Lateral system for row houses 6

[SleeplessEngineer]

I am not very familiar with residential buildings and wood construction (only been working in this area for 3 years, so forgive me if the answer is so obvious). This is what I have done for a single-family row house rear and one-story addition (20'x50'). For the short side, there is not really any shear wall segments that can be used for design due to the large opening in the rear wall. I have added 2 steel moment frames. The GC is not happy and argued over an hour and talking to the owner about engaging another engineer to redesign. He was arguing that in 20 years of his experience, no one asked for steel moment frame in single family. So, this got me thinking, am I missing some exceptions in IBC for row houses? As far as I know, row house should be designed independently for lateral forces (not allowed to rely on adjacent structure). Any tips or ideas for more economical solution will be appreciated. If I am indeed over-engineering, I would like to change ASAP. Otherwise, I don't mind losing clients. Thanks for reading.
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[human909]

If the diaphragm isn't designed to 0.99 without regarding the complexities and convolutions in the load path, there's potentially some reserve strength in the diaphragm to "find" a load path. So, hypothetically if you don't do a rigorous analysis of the load path if you keep a 50 plf excess capacity in the diaphragm it may work fine, if there's a load path and the results of that stress concentration is less than 50 plf.

But in many instances the diaphragm isn't designed AT ALL! This is particularly true in residential construction, but is also true in many peoples design approaches mid rise concrete slab structures.

(And sure there are plenty of case where explicitly calculating the diaphragm capacity just isn't necessary as it genuinely does have significant excess capacity and even a semi-experienced engineer can recognise that.)

 

[YoungGunner]

Going back to the Simpson strong wall idea - surely you have interior walls that could be used on upper levels for the cantilevered diaphragm methods and then just use the strong walls on the lowest level? I'm surprised to hear in your region that builders hate strong walls more than a steel frame, as that is opposite in my region.

 

[lexpatrie]

From the sound of it, per the OP, I guess not.

These things (Simpson strong walls) aren't that new, but I think a lot of builders are not familiar with them, steel they at least have a handle on when it will be shipped. They were pretty pricey back in the day when they were newer, if costs are more reasonable now, (or if it wasn't true, back then), don't know.

I'm not convinced the span limits on open front structure is being met with this design, which would mean the moment frames need to be relocated closer to the front/back elevations to satisfy that ratio, and the chosen steel has limited drift control anyway, W8 and HSS columns. Then there's that 2x4 to 2x6 transition halfway down the building as well. That's going to need some detailing "just" to save 2" on the floor plan. I wouldn't bother.

 

[LEC2018]

This thread was very helpful. I apparently am one of the only engineers in my city that prescribes Simpson Strong Walls or moment frames for these conditions, which occur frequently in a dense area. We got into a similar heated argument where a contractor suggested to our client to find another engineer because "they build this all the time and have never had an issue with using standard wall framing." I even showed them the calculations of the walls failing, not to mention the fact that they didn't even meet Code requirements due to minimum widths and aspect ratios.

For infills, our AHJ's practice for derelict structures is to demolish the interior structure and front/rear walls, but to leave the masonry party walls if there is an attached building that is to remain (the walls themselves are not structurally independent as two-wythe brick walls with a single foundation). In these cases and since 50% of those masonry walls technically belong to our client and is on our side of the property line, we will show diaphragm/shear wall connections to them as they are not allowed to be demolished in the event that the adjacent property is demo'd. Our AHJ has yet to deny this approach in either the review or inspection phases. Maybe that is what the OP is assuming with the design of the moment frames towards the bottom of the plan only.

OP, if you've found any other solutions, please let us know. I'm too being accused of over engineering in these cases.

 

[lexpatrie]

I'd imagine for the typical structure the "you can drive a truck through the building" style of design doesn't hold. This is like a shotgun building (enfilade is a prettier term) but even those styles had walls in the left-right direction.

