×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Live Load for Farm Building Trusses

Live Load for Farm Building Trusses

Live Load for Farm Building Trusses

(OP)
Hi guys,

I am checking the truss design drawings for a set of trusses that will be used to replace common trusses (4:12) for a farm building that were damaged by snow. The roof slope and surface (metal) allow for slide off credit as well as reduced snow load due to wind exposure. There is also an importance factor reduction.

In the TDD notes the specified live load is given as 63% of 37.6 psf ground snow load or 25.7 psf. It looks like this is the live load used in the design load cases without any additional roof live load (typically about 20 psf). Is this how the load is typically calculated when the snow load exceeds the roof live load? I remember hearing somewhere that roof live load was basically a construction load so perhaps that is how it is omitted from the load cases.

Also I am curious as to where the 63% might come from (Cs). I think that 80% is usual or perhaps 60% if there is certain exposure to the wind.

Thanks in advance.

RE: Live Load for Farm Building Trusses

Do you think you'd ever have 20psf of people standing up there when you have 26 psf of snow? I always justify it that way. I have a hard time seeing 20 psf worth of people trudging through 18" of snow.

RE: Live Load for Farm Building Trusses

(OP)
Actually I'm pretty sure it is nearly suicidal to be on a 4:12 metal roof with full snow load. I guess I was hoping that the truss industry might have a white paper or something that formalizes this reasoning since the building codes sometimes don't clearly differentiate between Lr and LL in the load cases.

Thanks again.

RE: Live Load for Farm Building Trusses

Around here (central Illinois) they will allow a designer to reduce the live load from 20 PSF to 16 PSF for ag buildings that humans are not normally in. But I've never seen it go lower than that.


In my experience, the majority of problems with ag trusses come from inadequate bracing. The truss designer comes up with a design that requires web braces (CLBs) But they never get installed in the field.

So bracing issues is what I typically focus on. I try to deal more with real-world problems than numbers.

RE: Live Load for Farm Building Trusses

charliealphabravo,
What code are you using? The load combinations in IBC and ASCE 7 differentiate between roof live load and live load. If I remember correctly, all of the combinations with snow have (Lr or S or R). Snow is not combined with roof live load.

If the truss designer used ASCE 7 to calculate the snow load, 0.67 is reasonable when I=0.8, Ce = 1, Ct = 1.2 and Cs = 1.
0.7 x 0.8 x 1.2 = 0.67

RE: Live Load for Farm Building Trusses

(OP)
Thanks that makes sense on the separation of Lr from snow load. I am using an old farm code for the existing trusses which refer to an old NBCC code (Canadian). Those codes seem to lump snow load and Lr together. Even the latest NBCC seems to do the same unless there is a provision I missed.

I wouldn't normally review the TDD with this level of scrutiny but I was curious how trusses designed under current codes would compare with my analysis of the original trusses under the codes that were in existence at the time. Would you guys care to take a gander at it too before I follow up with the designer?

My main question now is that I don't understand how they are getting 2x6 chords to take such high tension and compression loads. Perhaps they are using a low braced length or perhaps a repetitive use or wind load duration factor. Also I have never seen compressive member forces listed as negative. That seems to be a Canadian thing.

It doesn't look like the trusses have been through engineering yet and I have deleted the header and job specific info.

Thanks in advance.

RE: Live Load for Farm Building Trusses

To me, 63% seems like a huge snow reduction for a 4:12 pitch roof.

Mike McCann
MMC Engineering

RE: Live Load for Farm Building Trusses

You have me really curious now. Lots of questions/comments. I hope you don't mind.

I'm curious what role you play here. I'm guessing you're the engineer of record, and approving these trusses before they're built?


I agree that the forces seem high for 2x6 SPF lumber, if the truss is one ply. I don't see a note that says it's either one or two ply. Is it maybe in the top line that you cut off?

You might recall what I said earlier about CLBs. (Web braces) You'll notice that 2 CLBs are required on W5. No way would I send a truss out like that.

I know of a case in the Chicago area where a design like that went out to the field. The CLBs were not put on, and the web buckled badly under an unbalanced load during an ice storm. The webs eventually broke, which put a lot of bending stress on the BC splice. the splice plate eventually broke, and the trusses came down.

If I we designing that truss I'd upgrade that lumber to eliminate the double CLB. The single ones aren't usually a problem. But I've been in old ag buildings where 2 CLBs were required on webs, and they're always buckled badly.


You mentioned the braced length of the chords. The Mitek software (which I think this truss was designed with) shows purlin spacing right on the drawing. You can plainly see the purlins drawn in at 2' O.C. on the TC.

Since no purlins are shown on the BC, I wonder if they mistakenly designed it as a sheathed (Continuously braced) BC. Since there's wind uplift numbers on the drawing, there should be bracing specified for the BC. Maybe it's on the 2nd page that's been cut off?


I was wondering if you would consider emailing me the entire drawing so I could look it over. As much for curiosity as anything.

My email address is rsitruss (at) yahoo (dot) com


RE: Live Load for Farm Building Trusses

The 63% of ground snow load likely comes from using a Cb = 0.8 and a Cs = 0.79 where Cs = (60deg. - roof slope)/53deg.

For a farm building without defined attic space, no live load would be applied in addition to the snow load. There are minimum snow load values which you can't go below, even in low ground snow load areas. This ensures there is enough capacity for incidental live loads for repair/maintenance, etc... where persons may be on the roof (typically 1.0 kpa per NBCC)

The bottom chord bracing is specified on the summary sheet you attached to your post as bracing every 5.53 ft, or rigid ceiling applied.
The lateral bracing for the web members is very very common in the industry and as long as it is properly installed, will produce satisfactory structural performance.

It is the EOR's responsibility to ensure the overall stability by adding permanent bracing and ensure all bracing forces can be adequately restrained within the structure.

I recommend you review the TPIC manual for the design and fabrication of trusses, there is a lot of explanatory/background material in this document (TPIC = Truss Plate Institute of Canada)

RE: Live Load for Farm Building Trusses

(OP)
@Ron, Oh yes, sorry. I cut off the project specifics at the top indicating that it is a single ply truss. The number of trusses (29) is also consistent with 4' oc spacing and single ply trusses on a 120' long building.

@Canuck, Thanks that helps on the load combination issue. Also that looks like a reasonable source for the 63%.






RE: Live Load for Farm Building Trusses

Canuck67:
Per your quote:

"The lateral bracing for the web members is very very common in the industry and as long as it is properly installed, will produce satisfactory structural performance."

Of course it's common. But it's also very common for them to not be installed. That's why I brought it up.

I've been in a lot of attics over the past 30 years or so that I've been in the truss business. If a truss web required one CLB and it isn't installed, the web will typically buckle, but not a whole lot.

If a truss web required two CLB and they aren't installed, some of the webs I've seen have buckled to the point that they've either broken or pulled a plate loose from a joint.


So based on that real-world experience, I never send out a truss design that requires 2 rows of CLBs.


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources