Existing Wood Trusses are Failing...
Existing Wood Trusses are Failing...
(OP)
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?
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?






RE: Existing Wood Trusses are Failing...
What size are the plates? I have a 1980s text on the capacity of the plates. Perhaps modelling with a modified spring stiffness or nodal moment loads will give you more realistic load patterns. As you've already seen, our models don't always match reality. The designs we produce often work more because of the general rule that if a total system has sufficient capacity it will redistribute as needed in service to achieve stability.
Very interesting situation... Keen to see how it all works out!
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Existing Wood Trusses are Failing...
www.SlideRuleEra.net![[idea] idea](https://www.tipmaster.com/images/idea.gif)
www.VacuumTubeEra.net
RE: Existing Wood Trusses are Failing...
The trusses were probably designed for bottom chord bracing and it was likely shown on the truss plans, but the contractor might have left them out.
If I'm understanding your photo, the failed member failed in compression. Further, it looks like either a gambrel roof or a mansard. Is that correct? If so, both pick up large lateral wind loads.
I agree with YS....our models don't always equate to reality, and the most common error is that of connections not acting like they're modeled. Do as YS suggested and see how your distribution changes. Don't necessarily look for failure in that member, but look at its axial load relative to the other members. If it is higher than the others of the same configuration, then you have an anomaly that might be pointed toward a single side loading (wind from one direction, for instance), ponding on one side, etc.)
RE: Existing Wood Trusses are Failing...
Can you post a sketch of the truss, identify the loading mechanism, the failed and the survived members?
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
MDJ
www.windspeedbyzip.com
www.groundsnowbyzip.com
RE: Existing Wood Trusses are Failing...
I've seen a rectangular shaped house have one of its perimeter basement walls completly collapse leaving the house to "span" the long distance from one side to the other.
Wood structures have so much crazy redundancy that they don't always follow traditional stability logic.
RE: Existing Wood Trusses are Failing...
Attached is a jpg of the truss along with failed members from the program I am analyzing it in...
All I can determine is that the "other" members are overstressed but due to some of the modifications present (from the "handyman"... that just cracks me up every time I type it... I can hear it now "yeehaw I sur can fix yo probem... I'll nail me some 2x8 here and there and you's be good to go...) it isn't as noticeable on-site.
As to the cause I would think that even if this structure had experienced full DL and maybe full LL or WL I doubt it has ever seen current design load combinations DL + LL + WL (NDS - LFRD)...
I intend to add the lateral bracing and reinforce some of the members that fail in my model and go from there...
The buckled members appear to fail due to P-Crit axial loading...
If anyone has any design examples for wood truss plywood gussets that would be helpful also...
Thanks again,
RE: Existing Wood Trusses are Failing...
I have designed the modeled the truss with current loads applied and based the lumber grade on the effectiveness of the truss... but I have no way of knowing if the truss was properly designed in the first place and if it was truely designed for a clay tile roof... the owner says it was originally a clay tile roof but that wouldn't be the first time I've been lied too...
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
I have modeled everything as pin-pin everything as fixed-fixed and also fixed at continuous members (top & bottom chords) with pins at web members and TC or BC joints...
All with no luck... there where some additional memebers added (by the "handyman") and I am now wondering if that is what is throwing off my model?
RE: Existing Wood Trusses are Failing...
2)Space the nails at 2" o.c. for 2x4 members (2) rows for larger members. This will size your plywood.
3)The nail spacing generally governs the plywood size, however it is wise to check the shear length of the plywood at the heels along the plane of the top chord.
4)Fasten the 3/4" CDX plywood to both faces of the truss w/ clinched nails.
Example:
clinched nail value = 200#/nail (fictitious value)
F_topchord = 4000 comp. -> 2400/(.6)200 = 20 nails
F_btmChord = 3700 tension -> 2220/(.6)200 = 19 nails
Plywood will need to be 19*2= 38" wide spanning from the btm of the btm chord to the top of the top chord
FYI- truss designs generally run D+L and .6D+W to obtain the best stress reversals and uplift calculations.
RE: Existing Wood Trusses are Failing...
Old CA SE
RE: Existing Wood Trusses are Failing...
-Do you have a column or wall directly below the vertical ridge member? The stress ratios are high for members around the bottom chord panel joint.
