The Mystery of the Gap

I would have expected the windows to break before a concrete wall moves!

The contractors may be right! Time to call a specialist on foundations and basements rather than guess.

Hello AH,

Actually one window did break. It was a small window, maybe three feet square with a semi-circular top. It was located on the rear wall directly below the peak of the roof adjacent to several scissor trusses. I assumed that it broke as a result of roof deflection, but you may have a point. Perhaps internal pressure had something to do with it. The window panels cracked but did not fall out.

A foundation specialist may be a good idea, but the homeowner is on a tight budget and does not feel she can afford more experts. She continues to live under a roof which failed four years ago. She would like the original contractor to take responsibility for his work and repair the damage. It does not appear that this will happen. To date, the roof has not even been shored. It could fail at any time in the event of a significant snowfall.

Hello ba/ael

The gap could have been caused by floor slab shrinkage, and not noticed before the bang in the night.

The basement walls: Are they-

1. Reinforced Concrete
2. Concrete Block
3. Timber
4. Other - Please specify

If there is a gap between the floor slab and the wall, then if the problem has not been caused by slab shrinkage, the wall has moved.

In the case of (1) and (2) above, if the wall has moved, then the slab has been poured without proper reinforcing ties through both the slab into the foundation - thus the original work is defective.

In the case of (3) above, it needs to be established where the gap is:

4. Between the Wall and the slab

5. Between the slab and the foundation

If (4) above, then the foundation to wall tie rods have sheared off, or were not correctly placed to prevent movement.

if (5) above, then the slab has been poured without proper reinforcing ties through both the slab into the foundation - thus the original work is defective.

I think that the most probable situation is the house was built, and the basement slab later poured, without reinforcing ties poured into both the slab and the foundation.

This cheating is commonly done, and has been the cause of such problems.

I would carefully check the verticality of the house walls, as it is possible the foundation creep, if that has happened, may have indeed spread the scissor truss beyond its design stress, causing the failure of the truss at centre top joint.

To establish the cause is going to take a day or two, to fully inspect and write a report.

Are the occupiers the owners, and are they the original occupiers/owners?

If so, dependent on the age of the structure, and if the respective Contractors are still in business in the area or State, it may be possible to claim recompense from them.

I don't know if these folks are in a seismic (earthquake) zone, it may be that a mini-quake, or local ground movement has shifted the foundation of the wall, but if the slab and foundation were properly tied together, the whole structure would have moved, as a unit, no separation should have occurred.

We have seen similar happenings here, generally because of inferior construction (slab not tied to foundation = quite common), or local ground movement, or earthquake.

If inferior construction, it may be possible to claim from Contractors, but expensive to make the Lawsuit.

If local ground movement, possible claim under insurance policy.

If earthquake, possible claim under insurance policy.

We do not use scissor type trusses here, perhaps because they are not as strong as what we use, but all New Zealand is young geologically, local earth deformation occurs frequently, and all is in a seismic zone too.

That's my initial thoughts.

Kind Regards....

Hi SparkY,

Floor slab shrinkage is always a factor. It is common to find a small gap between wall and floor slab, but I have never been able to get my hand through the opening before.

The walls are concrete. Whether or not they are reinforced, I cannot say, but the usual practice in this area is to provide two 15M horizontal bars top and bottom...usually no vertical bars.

If the gap was caused by shrinkage, I would expect to see a gap at the corner. I did not think to check this at the time, but my guess is that there is no gap at the corner. Can be checked.

Standard residential practice in this area is to pour free floating slabs which are not tied to the wall or the foundation. This is not considered defective in this locale. It is certainly not considered cheating as it is the usual recommendation of our geotechnical engineers.

The house walls are likely not vertical, although I have not checked that. But when a scissor truss deflects, it necessarily deflects the top of wall outward.

