Tree falling on beam 1

[rheim]

A client wants to build a porch at the entrance of his house. He asked me to design the post and beam system of the porch using steel members, as he is afraid that one of the nearby trees might fall on the porch during a storm.
Does anyone have an idea how to determine the load resulting from a falling tree?

Thanks for any hints

 

[Robertmet]

What size trees ? A full sized , mature tree such as an oak has a lot more weight than most people think ! For safety and other considerations a tree should be as far from the house as it's height.Last year IIRC in this area we had a house condemned after a tree fell on it and another where an occupant was killed by a falling tree.I would take a very large beam to resist the fall of a large tree.Ask him to think of removing the tree !!

 

[civeng80]

Safer and cheaper to remove it.

 

[msquared48]

I have seen full grown evergreens slice a two story house in two. If he's that worried about them, at least have them topped if he does not want to remove them. Hopefully your client owns the trees so he can modify them?... Good luck.

Mike McCann
MMC Engineering

 

[CarlB]

I could see constructing the "post & beam" portion strong enough, but probably not the railing. Wouldn't have to be steel necessarily. No way to determine the load-depends on tree size, where & how it breaks, distance building to tree, wind speed when it's falling, type of branches (they may "cushion"), etc.

 

[Qshake]

Of course you know that it doesn't stop at the railing....you have to make sure that the load was transfered to the foundation and that means a no collapse for the porch itself.

I think this request is unreasonable and when shown the construction costs (back of envelop) it will be an eyeopener.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[rb1957]

in a battle between a porch and a tree, my money's on the tree everytime ... i don't think there is any reasonably accurate way to predict the load of a tree (unknown hieght, placement, type) falling in a storm (unknown intensity) ... if the tree's going to hit the porch, what'll it do to the house (unless it's an underground bunker) ?

either live with the tree (design the porch to code) or nuke it ...

 

[JStephen]

Seems like I've read in the past that in the design of roll cages on bulldozers and other heavy equipment, the design is done by assuming the cage has to absorb a specified amount of energy, rather than trying to predict all the forces on it. Of course, that's partly empirical, too.

Would it be acceptable to make the porch as strong as the rest of the house?

It's also not uncommon for big trees to drop pretty good sized brances onto a house, and that might be his concern rather than the whole tree falling.

 

[COEngineeer]

sure you might design in to take load from the tree, but I dont think the client has the money to design the porch to take lateral load from the tree . We are talking about all steel frame and beefy moment frame to resist the load of this falling tree. I say... forget it.

Never, but never question engineer's judgement

 

[Robertmet]

In this area ,Catskill Mtn/Pocono Mtn, a typical mature harvestable oak tree is about 60' high and weighs at least 3000 lbs !Add inertia to that and get out of the way !

 

[Lion06]

That would be a funny picture. The house gets crushed by the tree while the porch is still standing. What kind of thought process is that?!

 

[cvg]

3,000 lbs is on the light side. I would estimate nearly double that (just for the trunk)and more if you include all the branches.

 

[par060]

estimate the time of deceleration as the tree hits the structure ....like .001 sec...from there you can detemine a force.....its probably going to be much higher than you can hande with a porch structure

 

[msquared48]

I once had the opportunity to see a 24" (minimum) Douglas Fir conme down where we normally parked our car. That tree was 100 feet tall when we bucked it up the next two days. Using the volume of wood portion of the tree at about 105 cubic feet, and the density at 35#/ft3, the weight without the branches was 3700 pounds. Add the branches at the top 20% of the length, you are probably pushing 5 to 6000 pounds. Sometimes they come down slowly, sometimes not. It all depends on what's around them and the root system resistance relating to the wind direction.

Top the things. It's cheaper and quicker.

Mike McCann
MMC Engineering

 

[dgkhan]

How about stabilizing tree? Place two or three steel rings around stem at 1/4, 1/2, 3/4 height. Attach Cables to the ring and other end provide small footing... Like Guyed tower antennas.

 

[MiketheEngineer]

Save money on the porch design and build

Cut the tree

Buy a new tree and put away from the house

BTW - a number of people around here - 3 or 4 in the past year have been killed by falling branches -- not even the whole tree!!