-Why is no, or negligible, stress on members near the left end?
-The stress ratios on members around the 2nd bottom chord panel joint from the left don't make sense either.
-Is the right end supported on roller for this particular case?
Any one has better clues? Or I am plan wrong?
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Existing Wood Trusses are Failing...
I think there is no doubt it should have been braced. I don't think that pinned or fixed connections make much difference, as the member would have buckled laterally, not in the plane of the truss.
Defective timber, as Mike suggested above, could be involved. A lot of pine bark beetle infested logs were slipped into the market a few years back. Deterioration due to heat and humidity is another possibility, especially if the roof space is not vented. The white spots visible on some of the members suggests fungal attack.
RE: Existing Wood Trusses are Failing...
Hopefully you have modeled it without all the crap the handyman added. That should tell you what it "should have been". As hokie66 noted, removing the tile would make it more susceptible to uplift.
And then....you just might have a bad truss design!
RE: Existing Wood Trusses are Failing...
It maight not deserve the highest attention, as you can see the defects onsite, and make necessary modifications by judgement. But a valid analysis can check and backup your actions. From information you provided, the truss itself is inherently unstable, therefore member stress couldn't be predicted accurately. Someone before me has pointed out, that the reason for the truss to having survived for so long colud be attributed to the "hidden" redundancy built in the wood structures, however, it's not something you can rely upon. Also, someone has pointed out that the change in roofing material may have considerable impact on how the truss behaves. Bad wood theory? Difficult to image it happens on the pattern of one member on each truss.
How the truss was supported on the left end? Especially the upper panel joint directly above. Was it attached to something not shown, and/or could it be strengthened to take out the likely excessive lateral displacement?
Finally, with a limit budget, you may go ahead modify the roof system based on your best judgement. Otherwise, the truss merits a thorough structural evaluation, changes and $$$ are anticipated.
RE: Existing Wood Trusses are Failing...
The repair frame should be continuous across panels. Fasten the frame to one face of the truss with 10d common wire nails. Leave all of the existing truss members in place (of course you can remove the broken ones)
Nail qty is determined by the forces in the top and btm chords in the end panels of the frame.
The drawback is that sometimes the required frames are too large to install and there is no choice but to use plywood.
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
If your elevation is drawn to scale, and the span is 50', the height of that member is about 7.1 feet. The maximum slenderness ratio you can use for a compression member is 50 in CSA 086-01 (the Canadian timber code). For a width of 1.5", the maximum length of an unbraced compression member is 75" or 6'-3". The member should have been braced to meet the minimum standard.
Based on your geometry which I scaled, I believe the compression in Vertical #2 is about 1.7P where P is the panel point load, in this case w*2*50/8 = 12.5w where w is the uniform load per square foot.
If your roof load is 20 Dead + 25 Live = 45psf, P = 562# and the load in Vertical #2 = 956#, far too much load for such a long member.
In all probability, the original design required bracing but it simply wasn't installed. It happens all too frequently.
BA
RE: Existing Wood Trusses are Failing...
Mudflaps and BA seem to be correct that you have misidentified the member which failed, but it is hard to tell from the photo. It would be good if you can clarify for us, perhaps with another photo.
RE: Existing Wood Trusses are Failing...
I agree. When I first looked at the photo we have, I thought the "X" in the middle was some kind of cross bracing. After I stared at it for a while I realized that it was a web member from each truss which had buckled over until they met in the middle. That's one way of reducing your slenderness ratio!
I think the "handyman" added a couple of verticals to each truss but didn't remove the buckled members. Just as well! They may be carrying as much load as the repair.
It would be interesting to see a photo from the side, but it might end up looking like a forest of two by fours.
BA
RE: Existing Wood Trusses are Failing...
BA
RE: Existing Wood Trusses are Failing...
Can you provide location of the supports, and details on connections? Was there a change of support conditions after erection? I don't think the original designer/supplier/erector are all ignorant.
After re-exam the photo, I guess the top and bottom chords are still in position, means there is no obvious problem with lateral stability, but how and why so much compression gets into this member, if the chord is free to deflect in vertical direction?
RE: Existing Wood Trusses are Failing...
For the geometry of the two trusses in the photo to accommodate buckling of Vertical #2, the top and bottom chords would have to move about 2.7" closer together. Whether the top went down or the bottom went up or a combination of the two is not known.
