Force of Falling Tree on House

[mferg318]

I'm inspecting the foundation of a home that was damaged by (2) oak trees that fell on it during a hurricane last year. The roof and walls were significantly damaged. I am trying to determine if any of the cracking and settlement of the slab are a direct result of the falling trees. The insurance company has agreed to pay for all damage except for the foundation, because they claim the foundation issues pre-existed the storm damage. One idea I have is to calculate the force of the falling trees and determine if that force is indeed large enough to induce shear failure in a slab-on-grade. Anyone have experience with this type of situation or with this particular calculation?

 

[skeletron]

I would do the calculation the other way: find the max point load that can be resisted by the SOG and then back calculate to find the max weight of a falling object, then assess. But I think that is more of a physics homework problem. The tree weight would be pure guesswork.

 

[mferg318]

Most of the tree is still laying on the ground, so I can fairly accurately calculate the weight. But you are right, it is a physics problem, which is why I'm struggling with it...not the type of thing we usually deal with.

 

[phamENG]

Is the tree service using a crane? If so, you may be able to get a weight from the operator. Some rigs have built in load cells to prevent the operator from violating the weight/reach charts.

Unless the wall in question was sitting on a 4" slab, I doubt the tree caused the damage. Is this a wood stud wall? How many studs were likely engaged in resisting the impact of the tree? 2 or 3? What force is required to buckle all of the engaged studs? Is it greater or less than the dynamic impact of the tree on the wall? If greater than, the foundation damage is plausible - the studs would remain intact and the energy would be transferred to the foundation. If less than...the wall will buckle and the energy of the impact will be dissipated in the wall. The foundation will only have to deal with the static load of the tree, which probably isn't enough to crack the foundation.

I could be completely wrong. I know nothing of the size of the tree, the house, the foundation, or any of it. I just have my doubts about an average sized tree falling on an average house and breaking the foundation unless the foundation was already broken ahead of time.

 

[LittleInch]

This question is just the vertical version of an impact by a vehicle and about as hard to get an answer as so much depends on how much impact energy is absorbed over a certain distance.

I would go the other way around and try and see if you can estimate what the load/ force would need to be to break whatever framework is broken then use that as a force with an allowance for live load of maybe 2? to allow for the shock loading.

Some pictures would be nice!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WARose]

We've kicked around several different means of calculating impact from a fall before here. One example:

https://www.eng-tips.com/viewthread.cfm?qid=444175

Keep in mind though: in your case, it will take a calculation to get that velocity (outside of this method) because we are talking a rotational inertial problem (which isn't that difficult though).

Also speaking as someone who has done a lot of chainsawing: a lot of times the stump is restraining things to a degree. You cut blowdown off close to the stump and watch the stump flip back into the hole. But that probably doesn't matter to you as you are looking for a maximum here (at least that is what it sounds like).

 

[mferg318]

Good feedback so far. I think it's a good idea, as some have suggested, to do the calculation in reverse and see if the force required is feasible for a falling tree.

Let's say there was some pre-existing damage/cracking in the slab prior to the tree falling...it still seems disingenuous for the insurance company to say that the impact could not at least have exacerbated the foundation issues. I'm still real early on in the inspection, but one corner of the foundation (on the same side as the tree impacts) has settled roughly 4.5 inches. Unless I can find some other issue that would account for this excessive settlement, I don't know how I could dismiss the tree impact as a contributing factor. I understand it will be difficult to prove with hard numbers, but at some point engineering judgement has to factor in.

 

[Rabbit12]

The insurance company likely knows how difficult it is to prove what you are trying to prove thus they deny to cover. The insurance industry is a shady business at best.

They know most people probably can't afford to hire an engineer and lawyer to challenge them so they just bend over.