 

[Eng16080]

"they build this all the time and have never had an issue with using standard wall framing"

Typical comment from a builder with 30 years experience, I'll guess. Not that they would even know if there was an issue. Their involvement typically ends when they pack up their stuff and get in the truck. Also unlikely that their buildings have experienced a design load event.

I think you're doing the right thing, FWIW. As crazy as it is, this is the approach I take. I actually try to follow the code in terms of designing structures to resist lateral loads. I often get residential projects located on an ocean/lake with massive window walls. Sometimes I feel like I'm the only engineer designing these walls for in-plane and out-of-plane wind resistance. It's wild to sometimes see other engineer's plans which entirely neglect this aspect of the design.

For the townhouse homeowner undergoing a probably 1 million dollar remodel, a couple moment frames or some Simpson strong walls isn't going to bankrupt them and is a very cost-effective insurance policy whether they realize it or not.

 

[rowingengineer]

In instances like this, I do homework to see what the contractor has completed previously and what drawings he had access too. Often I find that there is a perfectly acceptable solution, just the builder has no idea how to express the solution in engineering terms.

I would ask for a few examples of similar projects with structural drawing and calcs? that you and your company can review, preferably buildings that the contractor has completed.

 

[GaStruct]

I am glad this thread is being discussed. I deal with this exact situation all the time - walls full of windows with no obvious LFRS option.

My process is:
1. I first try to use IRC or IBC prescriptive requirements where I can. Often the structure doesn't comply.
2. I try shear walls where they will fit. Often the aspect ratio doesn't comply.
3. I try Simpson products where I can. Contractors in my area are inept at coordinating this and it usually gets ignored/screwed up.
4. I use moment frames when all else doesn't work. This usually gets the contractor screaming at me and telling the owner I don't know what I'm talking about. Nothing new!

As mentioned above, I am convinced I am the only Engineer in my area that actually designs a lateral system and accounts for wind in-plane and out-of-plane. It's pretty impossible to compete with the Engineers that are somehow comfortable ignoring this.

Since most of these poorly designed structures are still standing it is extremely hard to convince anyone what is supposed to be required.

 

[lexpatrie]

Those moment frames are too close together to be plausible, plus effectively with no lateral system at the top of the house, there's a cantilever diaphragm on both ends of this house, the one at the top is way too long. I think I've already said this but it bears repeating.

Rowing - I gave you a gold star. That's a good way to look like a reasonable person.

And there's always the complain to the board that some other engineer is being negligent, if you believe that to be the case. I feel like the boards tend to take complaints from engineers as more sour grapes and attempts to eliminate a competitor, and when the public makes a complaint the sirens go off inside the building and they go all hands on it. But maybe I'm just being cynical.

ETA: Texas looks for professional engineers to lend expertise (some paid, some not), when it comes to board complaints, so you could consider that, at least in Texas, if you thik a lot of these are done wrong, you'd be in the position to present your objective findings.

 

[human909]

Those moment frames are too close together to be plausible, plus effectively with no lateral system at the top of the house, there's a cantilever diaphragm on both ends of this house, the one at the top is way too long. I think I've already said this but it bears repeating.

Could you please elaborate on that. I say that because I regularly use moment frames in circumstances like this or similar.

I've never had push back when I use moment frames, and they are fairly normal down in my end of the hemisphere. Though that is not to say we don't have a vast array of incompetence in construction here, just I haven't seen any reluctance to use moment frames.

I am glad this thread is being discussed. I deal with this exact situation all the time - walls full of windows with no obvious LFRS option

Slit windows are the fashion that I've seen some of the worst engineering around. Both horizontal and vertical.

(example image)

I recently saw one high in the wall of the ground floor. They had a decent lintel to support the floor above. But there was nothing to support the in plane wind load BELOW the window. A standard sill plate won't cut it when the span is 5m long and there is ~1m of tributary area! This was on a house build costing ~1.5mil.