The occupier is a lady who purchased the house approximately fifteen years ago. The house had been used as a display home and she did not get an inspection before purchasing. There are obvious flaws in the framing in the basement. For example, the teleposts slope west and the studs slope east. But I don't think that has anything to do with the current situation and I presume it was in evidence when she purchased the property.

The original contractor is a well known house builder who is very much still in business. The lady purchased new home warranty insurance which would have expired by now but for the fact that the issue has been in arbitration for the past two or three years. To date, no action has been taken to remedy the problem. Meanwhile, the lady is living in a risky building with a roof which could collapse at any time under a sudden heavy snowfall.

This area is in a minimal seismic zone. We are in the heart of the prairies. Our seismic values taken from the Alberta Building Code are 0, 1 and 0.05 for Z_a, Z_v and Zonal Velocity Ratio respectively.

I am not partial to scissor trusses either, but they seem to be favored by some of the local architects.

Thanks SparkY for your thoughts. I can assure you they are highly respected.

Hello again ba/ael

I've just had a further thought.

Edmonton is the Capital of Canada's Oil and Gas industry.

It may be possible that oil and/or gas extraction in a nearby zone, has caused local slumping.

Have a look at here: http://www.oilweek.com/maps_charts08.pdf

See if there is any local correlation between the house and oil/gas field underground.

It is a probable cause of ground movement.

We have had that here, from Gold Mines, Coal Mines, aggregate quarries, and other underground extractions, causing surface slumping/movement a mile away or more, from the vertical through the edge of the works below.

Such ground movement has generally been hotly denied by the mine or extraction company, at the request of their Lawyers and Insurers, but the proof is there, cracked walls, cracked brickwork, twisted and torm wallpaper, cracked reinforced concrete and so on.

Sometimes the local surface movement may take several years, after the extraction, before the surface slump or movement shows.

Kind Regards....

You are absolutely correct. Edmonton is the capital of Canada's Oil and Gas industry. I sometimes wish our government would slow down the pace of the oil sands development. But let us be realistic. If that had been the cause of this lady's problems, wouldn't the neighbors have suffered a similar fate? The fact is, they did not.

Four years ago, she experienced what she thought was an earthquake. Next day, she learned from the weather office that was not so. She spoke with her neighbors and found they had no such experience.

Perhaps it sounds absurd to suggest that the foundation walls moved as a result of pressure from the collapsing roof. But let's look at the facts. An eight foot high wall eight inches thick weighs 800 pounds per lineal foot (plf). Add to that another 200 plf for the stud wall and floor. So that is 1000 plf. If the wall is not doweled to the footing and the coefficient of friction between wall and footing is 0.5 (just a guess) then we would require a horizontal reaction of 500 plf to move the wall from its stationary position on the footing. A pressure of 125 pounds per square foot (psf) is required to move the wall.

Now, the question I am unable to answer is...could such a pressure develop from the roof falling two, maybe three inches before it landed on a partition. I suspect the answer is yes. Perhaps I need Fyz to provide some assistance on this one.

I don't think it's been brought up yet, but if so, please disregard this post. Was there a heavy snow load on the roof at the time of the collapse? I realize that local builders and architechs should have considered snow load when specifying truss design but this doesn't always occur.

Just a thought.

-John

Hi John,

I assumed incorrectly that the incident had happened under snow load but it did not. It happened on a very hot day in summer. Some windows were open for ventilation.

The gap between foundation wall and basement slab is at the front of the house. An unheated garage lies beyond that wall. The scissor trusses are covering the rear (approximately half) of the house. The front half of the house is framed with conventional wood trusses with horizontal bottom chords.

Hello again ba/ael

Quoted from your original Post: <"Four years ago an Edmonton lady and her son heard a loud noise in the middle of the night. She thought there had been an earthquake but none of her neighbors had felt anything.">

Quoted from your above Post: <"I assumed incorrectly that the incident had happened under snow load but it did not. It happened on a very hot day in summer. Some windows were open for ventilation.">

There appears to be a major discrepancy because "the middle of the night" is not the same as "a hot day in summer".

