Reading truss reactions 2

[JStructsteel]

Got a cut sheet for a girder truss from a truss mfg. Its simple span, but they have a group of reactions. Anyone know why, or how to read the reactions?

Obviously I can figure out the reaction if need be, just want to know what they are trying to report.

Their drawings suck, so it makes it that much harder to read too.

 

[kipfoot]

It's not unreasonable to have them send you a legible version of this. Especially since it seems like it would be a critical element in the design...something you need to get right.

This truss is designed to span continuously and is supported at the four locations where the diagram shows a top of wall under the truss: at each end and at the two interior points.

The reactions are at the bottom printed over the title block. R= 8893, R= 39706!, R= 41406!!, R= 15456

 

[JStructsteel]

Thanks, I missed the two interior walls they are going to use for bearing. Ugh.

Yes, those interior loads are massive.

These are preliminary, but it seems they would have some way to send a better drawing.

 

[oldrunner]

That might never happen. (ie, legible docs from truss guys). If it is a pdf, enlarge it on your monitor. Read all the notes as there usually some interesting ones -,

If you still can't read it, then don't approve the submittal.

 

[JStructsteel]

Another questions is some of the other truses have reactions like this : R=###, U=###, RL=###/-##, What is the RL=###/-##???

 

[XR250]

5-ply truss and a 41k truss reaction? Never seen such a thing. What is this truss supporting? Seems like a chunk of steel might be a better option.

 

[manstrom]

I'm not sure what you expected from a 60' long simple span truss. That design isn't feasible or constructable. How will they even ship that? How will the bearing even work at the ends?



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[JStructsteel]

XR250 - This is a truss girder supporting other trusses. I am relying on the truss professionals to design, I was just confused on the reactions, did not see they had interior bearing points.

Manstrom - Its not a simple span, they have interior bearing, which i was not expecting. Their drawings suck, so its hard to understand, thus the questions. Kipfoot pointed out above that they have interior bearing points. The design looks like it works according to the truss professionals, and based on that they must be able to build and ship. I will take their reactions and design proper columns to support at the bearings.

 

[XR250]

XR250 - This is a truss girder supporting other trusses. I am relying on the truss professionals to design, I was just confused on the reactions, did not see they had interior bearing points.

It does not always make sense to allow truss designers to dictate your structural system. They will try to make anything work with trusses - because, well, they want to sell you trusses. It does not always mean it is the best tool for the job.

 

[JStructsteel]

XR250, Why is a steel beam better than a truss in this application? Now maybe the contractor has to get a fabricator involved, potentially steel erector, welder, etc. At what point do you call 'BS' on the truss guys???

Im not trying to be difficult, but they provided a design, and I can provide supports for it. How do I know steel is the best tool? There are 1500 other trusses on this job, should i be looking to change some to just dimensional lumber, or some others to steel? They were engaged to provide a truss system, which they have done.

 

[manstrom]

The truss manufacturer will stamp the truss design based on the loads that you gave them. They had to add the interior bearing because a 60' clear span is not feasible. Frankly, that span is pushing it for deflection and the ability to put on a truck.

It is the SER's responsibility to ensure that the truss reactions and uplift are supported. The reactions IMO are enormous. A 40k reaction will need a big (custom) bearing plate and steel column and footing. Just because "it works" doesn't mean it will be a practical or cost effective solution. But it's your call. I would have tried to avoid such a heavy loading on a G.T.

When using trusses, it can be a black box because you may not have the design software to see what can work. For odd loading or spans, it is reasonable to reach out to a truss manufacturer and have them run a truss to see what is doable and what assumptions you can make. This eliminates coordination items like this.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[JStructsteel]

Again, it does work though. I have to design a column and a connection for it, which I would have to do for a steel beam anyway correct?

If I try and span 60' with steel, I'm looking at a 24-30" beam, which does not make sense, so I would add interior supports for a smaller beam. Then I would need a connection, and steel column and footing, and now forcing the contractor to also work with steel on the project.

This is a prelim design. This is the GT they provided.

 

[XR250]

I don’t think there are any hard and fast rules. Maybe this system makes the most sense in your situation – especially this late in the game. For me, if it takes more than 3-4 plies of a truss, I am thinking there is a better way to go. I feel that 5 plies of any wood member are too much.
At the end of the day, it is your call and you ultimately take responsibility for the project.
You would def. want to use the interior bearings for a steel beam.

 

[Celt83]

One thing to be mindful of which is often missed, you need to provide both a column for the reaction but more importantly you need to provide a stiff cap plate that gives them their required bearing length. For a 41 kip reaction you might be looking at needing a 24" wide cap plate to get their bearing to work out, you'll need to ask them how much bearing length they need to verify. Stiffeners, etc. for your cap plate will almost certainly be outside of the wall width so you may need to arch to add a bulkhead at the top of those interior bearing walls.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[JStructsteel]

Thanks XR250. I would be interested to hear any input from a Manufactured Truss Engineer too.

 

[rb1957]

Is anyone else scared as all crap about this question ?

JStructsteel, the suggestion is to engage a "proper" structural engineer, not to hope that someone might offer some thoughts here (based on a poor drawing).

"Surely" your project has a structural engineer ?

"Surely" the engineer understands what to do with these loads. As I see it, the truss guys are saying our truss will take the applied loads and deliver the reactions to the supporting structure (which is someone else's problem).

"Surely" you can get the truss design company to send you the "field of drawing" sketch (ie the drawing without the border) so you can read the damned thing.

How do you guys ensure that the five trusses shar the load ? glue ? nails ??

How do you attach to the diagonal members ? a nail plate on the two open faces ?? (good for 5 plies ?)

another day in paradise, or is paradise one day closer ?

 

[JStructsteel]

rb1957 I am not sure what you are getting at? Are you suggesting I am not a 'proper engineer'? Hopefully you are being facetious.

I am the engineer. I asked a question about the reactions about the truss. It was answered, and the supports will be designed accordingly. Nothing to be scared of dear.

As usual, not much help with your comment.

 

[Pmtottawa]

40 kips and 5 ply
I would be asking for different roof layout
5 piles will required bolts for assembly, which reduce net section. Have the manufacture provided with any assembly requirements.

60ft truss will require a special escort especially if its over 12ft high.

As others have said, truss man will design based on provided loads. In the end the eor approved them.

These reactions will require large bearings to meet perp to grain comp reactions.

Another thing I dont see is web bracing for webs over 6ft in length in compliance with tpi

 

[JStructsteel]

I will have a discussion with my client, who then can bring it up to the contractor. Perhaps a steel beam line will be more economical. Perhaps a wood beam line will be. Perhaps the truss is.

 

[rb1957]

JStructsteel,

apologies, I didn't understand that you were the engineer. Your original question seemed really "odd" to me.

Looking at the drawing with a bunch of effort I think I could figure out what they were saying. I hope you have a lot of paid time to do this ! Yes, their drawing does suck and the interpretation of the drawing leaves you open to liability (IMO). You're analyzing the supports for this truss ?

I raise a couple of technical issues, that others seemed to be concerned with.

You say "1500 other trusses" ??

Sorry that "as usual" I'm "not much help".

another day in paradise, or is paradise one day closer ?