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Live Load Reduction

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.   

RE: Live Load Reduction

i dont like Live Load reduction for roofs blush
 

RE: Live Load Reduction

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

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

RE: Live Load Reduction

(OP)
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.   

RE: Live Load Reduction

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

RE: Live Load Reduction

(OP)
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.

RE: Live Load Reduction

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.  

RE: Live Load Reduction

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

RE: Live Load Reduction

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.

RE: Live Load Reduction

(OP)
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.

 

RE: Live Load Reduction

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.  

RE: Live Load Reduction

(OP)
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.   

RE: Live Load Reduction

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

RE: Live Load Reduction

(OP)
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.

 

RE: Live Load Reduction

"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)?  
 

RE: Live Load Reduction

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  

RE: Live Load Reduction

(OP)
mijowe:

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

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.

RE: Live Load Reduction

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

RE: Live Load Reduction

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.  

RE: Live Load Reduction

I dont't think using live load reduction on one part of the truss and not on the whole thing is a good idea.
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 bigcheeks

RE: Live Load Reduction

I agree with most here in saying that while it's pretty clear that ASCE allows you to design roof live load members below 20 psf, I wouldn't.  No way I could justify the construction live load being less than 20 psf, so unless you're planning to shore the roof trusses or you're going to be on site 24/7 to make sure proper construction protocol is followed, stick with 20 psf and sleep a little easier at night.

RE: Live Load Reduction

Well...should I be nervous?  I've designed a 9.5 acre roof at 12 psf.

 

RE: Live Load Reduction

JAE, I would not be nervous (I have used the live load reduction many times, for floors and roofs). The building codes do not consider construction loads (See the IBC 2009 Section 1602 and ASCE 7-05 section 4.1 "Live Loads...do not include construction or..."). The contractor is responsible to build the structure per the design loads or hire an engineer to design support for increase construction loads. The most typical example from me is contractors using a higher strength concrete so they could start the walls and floors framing early. But that was when construction was booming.

Garth Dreger PE
AZ Phoenix area

RE: Live Load Reduction

Don't you guys have wind to consider?  Where I am, we never have to worry about roof live load, as the wind cases always control.

RE: Live Load Reduction

No comment...  bigsmile

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

RE: Live Load Reduction

So if it is not construction load, just what constitutes a roof live load of 20 psf?  Hail?  If so, I wouldn't reduce that.

RE: Live Load Reduction

The LL provisions are provided on the basis that there will be pockets of high loading in a general area of zero loading. The loading on any element must be considered "real" because the local high loading can be concentrated upon it. The immediate supports for it must be capable of supporting it.

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

hokie66,  Hail is a environmental load. Per the IBC 2009 and ASCE 7-05 "LIVE LOADS (ROOF). Those loads produced (1) during maintenance by workers, equipment and materials; and (2) during the life of the structure by movable objects such as planters and by people."

Garth Dreger PE
AZ Phoenix area

RE: Live Load Reduction

So for those of us in other countries not familiar with the IBC and ASCE documents, I wish someone would please explain a bit more about this.

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

Live load reduction is a reduction in live load that contributes to the actions on a member. If the roof design load is 20psf and you are considering the compression in a web member that is located near the mid-span than I would not use live load reduction for the compression in that member. As you say the purlin reaction on the truss would be the correct load.

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.

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