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Existing Wood Trusses are Failing...
3

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?
 

RE: Existing Wood Trusses are Failing...

Are you modelling the trusses as purely pinned?  They aren't... Timber truss plates do transfer minor moment loads, which can have a significant effect on the load patterns...

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...

StructuralJoe....those trusses were probably locally produced (Lakeland, Tampa, Brooksville, Dade City, etc.) so you can probably find the original truss drawings, even if the building department doesn't have them.  Many of the larger lumber companies have small truss manufacturing operations tied to them.

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...

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

Can you post a sketch of the truss, identify the loading mechanism, the failed and the survived members?
 

RE: Existing Wood Trusses are Failing...

If my memory serves me correctly, I believe that the Northwest received some bad lumber from Canada around that time.  I don't recall the particulars, but there was a big flap about it.  You might look into the grades provided.

Mike McCann
MMC Engineering

RE: Existing Wood Trusses are Failing...

Almost every roof truss failure I hear about during heavy wind loading happens because the web bracing was inadequate or not installed.



 

MDJ
www.windspeedbyzip.com
www.groundsnowbyzip.com

RE: Existing Wood Trusses are Failing...

Never ask "why" with wood.  I've seen so many things that should have failed but didn't.

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...

(OP)
Thanks everyone for all of the input,

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...

(OP)
Does anyone know of any procedure to determine the grade or strength of the wood in the truss?

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...

that looks like RamAdvanse

RE: Existing Wood Trusses are Failing...

(OP)
Thanks,

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...

To size the plywood you need to know the member forces.  1)Calculate the number of nails needed in each member based on member forces.  Multiply the nail values by 0.6 at the heels (Hankinson's formula).  
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...

Joe, your colored stress diagram doesn't indicate the same member that the picture shows as failed. The failed member in the picture is the green vertical to the left of the one your note points to. The failed vertical in the picture has a pair of knots in it, small to be sure, but close together, in a compression member. I'm guessing a P-delta failure due to localized warping in the area of the knots. Also check for slippage of the press plates on the top chord of the vertical although I think the "handy man" would have noticed that and fixed it with plywood gussets. I suggest you get on a ladder and closely inspect the top connection of the green and blue web members. Something is moving up there.

Old CA SE

RE: Existing Wood Trusses are Failing...

I have some difficulty to picture your analysis result.
-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...

The most intrigue point is question 2 above. Say there is 1 kip vertical load on the ridge, shouldn't the left vertical member take the entire reaction from the support below? However, your stress diagram shown the stress is close to zero, why?  

RE: Existing Wood Trusses are Failing...

I think kslee is on to something here... Several of your members appear to have "unlike" loads.  Are you sure there isn't anything wrong with your model?

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Existing Wood Trusses are Failing...

Correction: 1/2" CDX is typical not 3/4" as mentioned above.   

RE: Existing Wood Trusses are Failing...

lol... Make that "unlikely" loads in my previous post...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Existing Wood Trusses are Failing...

Assuming Joe has identified the correct member as the one which failed, that diagonal is a compression member in uplift loading.  If the trusses were designed for a tile roof as he has been told, removal of the tiles would have made that member more highly stressed in uplift than in the original design.

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...

Joe...all trusses in your area require a grade stamp on the lumber.  It should be visible.  It is a grade stamp from the Southern Pine Inspection Bureau.  The top and bottom chords are typically required to be at least No. 2 KD (kiln dried), while the diagonals may be No. 3 grade.

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...

StructuralJoe:

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...

You may consider having the local truss plant assist you with designing and repairing the trusses.  They will be able to model the existing truss for you.  Have them build a repair frame (partial truss) that will transfer the loads across the damaged sections.  This is also the preferred approach to truss repairs (using metal plates), especially when there is more than (2) damaged trusses in series.

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...

Unless it serves a special purpose, the direction of the diagonal in the left end panel shall be reversed to provide better stability. Also, at the right end panel, addition of a vertical my help to stiffen the truss. The analysis is straight forward, give it a try.

RE: Existing Wood Trusses are Failing...

Correction: To the left end panel, try "y", or "X".

RE: Existing Wood Trusses are Failing...

As noted by mudflaps, your photograph reveals that the second vertical from the left, shown in green on your elevation appears to be the member which buckled.  That makes sense as it is a compression member under gravity load whereas the member you flagged is a tension member.

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...

StructuralJoe,

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...

hokie66,

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...

StrJoe:

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...

kslee,

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...

The upper halves of the two members which failed clearly are bowed toward each other.  The bow may have been present from the beginning or could have occurred as a result of drying shrinkage.  This would have added eccentricity to these members when loaded and likely contributed to their failure.


BA

RE: Existing Wood Trusses are Failing...

Yes, it is very difficut to detect the 1.35" deflection as compared to the huge displacement of the buckled members, provides there is not much evidence on anticipated deformations (tear, pull, warp) around the bottom chord. Thus, I think it is imperative to set the investigation of load path on higher priority than laterl stability. Though ultimately it might be found that lacking of lateral braces has significant impact.

RE: Existing Wood Trusses are Failing...

Could you tell from your field exam if there could have been either excessive waning or an oversize knot at or near the point of failure in the two chord members?

Mike McCann
MMC Engineering

RE: Existing Wood Trusses are Failing...

Mike,

I assume you mean the two web members?  Right?


BA

RE: Existing Wood Trusses are Failing...

Mike McCann,

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...

Bruce:

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...

Mike,

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...

I have run no calculations, but base my comments on what could be.  

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...

Thanks, Mike for staying in the discussion.  Faulty lumber could be part of the problem, I agree.  But there are other possibilities.

You said...

Quote:

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.

It is not only a design problem.  Here is what the OP said:

Quote:

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

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...