If both statements are true, the only conclusion is that the loud noise occurred during a Solar Eclipse.

Meanwhile:

What type of roof covering is above the damaged truss:

1. Corrugated iron - Long-run galvanised/zincalume steel
   
2. Metal tray - long-run galvanised/zincalume steel
3. Corrugated aluminium - Long-run
   
4. Metal tray - long-run aluminium
   
5. Tiles metal sheet
   
6. Tiles Concrete
   
7. Tiles other - please specify
   
8. Rubber Membrane
   
9. Other, please specify

As you say the problem occurred on a very hot day, the gap may have nothing to do with the truss failure.

The problem may well have been caused by temperature expansion in the ceiling space, with consequent release/failure of the connective clamped spike plate/s.

A few photos of the failed truss joint/s, placed here at CR4, would help with analysis.

A) Has only the single truss failed ?

B) Are other trusses showing signs of failure ?

C) If yes to (B), are the failure signs at the same connective point of the truss/es ?

I appreciate this would require another site visit for you, but further information may result in establishment of the cause, as you appreciate.

Kind Regards....

You said "The engineer for the insurance company has proposed a repair method but it has been rejected by the original engineer and by the homeowner. Arbitration proceedings have been underway for at least two years with no end in sight."

I would also like to know what the insurance adjuster proposed for a solution. From the way you describe it, it's not a viable solution. Surely, with all parties involved, there is something that each of them could agree on, at least from a structural integrity standpoint. Now, costwise, to some of the parties, well... that's a different story.

Where do you come down with the parties involved (just as an oversear, or do you have a credible interest in, or siding with, one of the parties involved)?

Now, back to that hot summer day in Edmonton with an attractive lady.... (what's a fella to do?).

-John

The solution proposed by the insurer's engineer involves fixing the existing trusses with vertical tie rods each side of the king post and repairing other truss plates which have warped away from the wood members as a result of truss deformations.

The original engineer (and by that I mean the first engineer she retained to report on the problem four years ago) feels that it is not a proper solution and that the trusses should be replaced with new trusses.

The homeowner is convinced that the whole problem stems from foundation movement and will not let anyone in to fix the roof unless they also fix the foundation.

I do not believe the foundation movement, if in fact there is any, has anything to do with the truss failure. Notwithstanding, I do not understand the gap between the foundation wall and basement floor slab and was hoping CR4 could shed a little light on the subject.

The lady wanted to retain me to provide an engineering report, but when I read the first engineer's report, I felt he had pretty much nailed the reason for the roof failure so I declined the assignment.

But the gap continues to haunt me.

Hello again Sparkstation,

I do not know what time it was. Initially I thought she had said it was at night. Today she simply said she was taking a shower. Her son was working on his studies. She put on a robe and they both came out to see what the disturbance had been. They thought it had been an earthquake, but that proved to be wrong.

I believe the roof deck is covered with asphalt tiles.

I don't have photos, but will try to get them. I have seen photos of the failed trusses but did not actually climb up into the truss space myself. I would like temporary bracing installed before I do.

I believe that six adjacent trusses have failed.

Thanks for your input.

As a concrete finisher, is it possible the floor was poured at a very high slump, and the gap is shrinkage for that reason?

The Architect , the structure designer,the fabricator need to learn and pass tests on Arch/Hingepoit/bowstring/L/R ratios

Hello again ba/ael

A further thought re this situation.

Is the ball bulged at the centre of the length ?

If the movement was caused by the sudden increase in air pressure, from the dropped truss and a consequent shock wave, the wall should be arched or bowed, in a longitudinal direction, with the greatest gap now in the centre of the wall length.

If the gap was caused by shrinkage of the slab, then the gap distance should be approximately even, all along the wall length.