BA
RE: Existing Wood Trusses are Failing...
BA
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
I assume you mean the two web members? Right?
BA
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
I am not in a position to defend the Canadian lumber market because I know very little about it. Perhaps there was some serious skulduggery going on in that industry. I don't know.
What bothers me is your assumption that the problem addressed in this thread is in some way connected with bad lumber from Canada. I really thought you had more on the ball than that!
If you had taken the trouble to read my earlier posts, you would know that the design of Vertical #2 could not possibly have been justified, even if it had been grown in the good old U.S.A.
Mike, I really expected more from someone of your standing in the engineering community.
Bruce Alexander, P. Eng. (retired)
BA
RE: Existing Wood Trusses are Failing...
I try to keep my mind open to all possible reasons for any failure until the truth comes to the fore. Closing ones mind limits the truth. Sorry for the post. I won't make any more.
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
Keeping your mind open is great! Don't ever close it. Perhaps Canadian lumber was at the root of the problem. Who knows? Certainly not me!
Please don't be "sorry for the post". I admit that I was a little annoyed at you, but I want your continued input because, over the last few months, I have come to respect and value your opinion on a number of issues. Please do not drop out of this discussion.
I would invite you and all members of this forum to review the engineering aspects of this problem. If you do not agree that Vertical #2 was incapable of carrying the load, even with good lumber, please show me your calculations.
BA
RE: Existing Wood Trusses are Failing...
The EOR will need to sort through the local requirements and actual geometry of the truss to determine what was the cause. I was merely giving another alternative reason, possibly an additional one, and a follow up post to my original one with the actual information. I was not discounting your information, just adding to it.
That being said, I feel it is funny in a roof structure for only two trusses of many to fail if it is only a design problem. I still suspect that defective materials played a major part. It could also have been a local overload which happens here with snow driftring. But that depends on the location.
Sorry for the post. I got a little PO'd too.
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
You said...
It is not only a design problem. Here is what the OP said:
This suggests to me that one member, namely Vertical #2 had a problem. The problem was that it was too slender to safely sustain the load. Check it out. It is clearly incapable of carrying the load.
BA
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
I hear stories how home owners witnessed loud banging in their attic during heavy winds... when inspected, always a lack of bracing was discovered.
If this building was erected in 1985 the following links may add some insight as to the excessively bowed webs and failed members.
http:
http://ww
MDJ
www.windspeedbyzip.com
www.groundsnowbyzip.com
RE: Existing Wood Trusses are Failing...
1. I seem what looks like a ridge beam at the roof, above the truss, and beyond the break point.
2. I notice two five member joints in the bottom chord in the model, and see one in the foreground of the photo. I assume this is the right most joint of the sketch.
3. I see the added vertical, beyond the first five member joint. I see the broken diagonal after the added vertical.
Am I missing something?
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
1. What looks like a ridge beam is the top of Vertical #1 (numbering from left to right on the elevation).
2. The five member joint you see is directly under the ridge (which I believe has a height of 10.9')
3. The added vertical, actually two per truss is a repair immediately adjacent to Vertical #2. The additional vertical shown on the sketch is behind the photographer and does not appear at all.
That is the way I see it.
BA
RE: Existing Wood Trusses are Failing...
Just an FYI, I have posted three times to this and I haven't seen any go thru... not sure what is wrong but...
There is some confusion (my fault entirely) on the support locations, please review attached multi-page PDF...
In addition to clarify, it appears that permanent lateral bracing should have been applied and wasn't.
It also appears that possibly the roof wasn't originally design to support clay tile (i have placed 15psf SDL in addition to self weight).
Is it possible that the SSBCI in or before 1985 allowed for a great LL reduction that currently permitted? I used 20psf unreduced (i may be able to reduce the front slope LL due to slope angle)
In addition it may be possible that the design wind loads in 1984 were much less than currently required...
Also I believe a "stress increase" was permitted back then when multiple load cases were applied (ie DL + LL + WL)
All of this said and done I intend to first reinforce the members that don't meet current code.
Secondly I intend to add quite-a-bit of lateral reinforcing so that none of the member fail under any of the current loading combinations.