 

[JedClampett]

That insurance company is full of crap.
[ul]
[li]You have a perfectly good house,[/li]
[li]a tree fell on the house,[/li]
[li]it caused a great deal of damage to said house,[/li]
[li]including foundation damage.[/li]
[/ul]
Prove to me that the damage wasn't caused by the tree. Using that logic, maybe the wall studs were cracked already? Maybe the house was going to fall down in a strong breeze? We shouldn't have to pay anything!
I dealt with FEMA on hurricane damage on an office building. Extensive cracking, water leaks, spalling. They challenged me on the history of the cracks. "How did I know the cracks weren't pre-existing the hurricane?" I told them I had never seen the building before, there was a hurricane, logic tells me it was caused by the hurricane. Luckily a federal election was coming up, so we got the claim.

 

[phamENG]

4.5 inches!? I'd find it very hard to believe that the tree did that. How big is this tree? Unless it's a 1000 year old sequoia, it seems unlikely that it would break the foundation and cause it to settle 4.5". Is it possible? Sure...foundation was cantilevering over a void from a broken pipe or something and the impact was what did it in...but that's the edgiest of edge cases...

Are there any pictures of the house before the tree fell on it? A birthday party in the backyard? Google street view? That'll help you judge timing of the damage.

Don't worry about the insurance company. The insurance company and the homeowner have a contract that delineates what is and isn't covered by their policy. The vast majority of policies have clauses that exempt things that were damaged previously. Whether or not it gets covered is between the insurance company and the owner. Unless you're also an independent insurance adjuster/consultant that actively works to overturn insurance decisions (they exist, I've met a few), you're only concern should be on the plausible engineering explanation of what happened. Give the parties involved the information to make an informed decision.

 

[mferg318]

It is virtually impossible to prove that the damage was or was not there before the tree impact. Foundations are particularly hard to pinpoint the origin of damage. The damage to the rest of the house is intuitive...a tree fell on the roof, now the trusses are broken. But for whatever reason, the burden of proof is on the homeowner for the foundation. I do have (2) older inspection reports from the 90's, and neither report significant settlement, so that is a valuable piece of information.

 

[Tomfh]

I cut a tree down last year. A piece of the trunk about a meter long fell onto our driveway and cracked the slab. It caused a yield line failure and caused a half inch of localised deflection.

It wasn’t a huge piece of wood. Maybe a couple of hundred kg. It fell a meter or two.


As for finding the “force”, It’s not a force problem. It’s an energy problem.



Can you sketch the arrangement? How did the tree focus the load so that the foundation would shear?

 

[mferg318]

Also, I'm not saying that the tree impact definitely caused 4.5 inches of settlement. I'm simply pointing out that the impacts from 2 large oak trees certainly did not help, and it is plausible that the foundation could have continued to perform adequately if not for the storm. Without having seen in before the damage, all I can do is report the possibilities.

 

[phamENG]

mferg318 - can you post some more details? Size of the tree, exactly what foundation element was damaged (was it a slab, a footing, both?), how the load got there (was it just point loaded through a wall?), etc. Any pictures?

It could easily go either way.

 

[dhengr]

Mferg318:
Don’t forget that more than half the total weight of the tree was probably still supported by the stump, while it was falling, the full weight didn’t fall out of thin air. Limbs and leaves are bulky, look big, but they don’t weigh much. They do make for good sails in terms of picking up the toppling wind loads. The immediate tree trunk, across an exterior wall, or a few very large (not limber/flexible), and still mostly intact limbs punching through the roof and into a few walls do most of the damage. Unless you have a large limb punched right through the slab or a slab/found. crack immediately under the resting trunk, you probably have a pretty slim case. This is a real difficult problem to put any meaningful numbers on. As PhamENG suggests this problem requires some serious engineering experience and judgement to even start to point a meaningful finger at the falling tree. As the tree first hits the roof, its weight is spread over a large area and is fairly well distributed, vs. the normal roof design loads, thus, kinda breaking the fall. This weight goes down through several rafters/trusses and walls to the slab. If the cracks are new and emanate from one of these loaded walls there may be a case. What do the walls, plates and studs suggest happened, given their condition? You have to be able to point at a couple studs, a post, or some such, which really imparted the slab loading. Are the cracks new or do they have fl. paint in them, or normal living junk, like bread crumbs, fl. sweepings, dog hair, you name it? Normal, random cracks may have been enlarged, but that’s pretty tough to prove. Do they have before and after pictures which show the found. damage didn’t exist prior to the storm? Of course, if the found. damage is severe enough, they could not have lived with that as a preexisting condition. This has to be the kind of logical, well reasoned story you have to weave to bring the Insur. Co. along.

