question about home renovations

[Lion06]

I am sure many of you watch all of the home improvement shows (either by choice or force). I often see people just adding sliding patio doors or french doors out onto a new deck where there was only exterior wall.
Obviously a header is sized for this application.
The question I have is regarding the lateral resistance of the house. I have never seen any of these shows make any mention of the lateral system. How does one go about determining if the wall you are about to cut a 6'x8' hole in is a shearwall?? I would expect that whoever designed the house would have used every exterior wall of the house (solid walls, and solid panels between walls) as a shearwall in an effort to get the unit shear on each wall as low as possible.
That being said, if you start cutting holes in those shearwalls and making them essentially worthless, it seems to me that someone should be verifying this is acceptable rather than simply sizing a header for vertical loads.
Can someone tell me if I am missing something?

 

[Atomic25]

The answer to your question can be very detailed or simple...I'll go simple and say I have never seen a home's lateral system fail. If a house experiences enough lateral force on it, it will likely fail components before it fails the shear wall. Therefore, IF the house was originally designed with certain shear walls, then it's likely even if you take out an entire wall there will be enough redundancy in the system. I mean, legally you can design shear walls made of drywall, so there are likely some walls playing into the system that arent even being considered.

I would recommend asking this question on the ICC website's bulletin board. Many more code officials cruise that site and would be able to give you a better answer about the verification process. I'd think some locations will require permitting and may actually check this, but surely the majority don't...and I doubt anyone is going to force Mr. Average Joe Voter to go out an hire an engineer to verify if his house will fall down if he installs a patio door.

 

[Atomic25]

Oh yea, and also, another bothersome thing I always see on those home improvement shows is when they construct a 2 story vaulted room by just stacking two sets of stud walls on each other without designing a horizontally spanning header mid height. It just doesnt look like it can work, but it does all over the US

 

[UcfSE]

I don't agree that it is OK not to check the lateral system because there may or may not be some redunancy in the construction. That is what Atomic seems to imply. If not I apologize, but either way that is another thread.

I've often wondered the same about home shows. I suspect they don't show you all the engineer and legal requirements because it's too boring (?), but we the viewers don't know. I hope someone is checking all the structual and geotechincal requirements.

I like that part about "choice or forced". The forced part sounds familiar.

 

[Lion06]

UcfSE-
Thanks for reinforcing my concern. The next question I have is how would one go about checking without knowing how it was designed to begin with? Is it a matter of assuming all exterior walls can act as shearwalls and checking to make sure the remaining capacity is sufficient? I am just thinking that this seems like a monumental undertaking. You would have to verify anchorage of diaphragm to shearwalls, verify fastener pattern on shearwalls, blocked or unblocked shearwall, sheathing thickness, etc.... It seems like it is far more intrusive than is shown on t.v.

 

[gerhardl]

..StructuralEIT

I can assure you that you are not missing anything. Any skilled carpenter, framer or bricklayer (at least within Scandinavia) would ensure the wall was structural sound after such a remake, putting in extra steel or wood beams and supports if necessary.

To do otherwise is against law and rules in Scandinavia. And also in the US you have areas where the extra snowload on the roof (or a stormy day) could give some surprise after the sunny summerday when the TV-Team left....

 

[ctcray]

I would start by looking at the prescriptive requirements for Braced Wall Lines in the IRC and see if the minimums are still being met with the new opening in place. I don't thing the majority of homes in the United States are built with true shearwalls with hold-downs and the like. Maybe in high wind or seismic areas, but even in these areas, there are prescriptive "recipes" for residential construction.

That being said, I see lots of additions being put on where 20 foot sections of rear wall (at the end of the house, no less) are removed to open up to the addition. I don't think the code officials are even looking at bracing requirements when they issue a lot of these building permits. Usually they just want engineering on the 20 foot header. This is in Maryland, by the way.

It's difficult to force someone to get engineering there not being required to have.

 

[Lion06]

gerhardl-
With all due respect, I don't think many (if any) skilled carpenter, framer, or bricklayer is capable of assessing what I am talking about. "Structurally sound" is not necessarily what I am talking about. That, to me, means none of the existing framing is rotted and is in generally good condition. That does not mean, by default, that the "new" lateral system is capable of handling the loads.
I also have no (read little) doubt about the sizing of most headers over these openings for GRAVITY loads as this is pretty straightforward. The question I am asking is in regard to lateral loads.

