question about home renovations
question about home renovations
(OP)
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?
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?






RE: question about home renovations
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.
RE: question about home renovations
RE: question about home renovations
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.
RE: question about home renovations
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.
RE: question about home renovations
..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....
RE: question about home renovations
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.
RE: question about home renovations
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.
RE: question about home renovations
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?
RE: question about home renovations
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
RE: question about home renovations
"It's difficult to force someone to get engineering there not being required to have."
RE: question about home renovations
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.
RE: question about home renovations
http://www.swijetty.com
Sea Water Intake and Jetty Construction
RE: question about home renovations
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.
RE: question about home renovations
http://www.swijetty.com
Sea Water Intake and Jetty Construction
RE: question about home renovations
Fear of armchair engineers, contractors, and inspectors is at play. :)
RE: question about home renovations
RE: question about home renovations
RE: question about home renovations
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.
RE: question about home renovations
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.
RE: question about home renovations
Glass is not the best material to use to resist shear, but that does not mean that is does not have some shear capacity. I am just saying that having a patio door will not make the wall worthless. And a properly framed in patio door will not significantly reduce the shear capacity, unless you have a very small house and a very large door.
RE: question about home renovations
If you have a 30' wide x 9' high shearwall (assume an entire wall is available). Now you cut a 7' x 7' hole in the middle of that wall. How will you transfer shear across that opening to make this act as "one" wall? If you assume (2) shearwalls (which is perfectly reasonable), the unit shear will increase only marginally, but you have such a smaller "d" now that I would be amazed if a hold-down wasn't needed.
I don't think you can just cut an opening that size in the wall and assume that it will not significantly affect the lateral resistance of the building. Even if you look in Breyer's book, the shear on perforated shearwalls is considerably higher around the openings and is accounted for in the original design. Unless you tear off the siding and take the steps necessary to achieve this extra shear capacity, the segmented shearwall is the way to go. That being said, the "d" is getting cut down immensely as stated above and this has real consequences that can't be rationalized away, IMHO.
RE: question about home renovations
Do you think that everytime that a patio door is added the contractor takes measures to prevent this. No, it does not happen. That being said, Have you seen a house that has failed in this way prior to other significant failure?
RE: question about home renovations
Structural items are required to be ductile, something glass is not.
RE: question about home renovations
I don't agree that it's ok to do what you want because something else will fail first anyway. That only means that more than one member or system was incorrectly designed.
RE: question about home renovations
I agree that the door may have some shear resistance, but it is not something that can be counted on (there is no way to quantify it). Also, as csd72 points out, the glass is not ductile.
No, I don't think a contractor takes measures to account for this - that was the whole point of my initial post.
Also, I am not sure that just because I personally haven't seen this result in a catastrophic failure makes it "right". It is quite possible that there has been enough redundancy in the system to make up for it, or maybe the building hasn't experienced its design loads... either way, I don't think we, as engineers, can just say it is ok without investigation.
I have seen houses with cracks (in the drywall) radiating from the corners of the doors in situations like this. This may or may not be directly attributable to what we are talking about, but......
That being said, I was trying to figure out if these guys are doing something behind the scenes that we are not seeing or if this is a nightmare waiting to happen. If the first is true, I was curious as to what went in to checking this kind of thing (as you can see from some of my previous posts in this thread).
RE: question about home renovations
all the time, I get cold calls to look at existing residential foundations to sign-off on them for loan approvals. Our competition is rushing to shmooze me for all the "referrals" i give them.
RE: question about home renovations
RE: question about home renovations
regards,
Mike
RE: question about home renovations
Another emphisis he has that the code is only the minimum requirements and may not completely fill the bill.
One of his latest shows he had Weyerhaeuser come in and design a beam for the first floor that carried the load bearing wall for the second floor.
RE: question about home renovations
I have often wondered the same thing. I practice in Southern California and if we have a project that only moves a window/door even 6" within a wall that could possibly be a shear wall, we have to look at the lateral system that "might" be there. If we don't - the building official will ask us to. You never know when that 6" made the holdown that was there obsolete. The homeowner is always happy about this process but it is often less expensive than stripping down the whole house to verify for sure where the existing shear walls are (or if there are any, depending on the age of the house).
I'm sure that if these shows do look at the lateral system, it is just not shown. After all, the parts that would be very interesting to us, just isn't to everyone else.
RE: question about home renovations
RE: question about home renovations
1. calculate the load on the wall ignoring any contribution from internal walls.
2. Calculate the strength of the remaining wall using typical wall sheathing (note assumptions on sketch to be confirmed).
3. Calculate and specify the additional tie downs needed on either side of the opening. Also note edge nailing required to double studs.
csd
RE: question about home renovations