 

[Ron247]

More than one approach to representing your client.

[ul]
[li]How does the insurance company know the foundation issues predated the storm damage? Even if some cracks look old to them, there is such a thing as a crack was present before the storm but is a lot worse now. I equate that to I slightly scratched my car door does not mean I should not be reimbursed when another motorists scratches it worse.[/li]
[li]How close did the tree come to actually making contact with the slab? If if it did not, was there a fairly stiff T-post, Corner post or King Stud/Liner under it that helped pile-drive the slab. Even if the item broke/buckled, it provided a high axial force before it broke.[/li]
[li]I generally envision the impact as an open umbrella when the limbs hit first and thereby cushion the impact or the baseball bat style where the limbs did not slow anything down. Baseball bat is easier to prove than open umbrella.[/li]
[li]As long as the damaged slab area was ANYWHERE in the load path (start, finish or in between) it is possible it was damaged by the tree. Foundations and slabs are generally at the end of the load path or next to it.[/li]
[li]Most components in a building are not stress free. The slab probably has some tension, shear or bending stresses in it that were not enough to crack it. Even shrinkage stresses are still present in slabs that have cracked. The crack only relieved stress at the crack, not 5 feet away. The tree impact can crack the slab by supplying enough force to finish it off. For that reason, the impact from the tree does not have to be enough to constitute 100% of the break.[/li]
[li]Another view, there is a slight hollow under the slab but not enough yet to crack the foundation and settle. Along comes the evil tree and provides enough impact to finish it off. It is reasonable the area with the hollow would never break and sag without the tree impact AND it is reasonable that if the hollow got worse with time it would have eventually cracked and settled on its own.[/li]
[/ul]

 

[XR250]

That is going to be extremely hard to prove in my experience. I agree with others. Wood structures are really great at distributing impact loads over a large area.

 

[mferg318]

A few more details:

-There was previous inspection done by a firm hired by the insurance company not long after the storm. The report identifies foundation cracks, several of which they call "new" or "fresh" There are some pictures that appear to show cracks with "bright" concrete that are pretty clearly new. I haven't done enough investigation yet to try to trace the cracks back to other structural elements that that were impacted/damaged by the tree.

-4.5 inches of settlement would be pretty hard to live with. It is visible to the naked eye when you walk into the room. Of course, I really only have the homeowners word to go by that it wasn't like this prior to the storm.

-I agree with the general sentiment that it's hard to point a definitive finger at the tree impact as having caused ALL of the foundation damage. But I don't think its unreasonable to conclude that they caused some damage, or worsened previous damage, especially considered that the damage noted is on the same side of the house as the tree impact.

Hard to prove I know, but how much "proof" should be necessary in this situation?

 

[LittleInch]

It might be hard to prove but fascinating non the less.

I think this is boiling down to can you show a direct load path from the main trunk impact point, which is where you're going to get the highest point loads by far, to a stiff column that has then punctured or broken the slab?

If so then you stand a chance under the reasonable cause route. If not then it's just going to be an argument along the lines of well it wasn't showing any signs of collapse before the trees landed on it so....


Your post about the other inspection arrived whilst I was writing this and will aid the case for at least partial cost share.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[phamENG]

You might be surprised what people can live with. I worked on a house with 7 inches of differential settlement. This was a high end, waterfront home owned by a couple who demanded perfection in everything the contractors did. The house was about 80 years old, though, so the owners dismissed the slope as "character".

Now if we're talking about a 4.5" step in the middle of the room where the slab fractured, different story. But details on the situation are still pretty sparse.