Kind Regards....

good day sparky and all

i have to keep tghis one short so here goes.

out in sask the practice of not bringing t the inside basemwents finished wall studs right to the floor ieies in with your and other ideas. the reason for it is the strange ability of the earth to expand and contract so much that as they heave in the earth they can lift the rim joists plates right off the wall and havin not rebar to tie the poures or blocks to the foundation there is no place for them to go but up or sideways into the wall of the iside of the basement.. given the amount of displaced earth that is the provinces choice of revenue funding the investment in a hydrogeologists report might be the place to start then put out the word a class action is being launched over this problem. the roof joists cant do nything but buckle in the weakest part of their bow that is why there is only one window down out of the lot, they are in effect supporting the walls another freeze thaw or seismic bump will probably bring the rest of the deck on to the table.'da ber

Hello Sparkstation,

When I was at the site, I did not think to check whether the gap extended to the corner because it had not occurred to me that the movement might have been caused by internal pressure resulting from the roof collapse. Today, I spoke with the original engineer who intends to visit the site again. He will check on that very important aspect.

Thanks for your comments.

Hi friend,

Before I give any comment, I have one question for you, Do you use pumped well water at your house or may be your close neighbor's houses?

If the answer is YES, then that's the cause. This is due to "soil consolidation". Because the ground water is pumped out from the ground, then the soil stress is increase significantly beyond it's allowable stress to bear the building load and the soil mass become "compacted" and this cause your basement slab is settled down and leave a gap to the wall.

Hope this can explain your question.

Best Regards,

Seno (Civil Engineer)

Seno, I'd have to disagree, as most waterwels are artesian wells that go into the aquifer in the bedrock. The soil should not be affected in any way.

If they are pumping water from the soil, this is not a proper well and should be abandoned as soon as practical.

Hello Fierce Allegiance

Perhaps Bedrock in your location is different.

Bedrock is normally solid, with perhaps a few fractures, but contains very little freely accessible water which could be pumped.

Most pumpable water wells use water contained in an Aquifer composed of sand, gravel (Shingle) or other strata capable of allowing water to permeate freely in the stygian depths below.

As far as pumping water from the soil, I agree with you, because that water would not be regarded as safe to drink, without removing protozoans, bacteria and other nasty organisms.

Kind Regards....

In any case, the water comes from a public utility through water mains to each and every residence in the neighborhood. There are no water wells on the property or nearby. There is an artificial lake not far away but I can't see how that is particularly significant.

Another complaint the lady had was that she found two salamanders five or six inches long in her basement one day (not sure when). That would suggest they found access through the concrete walls (not sure where).

Hi ba/ael,

Going back to your original post, in part "I was able to put my fingers down this gap and feel the sand below the slab."

Was this house built on sand, pray tell? If so, therein may lie your/her problem(s).

-John

The soil in Alberta is predominantly clay. Local practice in house building is to place four to six inches of sand under slabs on grade. I believe the sand that I felt through the gap was likely placed by the contractor prior to pouring the basement slab.

In response to Sparkstation's question about time, I spoke to the homeowner today. She tells me that the incident took place about 12:00 noon on a hot summer day (possibly July or August of 2004). I had previously thought it was about midnight. I also assumed it was winter, or at least a period where snow would have been present on the roof. Just goes to show you...don't assume anything.

I have offered to visit the site with the original engineer to take a more comprehensive look at the house. If that visit should occur, I will be happy to report back to CR4. In the meantime, there doesn't seem to be much purpose in continuing this dialogue because we do not know all of the facts.

Thanks for your contributions so far.

No, the slumping of the roof will not cause a wall to move. The pressure energy in the air due to the slight change in volume of the internal space will not allow for the displacement of the soil to the outside of the house at foundation level nor will it allow for the displacement of a wall. Secondly, if the walls were moved by this pressure, then surely all the windows of the house would have been blown out? The one window that has broken, because it has stayed in place although cracked, generally indicates ground movement.

Check the alignment of the walls as already suggested as this will indicate how far the walls have moved.

Shrinkage of the order of 10mm is not the order of magnitude that is normally expected for concrete. This one should also be ruled out as a cause.

