Concentrated loads on bottom chord of exposed wood trusses

[PostFrameSE]

In Table 4-1 of ASCE 7-05, there is the provision that concentrated loads must be applied to a panel point or any point along primary structural members (which on a wood truss I would assume means everywhere on the bottom chord) when the trusses are exposed.

How is this load to be used with the Basic Combinations? Are these concentrated loads considered L sub r in the load combinations? If so, we're saying that the concentrated load will never be applied when there's snow load also. That doesn't seem right to me.

Do I need to look at this load combination with either full snow or full roof live load in combination with the concentrated load?

I'd appreciate some perspective here.

Thanks.

 

[civilperson]

The primary structural member is the beam, joist girder that supports the trusses and the concentrated load to be investigated is 2000 pounds. The bottom chord can only be loaded at a panel point or where a web connects.
These roof loads go into equations as Lr and include the addition of concentrated loads.

 

[kslee1000]

What is the nature of the concentrated live load (source & cause). Code specified live load usually refer to load on top of the roof such as human activities, so it is less likely to occur (full roof top live load) during event of snow. However, depending on the nature of the live load, the 2 (snow+live) may act together.

 

[csd72]

You should always try to take concentrated loads to the nodes in trusses.

 

[PostFrameSE]

Thanks for your replies. I am dealing with an all wood frame building with trusses spaced 8' apart. The bottom chord COULD be loaded anywhere by an employee in the facility (although I know it's best to do at a panel point csd72) and so I wonder if that is why the standard reads ".....or any point along primary structural members....."

Civilperson....I think I see what you're saying, but when I have a building with clear-span wood trusses, isn't the primary structural member the wood trusses themselves?

I'm with you kslee1000 that roof live loads and snow loads don't act simultaneously. Isn't this provision supposed to cover Mr. Farmer who wants to pull his tractor engine out in the middle of winter with 24" of snow on his roof and hangs it on my trusses? Those loads would act simultaneously.

I personally think that these concentrated live loads can NOT be looked at as Lr but must be added in addition to the load combinations. Does knowing the type of structure I'm dealing with civilperson change your response?

Last question.........assuming that these concentrated loads need to be applied anywhere on the truss in addition to the roof snow load, is there any way that I can contractually, or legally NOT do this by stating clearly that the trusses have not been designed to carry a large concentrated load? Would spray painting a sign on both sides of the bottom chord of the exposed trusses that warned anybody that they couldn't use the truss as a hoist be sufficient? I'm asking because anyone who has designed wood trusses will understand that a 2,000 concentrated load that is not acting at a panel point would be a killer, especially if it's in addition to snow loads.

Thanks.

 

[kslee1000]

Reply your last question first:

It seems to me there exists chances the trusses would/could be utilized for hoisting. If it is specifically requested by your client, your obligation is to advise him (by writing) wether it is permitted or not. Without his request, I don't think you need to go beyond the general scope of roof truss design. If the truss fail because the owner takes matter into his own hand without consultation with the designer, or a qualified structural engineer, he will bear the responsibility by himself. If you decide it is acceptable to hang weight and designed for it, load posting is wise, however, I don't think it is easy to handle, unless you have rigidly difined the location of load application.

Regarding to your question on "Tractor on Roof", the answer is similar to the argument above. Hope it helps.

 

[CJJS]

Try this link. Download the Load Guide. I have found this very helpful in interpreting code related issues as you have here.

http://www.sbcindustry.com/loads.php

 

[civilperson]

The truss is NOT the primary structural member referenced in the code, (support of the truss is primary). Loading the bottom chord is not a sure thing, inducing bending into a compression element. Loading at a panel point keeps the bending out of the chord.

 

[PostFrameSE]

Civilperson,

I think I see what you're saying. Since it does specifically address roof trusses AND these "primary structural members supporting roofs" I can agree that my wood trusses should have loads applied only at the panel points and that a wood truss isn't that "primary" member. Thanks for helping clear that up.

I understand well the benefits of applying loads at panel points to keep the bending out of the tension element (as is the case with a bottom chord) but my fear is that the general lay-person may not. However, code doesn't say that we need to look at wood trusses inbetween panel points so that issue goes away as kslee1000 pointed out.

