Lateral reinforcement for increased openings in houses and small buildings 2

[Montreal.eng.]

Hi ladies and gentlemen,

I would like to know your opinion and methodology when you get a project where the client wants to increase the size of a window or a door in the exterior walls of the homes, duplex, triplex. etc.
In Canada, houses and small buildings are governed by Part 9 of the National Building Code. I don't know what that translates to in the USA or other parts of the world. Where I work (Montreal, Quebec) the building code describes these areas as low risk of wind and earthquake forces, therefore there are no dedicated LRFS in standard houses and other small buildings that fall in Part 9. The code relies on the redundancy and achoring of the building via prescriptive methods to ensure proper connections. Even as newer homes are more and more "open space" with larger windows, the new version of the Code doesn't mention LRFS for these buildings. However, many municipalities demand an engineers plans for an existant opening surface increase.

My question is would you strictly reinforce for veritical loads and to what extent should an engineer keep using a certain part of a building code? How far would you go with your analysis and design? If a client wants to for example turn a patio door into an opening to welcome an extension or double the size of the windows in the back of the property, would you automatically calculate lateral forces design the new openings as moment frames or is that overkill? What if that property is 100 year old 2-3 storey townhouse? (many buildings being renovated in Montreal are that age and type).

Sorry for the long thread and thanks for your input.

 

[phamENG]

In the US (though it varies slightly state to state), we have LFRS requirements in the prescriptive code but they have limits. If the walls are taller than so many feet, wind speed higher than a certain value, distance between vertical elements, etc. If you violate any of them, an engineer has to design it.

For new openings, I usually compare it to the prescriptive code. If it still works, great. If not, then it requires a more detailed analysis and potential strengthening of the LFRS based on the existing building code which sets limits for demand increase as a result of an alteration or addition.

 

[skeletron]

I run up against this alot, while also trying to negotiate the current seismic requirements on an older structures not necessarily designed to that level. The bottom line for me is that the structure needs to work. You need to be able to rationalize that the changes aren't decreasing the existing stability or strength of the structure. Whether this is done through calculation estimates, detailed analysis, or even slide-rule type comparisons, I still think it's important to have a documented numerical rationale.

In your example, I would say windows are a minor issue. Contractor will still need to pull some sheathing, so there is an opportunity to re-nail, anchor, etc. For door openings it becomes a little more contentious in my area because it's usually a bigger portion of the wall and then you are removing sheathing below. That would be a situation where I would run a couple calcs to see if it's doable. I don't rely on "it's Part 9, so I can do Part 9 things" unless I'm really really really pressed to the limit. For open concept, etc. my opinion is that you no longer can rely on Part 9 because the loadpath is now changed and any redundancy is being removed.

 

[Montreal.eng.]

Thanks to both of you for your replies phamENG & skeletron.

phamENG - It seems the US code is clearer on what is expected when a modification is done to an existant structure compared to the Canadian Code. Like I said, I work in an area that is described as low wind and eq so the prescriptive method is to anchor the sill plate to the foundation at a certain max spacing, to nail the exterior sheathing to the walls with a prescribed spacing, to nail the studs to the top plate with x amount of nails, etc. The option to use engineered methods (Part 4) is there but it is optional and GCs do the minimum required (I don't blame them). In other words the almost the entirety of the house/building is used to resist lateral loads. That leaves the buildings with no clear LFRS to analyse and to reinforce.

skeletron -

"The bottom line for me is that the structure needs to work. You need to be able to rationalize that the changes aren't decreasing the existing stability or strength of the structure"

That's a good way to see it. The challenge that directly jumps to mind is how would I model or analyse the lateral resistance of an existing structure that has no dedicated system to resist lateral load to begin with in order to strengthen that system. Basically we're left with the option of designing reinforcements on "feel" alone.
That combined with an overly punitive professional governing body is not a good mix.

 

[skeletron]

There needs to be a dedicated system to resist lateral load, otherwise how does the lateral load path resolve?
Perhaps you intended to mean that the load system (of an older structure) is more the sum of the parts...in that case, you need to find how the lateral load path dumps load into the component that you are changing. Then create alternate load path(s) to handle that load.

 

[phamENG]

Here are a couple methods I use to do what skeletron is suggesting:

1) If the house is going through a major alteration with interior walls being moved, lots of changes to windows, etc., then I check it against the 'braced wall' requirements of our code. It's really quite similar to your prescriptive method. The construction is largely the same as the rest of the house, it's just a matter of making sure you have enough solid walls in the right places. If that doesn't work, I'll do a lateral analysis assuming each wall pier is a shear wall and find the loading in each pier. Then I'll do the same thing with the new, altered layout and see what the pier loads are. If they meet our existing building code (no more than a 5% increase, I think - I always I have to check that), then it's good. If not, I reinforce the places that exceed that limit.

2) If it's an addition or changing windows in one wall (common here on the beach), I focus on that LFRS line only. I look at the wind loads on the building and the tributary loads going to that wall line, and I design a new, engineered LFRS for that line only.

 

[Montreal.eng.]

phamENG - Thanks for the input. I think that's the way to go. Gradually moving towards the next more calculated and engineered requirement.

skeletron - This is what I mean.

Design requirements in NBC Article 9.23.16.2.,NBC Section 9.27. and NBC Section 9.29 simply address sheathing & gypsom thickness, min. nailing requirements, etc. If I want to be safe will interpret older buildings that often don't have exterior sheathing OR interior gyproc sheathing OR panel cladding as prescribed, as not being elligible for this section and have to be laterally reinforced using engineering methods (Part 4 or CWC Engineering Guide) in case of major modifications. For newer construction where a whole exterior wall will be large windows for example, I will design LFRS using line loads on that wall alone as phamENG mentioned.

You guys were helpful thanks again,

 

[Enable]

This thread reminds my of a shoring engineer I hired many moons ago to deal with a fairly complex site condition. He came to my site and said "I see your problem. I don't know how to fix it. So i'm going to leave". And the dude just up and left! He was a guy I had worked with for a while so it was funny (mildly irritating but mostly funny)

So in the same spirit: I see your problem. I have no great ideas. But instead of just leaving, here's what I got

I practice in Ontario and we have Part 11 which guides us through renovations. While not your code, we're neighbors so why not borrow a bit? Here's what the OBC says for Material Alteration or Repair of a building system (which I think your window enlargement would fall under)

(1) Where an existing building system is materially altered or repaired, the performance level of the building after the material alteration or repair shall be at least equal to the performance level of the building prior to the material alteration or repair. (bold is mine)

My read would be that you would have to evaluate the LFRS and ensure you've at least matched existing but you are not obligated, say, to sheath the entire wall to meet a prescriptive NBCC requirement if it was not already there. You could obviously do this with a moment frame but hell, depending on the existing, the enlarged window may suffice given sufficient anchorage.

BTW here is the definition of a building system

building system means a combination of elements or components that form a complete major division of construction in the design of a building or part of a building, including a structural or framing system, a waterproofing system, a drainage system, an exterior cladding system, a roofing system, a window system, a partition system, a corridor system, a stair system, a fire alarm and detection system, a sprinkler system or a heating, ventilation or air-conditioning system, a foundation system, a standpipe and hose system, a flooring system, a plumbing system, a sewage system or an electrical system.

You obviously still need to exercise some judgement. If the existing condition is such that the thing is darn near going to fall down, you probably have to fix it as a matter of ensuring public safety. But for a house that is of typical construction, that has been around a long time, and nothing too bad has happened so far you'd be pretty safe to apply Part 11.