Live Load Reduction
Live Load Reduction
(OP)
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.
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.






RE: Live Load Reduction
RE: Live Load Reduction
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Live Load Reduction
The slope of the trusses is 4:12, so I can see no reason for not reducing live load.
RE: Live Load Reduction
RE: Live Load Reduction
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.
RE: Live Load Reduction
Too many roofs with snow load problems...especially last winter in places like VA, that one might not think snow load was bad.
RE: Live Load Reduction
RE: Live Load Reduction
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.
RE: Live Load Reduction
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.
RE: Live Load Reduction
RE: Live Load Reduction
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.
RE: Live Load Reduction
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
RE: Live Load Reduction
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.
RE: Live Load Reduction
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)?
RE: Live Load Reduction
RE: Live Load Reduction
Thanks, this was my opinion as well. We just wanted to make sure that we were looking at it the correct way.
RE: Live Load Reduction
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.
RE: Live Load Reduction
Why are the purlins spaced so closely?
RE: Live Load Reduction
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.
RE: Live Load Reduction
Trusses have a lot of hard-to-determine secondary forces relating to the relative stiffness of truss....stiffening a member here and there, can drastically change load paths in a second order analysis.
Of course you are probably using software to model these effects so I'll shut the hell up
RE: Live Load Reduction
RE: Live Load Reduction
RE: Live Load Reduction
Garth Dreger PE
AZ Phoenix area
RE: Live Load Reduction
RE: Live Load Reduction
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Live Load Reduction
RE: Live Load Reduction
Any internal truss member that solely supports a low tributary area member, has that same tributary area by definition.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Live Load Reduction
Garth Dreger PE
AZ Phoenix area
RE: Live Load Reduction
1) Hail being an "environmental load", how much hail do you have to design for? Does that vary with location? Generally, areas without snow have the most hail.
2) For live loads, I understand the maintenance requirements, but planters and people? Would this not be only for a trafficable roof?
For comparison, in Australia the prescribed roof load for members supporting large inaccessible roof areas is only 5 psf. The loading goes up as the area goes down.
RE: Live Load Reduction
For web members that are located near a support which have much higher compression forces due to the accumulation of roof load that I would see it applicable to use area reduction.
I am overseas so will not comment on the use of UBC/ICC/ASCE or whatever code it is that governs this design.
I am actually surprised by the magnitude of the live load (20psf). As hokie mentioned, in Australia roofs which cannot be used for floor-like activities have a minimum design load specified in the codes as 5psf. Generally wind will be several times larger than this.