 

[Lion06]

ctcray-
Wow!! That seems almost negligent. Obviously not on any engineer's part (if one is not used), but.... my goodness that is scary. I would think taking out 20' of shearwall would change things drastically, from what was mentioned above to adding significant torsion by moving the center of rigidity (as a result of eliminating a large portion of the lateral resistance).
Also, I understand that many homes don't have hold downs, but that is more a function of the dead load on the shearwall and not that it is not a true shearwall, correct? Once you take out such a large wall, and throw more load into the other walls, isn't it quite possible that some of the other shearwalls that didn't require hold downs before will now require them as a result of the larger loads being dumped into them?

 

[csd72]

all you can do is recommend that they get it looked at, if they refuse to take your professional advise then thats their problem.

That said, if it is not in a hurricane/earthquake region then it will probably have much more racking resistance than it needs anyhow.

csd

 

[CJJS]

ctcray is exactly right

"It's difficult to force someone to get engineering there not being required to have."

 

[Ussuri]

I suspect they don't show you all the engineer and legal requirements because it's too boring

Saying that, I watched one of these home improvement programmes where a couple were rebuilding a fourteenth century castle in England. The interior was full of debris and plant life up to about the first floor level (second floor in the US). They dug all the rubbish out and arbitrarily shored up some interior walls.

At this point the camera crew legged it out of the building to the sound of creaking. Shortly after one of the interior walls collapsed after the shoring gave way. I think they were lucky the entire building didn't come down.

 

[COEngineeer]

Have you guys seen that show in North Carolina (trademark properties)? These guys flip houses in like 4 days. I am pretty sure they do not have permits and they tear a lot of things apart without PERMIT!

http://www.swijetty.com

Sea Water Intake and Jetty Construction

 

[PatBethea]

COEngineer,

Yeah, I get "forced" to watch them. They're pretty amazing with how fast they move. I think they're in South Carolina, actually. While I was watching late last night, I saw them tear into some trusses to create a vaulted ceiling. I was somewhat stunned.

 

[COEngineeer]

Well, of course you will make money if you dont have to pay for permit (no down time) and you dont have to pay engineer and architect fee. Just tear it apart, put new opening, add SQ. FT. and call it good. KACHING!!

http://www.swijetty.com

Sea Water Intake and Jetty Construction

 

[DarthSoilsGuy]

i like it when they do get the building inspector or a small time engineering consultant on the show. I haven't seen one of them comfortable working in front of the camera.
Fear of armchair engineers, contractors, and inspectors is at play.

 

[DWHA]

I don't agree that putting a patio or french doors into a house makes a shear wall worthless. In times of high shear you would expect that the door would be closed. (And this is a reasonable expectation, just like when you use load factors, you do not have a high weather load and a high live load at the same time.) If a door is installed properly, I believe that it will not significantly reduce the capacity of the wall when the door is closed.

 

[CJJS]

I would think that the glass would not perform well under shear.

 

[UcfSE]

Str'lEIT, that is exactly the problem you would have unless you were the engineer for that job already. Even if you have the plans, unless they are as-builts you never know what you really have. That is common to remodels and additions in general. You would have to have a way to verify what is there. If you assume something and it has a problem later on, guess who is in the fire for it.

With masonry you may get away with assuming every wall is a shear wall provided it is detailed properly. In that case you may just need a bar at the end of the wall which is typical, at least in my area. With wood, there's more to it as you know. If there is any uplift on the shear wall ends you'd need a holddown. So it's not quite as simple as saying every wall is a shear wall. That's part of why it also isn't as simple to say you have extra capacity with the interior walls. If there's no hold downs and the wrong screw pattern (if they even put screws in it!) for those gypsum walls then you have a problem. Yes there is some sort of capacity but you have no way to evaluate it with a given degree of confidence.

 

[wikidcool]

well, all walls will resist some shear, whether they were designed as shear walls or not. Not like the wind picks and chooses which walls to shove. Question is how much.

Like UcfSE said, all the unknowns are an inherent issue with additions and remodels. Here in CA, you DO have to get engineering to cut a hole in a shear wall usually.

Generally, it boils down to some educated guessing regarding construction of the era/region, what little site investigation you can accomplish pre-demo, and making some guesses with a whole lot of 'contractor to verify' on the plans.

One approach I've used is to calculate the CAPACITY of the wall (if possible) pre-modification, and then retrofit enough to maintain it - and not worry about what it may or may not have been designed to resist.