Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations waross on being selected by the Tek-Tips community for having the most helpful posts in the forums last week. Way to Go!

Existing Wood Trusses are Failing... 3

Status
Not open for further replies.
StructuralJoe

StructuralJoe

Structural
Jun 12, 2007
43
0
0
US
I have any interesting project...

It is a 1-story plaza in Hernando County, FL

It has wood roof trusses spanning ~50' between load-bearing CMU walls

It was built in 1985 and from discussions with the owner the roof originally was constructed of plywood sheathing with Clay Roof Tiles...

Upon inspection the roof has 1 member on each truss that has buckled, typical on every truss

The owner had originally contracted a "handyman" to provide "repairs" although no engineer specified the repairs.

Members where added randomly as shown in attached photos.

I have since been contracted and modeled the roof truss with all applied loads to current code...

It appears that the trusses have no horizontal bracing

My questions:

The only member that fails (on-site) does not fail upon analysis... but other members do, why?

Bad wood grade on every truss?
Load transfer?
Lack of proper permanent bracing?
Current wind loads to excessive?

I have modeled the wood grades to be the minimum that will work for the minimum loads applied and not fail any members... but I don't feel comfortable assuming that the trusses where originally designed to not have any lateral bracing (rat-runs)

Any thoughts on this process...

Any suggestions or recommendations?
 
Replies continue below

Recommended for you

Sort by date Sort by votes
Just a quick comment about modeling trusses in what appears to be RAM. They way I've always modeled a truss is to realease all but one member at each joint. You're releasing all the member at each joint. That may effect your results. Just something to think about. I'll attach a pdf of what I mean for some trusses I recently did.

Scott Shields
Ghafari Associates, LLC
 
 http://files.engineering.com/getfile.aspx?folder=4b95b091-7956-4225-9280-e15950e0f4d0&file=truss.pdf
Upvote 0 Downvote
Thanks Scott,

As you may have noticed I am very new to RAM Advanse. I have used the truss design in Staad and never encountered this problem.

After my last post the REP from RAM finally returned my call about the "instabilities" at certain nodes. Staad used to understand that this was a planar element and never have this issue.

In all of my most recent models I have left one member fixed and the bending moment still looks correct.

One of my questions to RAM that hasn't been answered, does this or would this affect the generated k values? I have set k to 1 for all of my members manually but it would be useful to know in the future.

Thanks and have a great day,
 
Upvote 0 Downvote
Joe...develop the shear in the fasteners and use the glue to keep the members from warping and keep in contact. Adds a little safety factor as well.

Good luck...now go watch the mermaids after a few more beers.
 
Upvote 0 Downvote
StructuralJoe,

This is a very interesting post. I have had little luck with wood truss analysis making any sense in Ram Advanse. The couple of trusses I had to back analyze had the Mitek plates and I could not get the truss to work even with all nodes fixed.

Mark1234,

Do you have any insite into this. How does the Mitek software handle the connection plates? Is there something else in the analysis that makes the members stronger than RamAdvanse, like reduced member length?

 
Upvote 0 Downvote
Thanks Mike,

Better yet can you use a dxf file?

See attached

Design Criteria:
Selfweight
15psf TCSDL
16psf Front Top Chord LL (due to roof slope)
20psf Rear Top Chord LL
5psf BCSDL

There is also 2 small point loads for an A/C unit but I doubt that is critical

120mph wind zone
Category 2
Exposure B
I=1.0
Fully Enclosed => 0.18

Let me know what you get,

Thanks
 
 http://files.engineering.com/getfile.aspx?folder=09ff45b2-1627-4b6a-ae80-83b0483ec0fa&file=Truss_Output.DXF
Upvote 0 Downvote
Ok...

Please see
The truss was run 3 different ways.

#1) all 2x4 web and chords with additional 20 psf in open areas.

#2) all 2x4 web and chords w/out additional 20 psf

#3) all 2x4 webs and 2x6 top and bottom chords.

I wasn't sure what you documented in the field for top and bottom chords. In all reality the truss is not ever seing the 20 psf LL, however, web 3 is still broken. It suggests that wind load caused this to happen because there was no lateral bracing.

StructuralJoe,

I wish we could get our hands on the Mitek software however they do not give it to structural engineers. It is not free, either, for truss manufacturer's. I believe they pay quite a bit for it. My brother-in-law is nice enough to run trusses for me when I need it.

Hope it helps.

MDJ
 
Upvote 0 Downvote
Mark,

You assumed, as I did that the left hand support is on the first vertical. I believe it is one panel point over, i.e. it is on the second vertical from the left.

That means the second vertical is taking the entire reaction of the truss.


BA
 
Upvote 0 Downvote
Bruce, you are correct, sorry mark for leaving that out.

So the left support underneath R3

The top and bottom chord are both 2x6 and the bottom chord is spliced at nodes 17,15,& 13 directly under the vertical

I had the Top Chord from node 7 to 11 fail (strengthened with #2 SP 2x4

I have the bottom chord fail from node 15 to 11 (same repair)

I had W6 fail, although it was fine when I was on site.
I strengthened it the same way as above.

I added lateral bracing at midspan of Members 4, 6, 7 & 9

I repaired member W4 because it is the one that failed completely.


I think that covers it, thanks for the back-check Mark, it's really useful...

Joe
 
Upvote 0 Downvote
Can you clarify. Did the truss calculate to be okay with the Mitek software and not Ram Advanse?

As you can see with my previous post I have had similar experience.

 
Upvote 0 Downvote
Truss webs in compression are designed w/ an effective length of .8* unbraced length.

The TPI standard details the requirements for truss member design.

Mark1234, you are going to get your friend w/ the Mitek software into trouble, careful.
 
Upvote 0 Downvote
Agree with mudflaps et. al that it is the red web members that failed. They're compression under gravity and they popped. They are probably still carrying the compression load so be very careful about removing them!
 
Upvote 0 Downvote
Wood graded by the pre-1991 standard has to be designed with the NDS rules in effect at that time. If these trusses were built in 1985 they should not be evaluated using current NDS formulas. The American Wood Council has a posting on this at Before I retired I used RAM Advanse and it did not have a way of analyzing wood by the old formulas; has that changed or are you doing a manual check?



 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

m1208
  • Locked
  • Question
wood trusses subject to house fire
Replies
4
Views
12
dvd
dvd
reverbz
  • Question
Information for Wood Log Beams and Trusses 2
Replies
4
Views
22
Craig_H
Craig_H
RontheRedneck
  • Locked
  • Question
Fall arrest anchors on wood trusses 2
Replies
17
Views
18
jerseyshore
jerseyshore
mfstructural
  • Locked
  • Question
Reinforcing of Existing Wood trusses for RTU
Replies
15
Views
16
XR250
XR250
lexpatrie
  • Locked
  • Question
Reinforcing existing wood joist
Replies
1
Views
17
phamENG
phamENG
Back
Top