16' span, but it shouldn't be the same?

The 16' is a weirdly custom fit, but it's the span between two of our building steel beams that this is getting attached to, so custom it is.

@TimMc you're absolutely right and if I had the distributed load table and deflection point and a comparison of the two I would be totally on board. In all honesty the architect shouldn't have given the original load capacity of the 16' span and given me the finished as built stating the span would support the needed load with a capacity up to #. We specified a load and they specified the 16' box truss, then changed their mind when it came to pricing. That's a horse of a different color to me.

Certainly not trying to overthink, but in a forever until the building isn't here anymore install we always think of more not less. We've built our facility to always do more tomorrow than today so any change in a specified capacity is significant in that respect.

 

[Amiers]

As far as putting a fixture center center you can pipe and borough off the top cords on either side of the center and then run another pipe across it.

 

[MRW Lights]

As far as putting a fixture center center you can pipe and borough off the top cords on either side of the center and then run another pipe across it.

Certainly a possiblity, the larger concern and actual plan is a TV Monitor at center, which theoretically could be the same, but still a pain for a permanent install.

 

[TimMc]

The manufacturer information conveniently wasn't included in this draft. Very much an architect that thinks we're just a school and we don't know better. So when the job was specified with a weight capacity of a 16' span of truss and the installation is 2 pieces of 8' truss with only a bolt plate at the center I see a point of failure. As it's a permanent install I can't put something above heads with "it's the same capacity with or without bolts" that can't actually be true? Those bolts have a failure point and the span would have a different allowable deflection with a break in the span than without. Though theoretically the deflection allowance I would expect should actually become stronger with a bolt plate in the center opposed to the full 16' span, but it shouldn't be the same?

The 16' is a weirdly custom fit, but it's the span between two of our building steel beams that this is getting attached to, so custom it is.

@TimMc you're absolutely right and if I had the distributed load table and deflection point and a comparison of the two I would be totally on board. In all honesty the architect shouldn't have given the original load capacity of the 16' span and given me the finished as built stating the span would support the needed load with a capacity up to #. We specified a load and they specified the 16' box truss, then changed their mind when it came to pricing. That's a horse of a different color to me.

Certainly not trying to overthink, but in a forever until the building isn't here anymore install we always think of more not less. We've built our facility to always do more tomorrow than today so any change in a specified capacity is significant in that respect.

Grade 8 bolts are likely stronger than the gusset plates or the welds holding the gussets to the chords. There are accepted ways to lock fasteners that the architect or engineer can specify.

From Thee Armchair Consultancy my guess is your custom 16' truss is pushing something over-budget. Is there a viable trade-off you can offer to get the truss you want?

 

[MRW Lights]

Grade 8 bolts are likely stronger than the gusset plates or the welds holding the gussets to the chords. There are accepted ways to lock fasteners that the architect or engineer can specify.

From Thee Armchair Consultancy my guess is your custom 16' truss is pushing something over-budget. Is there a viable trade-off you can offer to get the truss you want?

I don't even think the project is over budget, it was bid with the budget for the 16' truss and the savings for splitting it is approximate to 1% of the budget. If it comes down to squabling over 1% in change orders we can shake the seat cushions for that... We originally specified a smaller basic ladder truss with the intention of 3 cyc panels and a smaller TV monitor anyway. They bid us for the box truss because it was easier to install in the location (which is technically true) so we said sure. It'll all be well and good in the end, I can definitely work with the split truss if that's the way it goes. I'd return with photos if I thought it was going to be completed soon, but I expect the forum to have long forgotten this thread by the time supply chains allow us to get the material. We started renovations last week and I expect a solid 3 months before it's done, when realistically it's about a total of 2 weeks worth of labor...

 

[almorton]

This box truss holding our revolve. The prosc is 24'

 

[JimmyM]

Obviously I’m not a rigger and this may go against the look but one compromise might be an 8’ flanked by 4’, so your center is uninterrupted and it’s still not custom, would probably be a hair more expensive. I assume based on my reading in the thread that this configuration would be sturdy enough for this use.(obligatory not a rigger again.)

 

[NJLX]

Obviously I’m not a tigger and this may go against the look but one compromise might be an 8’ flanked by 4’, so your center is uninterrupted and it’s still not custom, would probably be a hair more expensive. I assume based on my reading in the thread that this configuration would be sturdy enough for this use.(obligatory not a rigger again.)

I'd go for a 10' stick and a 6' stick, if you want to regain the hang point on center(all things being equal, 2 sticks of truss is probably cheaper than 3)

 

[FMEng]

It seems to me that the architect has, or needs to have, a structural engineer on the team that should weigh in on this. Or, the manufacturer's in-house engineer can answer it. I would ask for the opinion in writing, with a PE stamp on it. Neither you, the architect, or anyone here are qualified to decide this.

 

[MRW Lights]

It seems to me that the architect has, or needs to have, a structural engineer on the team that should weigh in on this. Or, the manufacturer's in-house engineer can answer it. I would ask for the opinion in writing, with a PE stamp on it. Neither you, the architect, or anyone here are qualified to decide this.

Exactly this! I simply couldn’t take the “it’s the same, only cheaper” I may not be a structural engineer, but any amount of experience has taught me cheaper is almost always never the same…

Now in total we’re looking at starting with about 250lbs in fixtures and cable across the entire span in the initial design of the space so we’re no where near what any rated capacity should be on a truss of this nature, but as you said I’m not qualified to make that call so I want someone who is to say what it should be.

 

[egilson1]

A solid 16’ custom stick and (2) 8’ bolted sticks will indeed have the same load rating. We use grade 8 bolts not for their holding strength but to last longer after repetitive use. The joint is no stronger or weaker than the rest of the truss. I deal with this all day every day.

I’d be happy to talk with you directly to further explain if that would be helpful.

 

[microstar]

Here's another way to look at it...

The architect proposed a better solution to ladder truss by going to a box truss with the weight capacity that was needed.
He then saved the project money by going from a custom length to two standard lengths. The poster probably does not know the status of the budget but the architect probably does.
The architect then correctly stated that two truss sections bolted together were the same as one piece the same length. If he was not to be believed, then he should have been asked for verification.
Apparently he did state the capacity of the truss.

The poster disagreed or had an issue with most of these decisions. The decisions all turned out to be correct.
Kudos to the architect ".....that thinks we're just a school and we don't know better". Sometimes the person with the little stamp does know what they are doing
(and may well have consulted a structural engineer).

 

[RonaldBeal]

So my question was… wouldn’t that effect the weight capacity of the span? He says no it’s the same weight capacity. I said wouldn’t it still need a center support to avoid flexing and he claims a 16’ span split in half would be stronger in the middle.

The Architect is correct. It is the same weight capacity, and will certainly be cheaper than having a custom 16' fabricated over off the shelf solutions.
Look at this example for Thomas truss:
https://www.jthomaseng.com/products/aa/general-purpose-truss-12-×-12
Obviously the ratings will vary depending on the mfg and grade of truss (light duty, medium duty, heavy duty, etc...)
In the link above, a 20' span is rated for 1550 lbs at center with 1.1 inch of deflection. 16' will have even higher load at center...

If you "need" the center clear of the gusset plates, ypu could do a 10' and 6', or 4',8',4', and all will have the same strength.

 

[MRW Lights]

I need to take a step back and say I hope I haven't misrepresented the architect on the project or the process and am happy to have had @egilson1 provide the answer my analytical brain was after and for all the times I've used pieces of truss built like an erector set bolted together in all kinds of shapes it wasn't ever that I didn't trust it, but I honestly didn't know that in this given construction it doesn't have a direct effect on the overall capacity of the span bolting the two sections together, but now it makes more sense. We'll be getting a load table for the ordered piece of truss, but it meets or rather exceeds the requirements of the project as specified and that's the goal.

We had a construction meeting today and I have all good news and updates to come back to you all with, that give a lot of clarity.

to @microstar 's points - The box truss was a welcome change, we didn't "need" it in the original plan for the space, but we're honestly happy to have it and ultimately weirdly saved us money due to the mounting and custom size, which is fine. I do know the scope of the budget and it turns out the savings from making it two pieces was a happy circumstance of the manufacturer's suggestion who was concerned about the look of the truss having a "natural" yet minor arc beyond a span of 10'... but in our case it's fine and won't effect photo metrics. Once we learned the change order was presented for esthetics and not functionality we all got on the same page. Ironically we're keeping the 16' span for esthetics and did offer the solution of doing a 10' and 6' span or combination thereof to move the center point and it was figured that other combinations didn't save enough money over doing the full length.

Overall this project is saving itself money moreover than not, but now that I've put that out there I'm sure that will change and we will eventually need to find other ways to cut as with every project, but for now we're on schedule and at budget...

 

[gbdesign]

It hasn't been mentioned, but how is this truss being hung?

 

[MRW Lights]

It hasn't been mentioned, but how is this truss being hung?

Perfect timing I was just stopping through CB for my lunch break...

This truss will be mounted to steel plates at either end and those plates are mounted to the building steel as it's conveniently an exterior wall. Approximately 16' above the floor at center. Final placement is being determined as we open the wall in the next couple of weeks.

 

[Richard Young]

We’re building out a new studio in our broadcast facility and on a wall I have spec’d a 16’ span of aluminum box truss. The architect has kindly offered to save some money by bolting two pieces of 8’ truss together.

So my question was… wouldn’t that effect the weight capacity of the span? He says no it’s the same weight capacity. I said wouldn’t it still need a center support to avoid flexing and he claims a 16’ span split in half would be stronger in the middle.

Additional information- this is a key light truss similar to an FOH position, we’ll never be near the capacity or should cause any amount of flex at the center, but still these are the questions. Also, not being able to hang anything on center is a personal pet peeve, but I can deal with it.

Hoping I can learn more about span length of box truss and effective weight capacity.

Why unless truss is a “look” you want why use truss at all in a fixed studio position?
A 1-1/2” schedule 40 pipe grid put together with speed rail fittings is far more useful than a hunk of truss