Live Load Reduction

[OHIOMatt]

I have a project were I am designing steel trusses that span 80'. The trusses are spaced at 16' o.c. so the total tributary area for the truss is 1280 square feet. The trusses support purlins which will be placed at 2'-0" o.c. The pulins have a tributary area of only 32 square feet meaning that they are designe for 20 psf.

For webs that are perpendicular to the chords, should they be designed for 20 psf or can they be designed for 12 psf based upon the total area of the truss? It seems logical that they should be able to support the reaction of the pulin but I just wanted to get confirmation that I am not over thinking this.

 

[ToadJones]

i dont like Live Load reduction for roofs

 

[msquared48]

If the live load is snow, no LLR is allowed.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[OHIOMatt]

Snow load is minimum (5 psf or 0 psf), I am specifically speaking of live load.

The slope of the trusses is 4:12, so I can see no reason for not reducing live load.

 

[vandede427]

I'd keep that extra 8psf in your back pocket as insurance. Use it later if you get desperate.

 

[OHIOMatt]

I would really have a hard time explaining to my client that I increased the minimum required live load by 67% (and thus cost) for "just in case reasons". Again were is the load going to come from? There is minimal to no snow, the roof is 4:12 so there is no chance of ponding.

The question, is do you design individual elements of the truss for the code allowed live load reduction, or do you interpret these as individual members with smaller tributary areas and thus a higher code minimum live load.

I did not start this thread to be a debate on the merits of the code allowed live load reductions.

 

[ToadJones]

My reason for not liking LLR was what Mike said.
Too many roofs with snow load problems...especially last winter in places like VA, that one might not think snow load was bad.

 

[nutte]

Per the code, the truss can be designed for 12 psf.

 

[dcarr82775]

I never reduce roof LL. An increased load can come from roofers stacking an entire roof worth of material in one area. I've never seen roofer who spreads his material out.

If you really want to reduce the LL I would design the truss for the reduced load and then check the webs at isolated locations for the purlin loads.

 

[OHIOMatt]

If we start getting snow in places such as Southern Texas and Phoenix, AZ, I would assume we would have much greater concerns than roof collapses.

Once again, how do I tell a client that has never seen a snow flake that I increased his minimum roof capacity by 67%?

This is a big country that is basically governed by a common model building code. The code provisions exist for a reason. I would appreciate some input on the original question.

I have desinged structures in all 50 states. My approach to a design is different based upon expected reasonable conditions in a given location, while never going below the code prescribed minimums. I build some conservatism into the designs, but part of being an engineer is to make designs practical and cost effective. Why put steel in a location were it will never be used.

 

[ToadJones]

if you have a web member that is perpendicular to the top chord at a panel point where the purlin is landing and there are also web diagonals at the same panel point, chances are the load in the vertical will be VERY low....maybe even a zero-force member for a first order analysis.

 

[OHIOMatt]

Thank you for getting back on topic.

The web member is perpendicular to the chord, but the diagonal frame into it at the bottom chord, while the purlin frames into the top chord. The load in the web member is esentially the reaction from the purlin.

 

[msquared48]

Kinda got a feeling where this is leading here, and it is a much broader topic.

No offense intended, but I get the feeling that the client is calling the engineering design shots here, and that is dangerous, very dangerous. In this economy, I would expect the scenario of "do it my way or I will find some else who will". This is beginning to smell of it.

Professionally, I would use my judgement as a structural engineer to solve the problem, not relenting to the economic pressures of a non-engineer. You must always remember who he is going to sue if and when the thing fails, and this type of individual will do that at the drop of a hat.

Bottom line is, if you can justify the reduction in a court of law, fine. Otherwise stick to your guns and fire the client if necessary to CYA. I am speaking here as the owner of my own firm, but also one that has no work on the desk at the moment. That being said, I would still walk away if warranted.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[OHIOMatt]

msquared48

I am getting no pressure or from the client. They shopped one of our designs once and found that what we where providing was efficient and cost effective and have made no efforts to reduce our fees or challenge our designs since.

But as a part of this, I constantly challenge myself and the other engineers in my company to make sure that we are providing the best possible solution.

The question above came about as I and another principle are having a bit of a disagreement and I was hoping to get other opinions. As of now, the client is not aware that we could be looking at a potential cost savings, if we design the webs in question to reduced live load as opposed to standard 20 psf.

At this point, I am not even sure that I would take advantage of the potential savings. I am just trying to figure out how others would approach the problem. I do not know how to get to a solution if I do not know where I am starting.

Nothing going on in the background, just wanting to make sure that I am seeing the solution in a reasonable manner. I want to provide a design that meets the minimum code loads (more when we feel the code minimum is likely to be exceeded), is economical, and provides the best value for the client.

 

[mijowe]

"The load in the web member is essentially the reaction from the purlin."

If the source of the load is 100% (or close to it) from the purlin, then you could only use the TA of the purlins for the reduction of the web.

Any significant savings that you see by using a reduction will be in the chords and the diagonal web members, which you plan to do anyhow.

If you have 15psf DL, the load to your web would be

(15 +20) x 32 = 1120 lbs vs
(15 + 12) x 32 = 864 lbs

So when you consider fabrication shipping and erection, I can’t imagine there would be a huge diff in the webs.

Are you putting any bending into the chords from the purlins (or do they all fall on panel points)?

 

[mijowe]

Are you using a Kll of 2 for interior beams, or 1 for "All other members identified above"? I am not calculating a reduction to the 12 if I use the latter

 

[OHIOMatt]

mijowe:

Thanks, this was my opinion as well. We just wanted to make sure that we were looking at it the correct way.

 

[JAE]

I'd agree with mijowe - each element along the load path has a tributary width associated with it.

So the purlin forces applied to the individual web member should be based upon that web member's tributary area - not the purlins...they may be the same, but in the case of two purlins coming in from either side of the truss, the web member's TA would be twice that of an individual purlin.

I think what many above were intimating was that you can really spend a lot of time chasing down tributary areas to save a few dollars on web plate thicknesses and perhaps even find that the savings is minimal.

 

[ToadJones]

dumb question....
Why are the purlins spaced so closely?

 

[ToadJones]

Does the code really imply that the LLR be taken on a per-member basis?
I somehow doubt this is the intent...I could be wrong. But, I think if you can justify reducing the live load, then it applies to the truss as a whole.
Also, keep in mind your construction live loads....can't reduce those!

Matt....you have really done work in all 50 states?
That is quite impressive.