Fortunately, the scissor truss will still stay in place under snow loading as the king post is a tie for wind load. Unfortunately, the top members are now a little bit pushed upwards and outwards with respect to each other and the top horizontal tie is working more in tension. Not a good state of affairs. The timber will be quite forgiving in terms of redistribution of load but the connections won't. Hence, any tension ties will the new weak points.

Living in a cold climate such as Edmonton, is it possible that the internal wall has been pushed out at foundation level by frost? Perhaps an infiltration of water followed by a frost? What time of year did the original incident occur? Frost is one of those almost unstoppable forces (at least at the scale of housing).

I spoke with the lady a few minutes ago. She tells me that the incident occurred on a very hot day in the summer of 2004. I had assumed that the roof was likely loaded with snow, but that is not the case. It certainly strains the credibility of my earlier theory. Some of the windows, apparently, were open.

The king post is a necessary element for gravity load. Snow load produces tension in the bottom chord and since the bottom chord is not horizontal, a tensile force is required from the king post for equilibrium of that joint. The original engineering report indicates a calculated tension of 1730 pounds at maximum design load.

Temporary bracing should be installed under the high point of the bottom chords to protect the occupant from excessive snow load. To date, this has not been done.

What actually is the real problem??. The gap itself or just an explanation? Or is it the roof collapsing??

If the truss members pulled apart too much you might want to close the gap by jacking up the roof at the centre point - or connecting a horizontal winch cable to each truss at the wall plate and winding in the ends until the gap closes - then bolt or weld the gussets in place when everything is lined up.

If everything was fine until the 'noise' and Ok since, then it seems you only need to install some strong gussets to reinforce the truss (more than one??) at the point it failed to prevent further settling.

If the building has not collapsed after 4 years there is nothing much wrong.

What actually is the real problem??. The gap itself or just an explanation? Or is it the roof collapsing??

The problem is that the lady has been convinced that there is more to this than just a roof failure. She has been convinced by parties unknown that the whole issue was caused by foundation movement. The only foundation movement I could find was the gap described above and, at the time I did not believe it warranted remedial action.

The engineer for the insurance company has proposed a repair method but it has been rejected by the original engineer and by the homeowner. Arbitration proceedings have been underway for at least two years with no end in sight.

My immediate concern is that the roof be temporarily braced down to the Main Floor for protection of the homeowner. Then the parties can take as long as they wish to come to agreement.

Beyond that, I would dearly love to find a credible explanation.

If I recall the loads for a vaulted ceiling correctly the weight of the ceiling and roof will put a lateral load on the walls along with the vertical load. If a truss failed then the lateral load might have been raised to the point where the foundation was pushed out a bit? I saw this happen in a church I attended as a kid.

It's tough to tell without measurements but it sounds like the walls are going to have to be pulled in somehow. It might not be possible to repair either.

I think you need to get a good architect and / or structural engineer involved. This job may require a lot of "deconstruction" before the repairs can begin.

BA/AEL,

I confess I am having difficulty in trying to visualize the deflections of the truss members that caused the separation of the center vertical from the main, uppermost diagonals. The only thing I can think of is that these diagonals deflected in bending (excessive snow load ?) and forced down the two short vertical members at the centers of the diagonals. In turn, that may have forced down the lower diagonals. But this implies that all the joints allowed rotation or slippage, especially the lowest center joint, thus adding to the list of repairs necessary.

The basement floor-wall gap is the least of the worries, especially if there is no noticeable wall tilt. Any of the commercially available, flexible sealants will do to close the joints to prevent occupancy by insects, or ground water seepage.

Devising some sort of jacking arrangement to restore the original truss configuration will be the main problem if there are no central columns against which to place a jack.

If the attic area is clear, perhaps a horizontal cable tie with a turnbuckle to apply inward tension to the two outermost lower joints of the truss might work to pull the outer ends of the two main diagonals together, thereby lifting the center joint of the truss. The basement wall-floor joint was probably opened long before the truss problem occurred. Molefex

Molefex,

Sorry I don't have a photograph to show you but here is a sketch of the joint in question. You can see that the truss plates have been misplaced so that they don't lap sufficiently with the king post.