CJSchwartz, I looked at the load guide and didn't see how it would tell me if the load was to be added concurrently with snow loads or not. I think it about has to be.

Thanks for your feedback everybody.

 

[JAE]

I'd agree that the roof truss isn't necessarily the primary member - the category that is stated here has the following elements:

1. EXPOSED roof member - i.e. no sheathing and the members are visible from the floor.

2. Exposed to a WORK FLOOR - if this isn't a "work" floor where business operations occur where hoisting might be required, then this whole load category doesn't apply.

3. It is applicable to roof trusses OR primary members. For trusses - just at the panel points.

4. It is listed in the live load table for Roof Members. Therefore it should be applied in your load combinations as Lr.

 

[PostFrameSE]

I just re-read the 2006 IBC and Section 1607.11 reads "The structural supports of roofs and marquees shall be designed to resist wind and, where applicable, snow and earthquake loads, in addition to the dead load of construction and the appropriate live loads as prescribed in this section, or as set forth in Table 1607.1." I think the words "in addition to" make it fairly clear that the extra loads, primarily the 2,000lb concentrated load that I'm concerned about, are not to replace the snow component.

I'm not wanting to beat this horse until there's nothing left, but as you guys have helped shape my thought process, I've come to the conclusion that the load can NOT replace Lr or snow in the load combinations.

There are clearly provisions in the load combinations for the unlikely event of full snow and full wind occuring at the same time, but there is no provision for the reduction of "interior roof live loads" in connection with exterior roof live loads or snow loads.

Thanks again for your help!

 

[haynewp]

I read through 7-05 and I think it should be applied as an occupancy live load "L" according to the definitions in section 4.1, so it would be in combination with "S" or "R".

However, by the exclusion of section 4.3 it does not have to be concurrent with "Lr". Which would make sense that there is not an occupancy point load hanging from the roof at the same time it is under the construction load of "Lr".

 

[JAE]

I still think it is a "roof" load as it is listed under "Roof" loads in the table.

Given that, I wouldn't add the concentrated load to the uniform roof load above - simply use it in separate load combinations as Lr. (i.e. wherever Lr occurs in a load combination, include a separate concentrated Lr combination in addition to your uniform load Lr combination).

 

[haynewp]

If it was meant to be considered a roof load "Lr", then it doesn't really coincide with the definition as set in section 4.1 since I think it is occupancy applied (repair garage = motor hanging from the roof).

ROOF LIVE LOAD: A load on a roof produced (1) during
maintenance by workers, equipment, and materials and (2) during the life of the structure by movable objects, such as planters or other similar small decorative appurtenances that are not occupancy related.

 

[CJJS]

IBC 2009 1605.3.1

"Exceptions:
1. Crane hook loads need not be combined with roof live load or with more than three-fourths of the snow load or one-half of the wind load.
2. Flat roof snow loads of 30 psf (1.44 kN/m2) or less and roof live loads of 30 psf or less need not be combined with seismic loads. Where flat roof snow loads exceed 30 psf (1.44 kN/m2), 20 percent shall be combined with seismic loads."

I think the engine hoist would qualify as a crane hook.

 

[PostFrameSE]

Great point CJSchwartz!

I like that. Similarly it is in the 2006 IBC 1605.3.1 exceptions. THAT makes sense to me. Not sure why I didn't see that before.

Haynewp, I agree with your points as well.

I know that many of us have differing opinions on this subject for various and substantiated reasons, but I'll sign off by saying that my conclusion of the matter is that for my particular type of construction, when wood trusses are exposed to a manufacturing, storage warehouse, or repair garage area that I will use 3/4 of the snow load in conjunction with 2,000lb concentrated loads at the panel points only.

I appreciate everybody's viewpoints on this as you have all helped me tremendously to solidify in my mind a reasonable approach with good justification for handling the concentrated load provisions with these types of projects.

Thanks again!

 

[kslee1000]

I think you are pretty close to wrap up this subject. However, you may want to go one step further to find out why IBC calls for (3/4 snow + L). I am woking in a industry setting, the cranes stop operation only if the roof cave in, and we are in a heavy snow region.

Can anyone please point out the reasoning of IBC?