OK.  Misread it.  Probably is a design problem then.

Mike McCann
MMC Engineering

RE: Existing Wood Trusses are Failing...

When I see anomoly's like this it has always been to lack of truss bracing.

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://en.wikipedia.org/wiki/Storm_of_the_Century_(1993)
http://www.tboblogs.com/index.php/news/story/no-name-storm/

MDJ
www.windspeedbyzip.com
www.groundsnowbyzip.com

RE: Existing Wood Trusses are Failing...

I have to admit that when comparing the computer model with the photo, they do not seem to match.

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...

Mike,

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...

(OP)
Group, thank you everyone for all of your input.

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...

Joe,

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...

StructuralJoe,

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...

I agree with BAretired that the most likely cause of the problem is compression members that are too slender. Of course bracing will resolve that, and is certainly the way the truss system should have been originally designed/installed. However... just because bracing "should" have been used does not necessary mean that adding bracing is only way to address the situation as it exists.

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
www.VacuumTubeEra.net r2d2

RE: Existing Wood Trusses are Failing...

StructuralJoe;

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...

(OP)
Mike, correct on the support, that was one of many iterations to determine what is happening... great catch thou

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...

I wondered about the supports too.  At each support there is a horizontal and vertical roller.  That would be the equivalent of a hinge at each end.  Based on that, there should be no stress in the bottom chord at all, so I concluded that I must be misinterpreting the boundary conditions.


BA

RE: Existing Wood Trusses are Failing...

(OP)
I applied a line load of 5psf x 2' = 10plf to the bottom chord to acount for ceiling, MEP etc...

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...

I have specified "Liquid Nails" products (or equal) for attic truss repairs (in South Carolina) - there are several such as this one:
http://www.liquidnails.com/products/product.jsp?productId=36#msdsTech
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
www.VacuumTubeEra.net r2d2

RE: Existing Wood Trusses are Failing...

(OP)
The comment about supports brought up another thought entirely...

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...

Using a pin at the left end of a truss and a roller at the right should give precisely the same member forces throughout as a roller at left end and a pin at right.


BA

RE: Existing Wood Trusses are Failing...

One thing I don't under stand - how could the program let you have Roller-Roller support without flagging (global instability). I wouldn't trust that output.  

RE: Existing Wood Trusses are Failing...

kslee,

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...

I see. Thanks.

RE: Existing Wood Trusses are Failing...

(OP)
Bruce, I thought the same thing at first but because the truss is not symetric the forces do change...

 

RE: Existing Wood Trusses are Failing...

Sorry Joe.  If the forces change then you are doing something wrong.  The only think that should change is the horizontal deflection of each node.


BA

RE: Existing Wood Trusses are Failing...

(OP)
Bruce, it's possible... I will double check my results...

RE: Existing Wood Trusses are Failing...

For gravity load alone, it shouldn't make any difference. If lateral load is involved, the reactions could change.

RE: Existing Wood Trusses are Failing...

Joe,

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...

Joe,

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...

(OP)
This change of failed members occurs in combinations involving wind loads which are perpindicular to the roof surface... therefore yes there are horizontal forces...

RE: Existing Wood Trusses are Failing...

If there are horizontal forces involved, find the horizontal reaction 'H' at the pinned end.  Assume H/2 at each end and modify the forces in the bottom chord by H/2.  I believe that the bottom chord is the only member which should change as a result of switching the pin and roller. Others may wish to comment.


BA

RE: Existing Wood Trusses are Failing...

Internal force changes in both top & bottom chords, since the chord segments at ends have different tilt. Also, for pin-pin, I afraid the horizontal reaction is defferent at each end as well, because the truss is not symmetrical.

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...

I believe you could check this graphically with a Maxwell Diagram, and rather quickly.

Mike McCann
MMC Engineering

RE: Existing Wood Trusses are Failing...

(OP)
The change was minor enough that it isn't a big deal... I have modeled it pin-roller, roller-pin and pin-pin and my fixes still work... so no biggie

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 all joints of the truss are considered to be pinned, then a unit load applied at each end of the bottom chord affects only the bottom chord.  No other member should be affected.  If joints are considered moment connections, then all members will be affected.

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...

Cheers :)

RE: Existing Wood Trusses are Failing...

Joe...agree with SRA...use liquid nails.  I've used it before (I'm in Florida...NE)

BA and Mike really hung in there on this one.  Virtual beer for both!

RE: Existing Wood Trusses are Failing...

(OP)
Ron, I plan to, it is a little confusing, where are the calculations for "glue"?

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...

Joe,

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...

H*&^...  Just rip off the roof and and install new, properly designed trusses... I'm gonna have that beer, or two.  

Mike McCann
MMC Engineering

RE: Existing Wood Trusses are Failing...

Hey Mike, wait for me.  I'm getting thirsty too!


BA

RE: Existing Wood Trusses are Failing...

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
 

RE: Existing Wood Trusses are Failing...

(OP)
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,

RE: Existing Wood Trusses are Failing...

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.

RE: Existing Wood Trusses are Failing...

StructuralJoe,

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...

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?

 

RE: Existing Wood Trusses are Failing...

(OP)
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

RE: Existing Wood Trusses are Failing...

Ok...

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...

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

RE: Existing Wood Trusses are Failing...

(OP)
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

RE: Existing Wood Trusses are Failing...

I guess my beer break was too long.  What happened?

Mike McCann
MMC Engineering

RE: Existing Wood Trusses are Failing...

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.

 

RE: Existing Wood Trusses are Failing...

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.

RE: Existing Wood Trusses are Failing...

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!

RE: Existing Wood Trusses are Failing...

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 http://www.awc.org/HelpOutreach/FAQfiles/In-Grade_testing.html
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?

 

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