The bite was found to be 1.5" at the centre and about 1.1" at the edge of plate. The king post has separated about 3/4" from the underside of top chord and you can see striations where the teeth of the truss plate slipped. The truss plates have curled slightly away from the wood.

Under snow load, the king post is in pure tension and the failure is a tensile failure. Some of the trusses landed on partitions between Main Floor and roof. A china cabinet fell over and several items were broken. These events are probably what felt and sounded like an earthquake.

According to the engineer who wrote the report, a number of other joints are damaged as a result of truss deformations. That is why he wants complete replacement of the trusses rather than a repair of the existing trusses.

I completely agree with you that the floor-wall gap is the least of the worries. But the lady firmly believes that a foundation wall remote from the collapsed trusses is somehow responsible for the failure. She has not allowed anyone in to repair the roof unless they agree to repair the foundation as well.

I did not see evidence of foundation movement other the the gap to which I have referred. I cannot account for that movement other than by the sudden increase in air pressure resulting from the falling roof, but I admit it sounds a bit far fetched, particularly as it happened in summer without snow load. The dead load of the roof is probably in the order of 12 pounds per square foot.

The engineer who did the original report seems to be making reasonable conclusions i.e. change the trusses. The main rafters (the top members of the truss) are probably working in bending now which is not what they were designed for (apart from between nodes).

I think that the home owner can change the trusses as originally suggested but the form of the truss should be such that it is less susceptible to foundation movement. An A-frame could be better with stronger rafters. It could be done tastefully so that she has better attic space too.

ba/ael,

Decipher the diction of post #14 and you will have the cause and the scissor truss is inappropriate to it's environment.

Replacing the truss system due environmental factors is the correct answer.

I visited the residence a couple of days ago to get a better look at the slab to wall junction. The mystery of the gap is no longer a mystery.

The picture at the right shows the junction of the foundation wall and slab. A stud wall is seen about 1/2" clear of the inside concrete face of wall. I checked in several places and found that the gap was uneven below the bottom plate of the wall. In looking at the concrete finish, I concluded that the slab must have been poured after the stud wall was built.

Concrete partially filled the gap below the plate but not entirely. When I extended my fingers below the plate, the width of void was variable. The slab does not end in a formed edge but in a random contour, just the way the concrete flowed when it was placed.

Here is another shot in a different location. The black surface is the inside face of wall.

Normal practice is to pour the slab first, using the exterior wall as a form for the edge of slab. In this case, for whatever reason, that was not done.

So far as can be seen, there is no evidence of foundation movement, contrary to what the owner has been told by several foundation contractors. She does not believe me, however and was quite hostile when this was suggested.

I would like to show some pictures of the roof structure, but I will do this in a separate post as I have not always been completely successful in transferring pictures into the CR4 editor.

Hello ba/ael

It is evident that the house was not built in the correct manner, with the slab and foundation walls being a single poured and reinforced unit.

Perhaps it may have been done during the Winter, with the slab poured once the building was closed in.

That way, the foundation wall could have been easily covered, to prevent frost damage, whilst the slab, after pouring, would have protection from the elements by walls and roof.

Kind Regards....

The residence in question has scissor trusses in the rear half and conventional trusses in the front half. Here are a few shots of the scissor trusses. See the original post for the shape of a typical scissor truss.

Hello again, ba/ael

It is now easy to see what caused the truss/es failure.

The nail plate is not large enough for the job.

Evidently the incorrect nail plate was used, which is immediately obvious from your excellent photos.

The Truss makers are responsible for the failure, because of faulty assembly, but the home owner does not have a contract with the truss designers, just the building contractor.

However, if that Truss Company is honourable, they may be persuaded to contribute towards replacement costs.

Kind Regards....