Also there has been some question as to the orientation and the members that have failed. 1 member in each truss has completely failed... another member in that truss (opposite)fails much higher... I wonder if the longer legth member bowed but didn't break whereas the shorter member (when load was subsequently dumped into it) cracked much easier... neither of which is here nor there... i intend to bring these trusses up to meet current code regardless...
Thanks especially to Mike and Bruce, I greatly appreciate both of your inputs! And also everyone else involved.
Thanks, Joe
RE: Existing Wood Trusses are Failing...
Thanks for the response. Mike and I can go and have a beer now.
The last page on your PDF shows that the truss is higher than I had thought. The left support is at Vertical #2 where I had assumed it to be at Vertical #1. This makes the stress in Vertical #2 considerably more serious than I had calculated.
I assume that the ratios on your last page are F(calc)/F(allowable). If that is right, you have some pretty impressive ratios.
Thanks again, Joe.
BA
RE: Existing Wood Trusses are Failing...
When I looked your second picture, I can tell without any doubt that the vertical green member will buckle without bracing. The blue one is a tension member at vertical loads therefore it can't buckle. I don't think the failure has anything to do with the lateral wind loads.
RE: Existing Wood Trusses are Failing...
IMHO, rather than retrofitted bracing, requiring accurate placement to work properly, a less sophisticated approach is probably more cost effective:
Use glue and deck screws to "sister" a new, near full length member (2x4, probably) to any currently unbraced compression member that is too slender (greater that 6' 3" unbraced length). Doing this would increase resistance to buckling by merely doubling the width of the compression member. Members that are buckled could have a pair added, one on each side.
Advantages are:
1. Truss members needing reinforcement can be determined in advance and the lumber cut and drilled (for screws) before it is taken into the attic. (Assuming that lumber approx. 12' long can be taken into the attic).
2. The work is repetitive, chance for installation error is reduced. Inspection and acceptance of the work are also simple.
3. In the photo the attic is already "crowded". Once installed, there is still reasonable (future) access to all parts of the attic. - better than with a bracing system installed.
www.SlideRuleEra.net![[idea] idea](https://www.tipmaster.com/images/idea.gif)
www.VacuumTubeEra.net
RE: Existing Wood Trusses are Failing...
Not time for a beer yet...
Sorry to belabor the point here, but I still have a couple of concerns with the computer model:
1. Are all the joints really pinned? Normally the top and bottom chord are continuous over some joints, but not at the splice points.
2. You have shown roller supports at both supports in edach direction. I do not agree with this. The vertical direction should be set so no vertical translation can occur. The horizontal direction should be set so no translation can occur at one joint, but can at the other.
See what happens to the deflections and member moments/shears after these adjustments are made.
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
To further clarify:
I have modeled supports as pin-pin and pin-roller
I have modeled all points pin-pin
I have modeled all points fix-fix
I have modeled points at continuous Chords to be fixed and splice locations to be pinned - along with all web members pinned (I did record splice points on site)
I have modeled with lateral bracing
I have modeled without lateral bracing
I have modeled with some members laterally braced and other members "built-up"
I have modeled with 75% of LL and WL
I have modeled with 5psf TC SDL + SW instead of 15psf (the non-clay tile option) and 75% LL & WL
To note SlideRuleEra option that is what I intend, but under DL+LL+WL I will still need some additional "rat-runs" to strength members that will undergo compression...
I intend to use all #2 southern pine 2x4 so size and length shouldn't be too cumbersome... and for this space rat-runs shouldn't be to difficult IMHO...
Thanks again to everyone posts... I may upload my intended repairs to hear additional input...
Does anyone know of a particular "GLUE" to specify for attic spaces here is Florida? I plan to develop the forces using screws but glue would be a good way for me to CYA...
RE: Existing Wood Trusses are Failing...
BA
RE: Existing Wood Trusses are Failing...
I have seen this on current truss shop drawings...
Technically it is a has resistance at both end due to the coff. of friction on the truss bearing of the CMU and also the tie-down in shear... that's why i did it both ways...
Thanks, Joe
RE: Existing Wood Trusses are Failing...
http://ww
Can be applied in temperatures up to 100 deg F. with a maximum service temperature of 140 deg F. That should be high enough for a ventilated attic.
You will have a large area of contact - properly applied the glue will be better than screws. Will also help to compensate if the existing members are of inferior wood.
www.SlideRuleEra.net![[idea] idea](https://www.tipmaster.com/images/idea.gif)
www.VacuumTubeEra.net
RE: Existing Wood Trusses are Failing...
I ran pinned-pinned and pinned-roller... and compared the results...
Then after the discussion I just tried roller-pinned... the stresses change entirely for the entire model... other members fail that never before did...
very odd... any thoughts... in the dozens of truss shop drawings I have review I have never seen a roller pin always a pin-roller...
RE: Existing Wood Trusses are Failing...
BA
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
The way he show it, it is not unstable. It is redundant because he has two roller supports each end which is equivalent to pinned each end...redundant by one.
BA
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
BA
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
It just occurred to me that if you are applying horizontal loads then you will get different loads in some members. What I said earlier was correct for gravity loads.
For example, if you apply a horizontal load to the left end of the bottom chord and that node is pinned, no force will go into the chord. If the R.E. is pinned, the same force will produce a compression throughout the Bottom Chord.
BA
RE: Existing Wood Trusses are Failing...
As mentioned by BAretired, you should get the same results from pin-roller and roller-pin.
The horizontal deflections/reactions come from the wind loads. If you are only running one load case (wind from the right for example) then you will see different results in the pin-roller roller-pin models. You need to run multiple load cases (wind from the right, wind from the left, and wind parrallel to the ridge.
Also, as mentioned earlier, you will get the most important stress reversals for the .6D+W load case.
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
BA
RE: Existing Wood Trusses are Failing...
If you have confidence in your program, trust the results it provides. It's not worth the time & efforts to solve by hand for a statically indeterminate truss, unless you want to refresh the memory of those good, old colledge days. :)
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
No need to relive the good ol college days... too much beer involved ;)
Thanks everyone for the great posts and take care!
RE: Existing Wood Trusses are Failing...
If the horizontal resistance is deemed to come from the roof diaphragm, then the truss could be modeled with a series of unit loads, one at each node, each parallel to the top chord. You would then apply just enough of that load to eliminate the horizontal reaction at the pinned end and combine forces for the final result.
I don't know of anyone who would go to such lengths, but perhaps there are some.
BA
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
BA and Mike really hung in there on this one. Virtual beer for both!
RE: Existing Wood Trusses are Failing...
Should we just trust the maufacture or can the bond be calculated similar to shear flow? If shear flow where can the shear strength of the adhesive be found?
I will most likely develop the force in shear thru the screws and use the glue as addition F.S.
Virtual Beer for everyone!
RE: Existing Wood Trusses are Failing...
I think taking all the load in screws is the right approach. The temperature in the attic space can exceed 140F in Florida. Liquid Nails does degrade with time and temperature. The manufacturer says the expected life is 20 years. I think you need the solvent based glue, not the water based one.
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
BA
RE: Existing Wood Trusses are Failing...
Scott Shields
Ghafari Associates, LLC
RE: Existing Wood Trusses are Failing...
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,
RE: Existing Wood Trusses are Failing...
Good luck...now go watch the mermaids after a few more beers.
RE: Existing Wood Trusses are Failing...
If you want to post a sketch of the truss profile with dimensions I will run it for you with Mitek software with all the load cases used today.
You mentioned the roof once had clay tiles. It would be interesting to know when those clay tiles were removed...
BTW... from your analysis, why do you think the web members failed?
MDJ
www.windspeedbyzip.com
www.groundsnowbyzip.com
RE: Existing Wood Trusses are Failing...
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?
RE: Existing Wood Trusses are Failing...
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
RE: Existing Wood Trusses are Failing...
Please see http://www.windspeedbyzip.com/MI2000-MARK%20J.pdf
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
www.windspeedbyzip.com
www.groundsnowbyzip.com
RE: Existing Wood Trusses are Failing...
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
RE: Existing Wood Trusses are Failing...
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
RE: Existing Wood Trusses are Failing...
Mike McCann
MMC Engineering
RE: Existing Wood Trusses are Failing...
As you can see with my previous post I have had similar experience.
RE: Existing Wood Trusses are Failing...
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.
RE: Existing Wood Trusses are Failing...
Why is that?
MDJ
www.windspeedbyzip.com
www.groundsnowbyzip.com
RE: Existing Wood Trusses are Failing...
RE: Existing Wood Trusses are Failing...
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?