Hello Sparkstation,

I agree with you completely. Four years ago, the truss company accepted responsibility and agreed to repair the faulty trusses. The insurance company retained an engineer to come up with an appropriate repair. His proposal was to add steel tension rods, one on each side of each truss at the king post, thereby providing the necessary tensile force to replace the inadequately connected king post.

The engineer representing the homeowner felt that the repair was not acceptable and wanted replacement of the faulty trusses. This, of course would add considerably to the cost of remedial measures.

Both parties agreed to arbitration and I believe some preliminary meetings have taken place, but to date, no arbitration hearing has been held. One of the problems was that the lady who owns the home called several contractors to give her a price on fixing the roof and when they arrived, they apparently told her (and I was not present during these conversations) that there was no point fixing the roof unless the foundation was underpinned (at a cost of about $150,000 according to her). One of them reportedly said that the house was beyond repair and should be bulldozed to the ground.

In my opinion, these contractors are recommending expensive underpinning measures which are completely unnecessary.

I spoke with a representative of Alberta New Home Warranty and he advised me that they had tried to gain access to her home to fix the roof but the lady would not allow anyone in to carry out roof repairs. When I questioned her about this, she responded that "fixing the roof won't do any good unless they fix the foundation too". I could not persuade her that there was nothing wrong with the foundation.

When I visited her house last Thursday, I took particular note of the slab on grade in the basement. Her engineer was present. I suggested that the finish on the concrete slab in the basement indicated that the slab was poured after the stud wall was built. She and her sister (who was present through all of this) became extremely agitated at that suggestion. This led to confrontation and nothing useful followed.

My concern is that the roof is in danger of collapse under heavy snow load. If the insurer would agree to provide temporary shoring, they could argue the case for as long as they want. But for now, and for the last several years, the owner has been exposed to a danger which is not acceptable by any reasonable standard. The fact that she does not realize this is not helpful.

I may have to notify the city housing inspection department if something is not done very soon. I do not want to wake up one morning and read the headline "lady killed in roof collapse".

ba/ael,

I found your post and your failure of the truss web connection to the top chord is very similar to the one I have been retained to investigate. Please see the attached photos. The top connection of the web vertical at the crown of the truss (at the top chord, at the center of the span) pull out of timber material and the plate of the connection appeared to have been pried out. The bottom connection plate of the same vertical web member at the bottom chord showed no signs of distress (this is at the center of the span). It appears that the required design loads for the trusses were not very conservative (the design loads were either low or had very light excess over what was actually going to be applied to the roof truss). It appears that more dead load was applied to the truss than what the truss was designed for. The connection in your situation appears to have very small lap with the vertical web member. The condition that I have more lap, but the span of the truss was approximately 48-feet and the DL = 25 psf or more, therefore the forces were great and thus the lap may have been larger than your but not enough. What are your thoughts on my truss? What was your final conclusion on the cause of the failure of your truss? Did you find any documentation on the required lap of the connection plate, the allowable loads on the connection plate, the allowable loads on the timber members, and does the TPI address this in the design code that TPI publishes?

Blessings,

Jim

In my case, I concluded the truss plates had insufficient lap with the vertical member and substantially less than specified by the fabricator, so it could be said to be a fabricator's error. The gap between the basement slab and the foundation wall was filled as part of the remedial work, but I was of the opinion that it was unrelated to the roof failure.

I have seen two other failures similar to yours. In both cases, the humidity was very high in the truss space.

In one case, a pig barn roof, the farmer had suspended a feeder trough from the middle of each truss at the bottom chord, a load which they were not designed to carry. That plus a heavy snow load brought the entire roof down. Grooves from the truss plates could be seen on each face of the top of the king post.

In the other case, the roof was covering a greenhouse attached to a residence. The humidity was kept constantly high and the roof collapsed under a heavy snow load, but not greater than design snow load for the locale. I suspected that the humidity was at least partially instrumental in both failures.

Thank you.

Jim