Complex Residential Structures 3

[Flotsam7018]

Primarily the residential structures I am involved in are complex - hence why they call us. Lots of different roof planes and overbuilds, horizontal/vertical offsets everywhere, large openings - the usual. I happen to also be in a state where building things to code is still an idea many are just beginning to comprehend.

The methods laid out by Malone/Rice give an engineer the feels goods - they adhere to statics, not just intuition. However, we typically neither have the time , budget, or contractors in our good graces to follow all of these procedures and resulting details at every instance where an anomaly occurs. Typically, I identify the most egregious aspects of the structure from a load path perspective and focus on those areas, determining other, less offensive areas to be acceptable without specialized analysis/detailing. Even this approach occasionally gets push back, either from spending too much time or developing details that are atypical for this area.

I'm curious about other peoples approach towards complex residential structures and how you achieve a design that doesn't require an inordinate amount of time, doesn't make the contractors complain to everyone in town, and also makes you feel that you've done your job.

 

[phamENG]

I don't/ can't. So I charge what I need to charge to do what I need to do and design what I need to design. I listen to reasonable concerns, ignore their gripes, and move on to the next. I'm still in business, so I guess it works.

 

[Just Some Nerd]

I'm curious about other peoples approach towards complex residential structures and how you achieve a design that doesn't require an inordinate amount of time, doesn't make the contractors complain to everyone in town, and also makes you feel that you've done your job.

This is almost exactly the proverbial "Good, Fast, Cheap - Pick Two" scenario

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Why yes, I do in fact have no idea what I'm talking about

 

[milkshakelake]

The problem with irregular wood structures is that it takes a ton of time to design compared to steel/concrete if you really do it the right way per Malone/Rice. There will be connection detailing that nobody will ever follow in real life. In my limited experience, the inspectors don't make a big deal about these load path connections either, but that might be a regional thing. I just do what I have to do, which is make a solid, legally justifiable, rigorous set, and be lenient on the actual construction.

For actual construction, I'm not a stickler for having a completely connected diaphragm because that's really hard to do in real life. I do pay a lot of attention IRL to holddowns, double sided shear walls, and other important elements.

I haven't yet figured out how to make this work cost-wise. Irregular wood takes a ton of time and I can't charge more than competitors, and I also can't turn down these jobs because I need to keep the lights on. One thing that helps save drafting time is to make general details that can adapt to different situations, like a really nice shear wall detail that considers bottom story, upper stories, FTAO openings, joists parallel/perpendicular, etc. but I haven't figured out a way to make it work for irregular diaphragms yet. I note those things on plan. I have typical details for supports on podiums that I copy from job to job, and for things like slanted joists for weird facades. That's all for saving drafting time; not sure about engineering time.

 

[manstrom]

I assume you are referring to lateral design mostly. There are some assumptions in Malone's book (and wood design in general) that are safe and simplify design, but are conservative when considering conventional construction methods. For example, the idea that there is one chord in a wood diaphragm and the floor joists/studs are neglected is very conservative. Glue in diaphragms is neglected, gypcrete is neglected. In reality, the doors and windows in a structure are going to contribute some lateral restraint to a structure.

Having done hundreds of 4-5+ story wood structures with no major failures (knock on wood) I am comfortable with what I consider to be "conventional construction". When you start getting away from conventional details, that's where you need to be more conservative with your designs. Modern custom homes come to mind where there are irregular planes and lots of glass. In this case, the builder should expect to pay more as they branch away from conventional details.

Another downside of tricky wood designs is that there is no good software for lateral. It ends up being done by hand or spreadsheet.

 

[milkshakelake]

@manstrom In terms of conventional designs, how did you do it for irregular diaphragms? We end up calculating a lot of forces around openings and setbacks, and how things connect to each other, and it's wildly different from project to project. Maybe after a hundred designs, you get a feel for how large these forces are, and you specify typical connections for maximum forces at typical conditions. We do that for cold formed steel.

 

[manstrom]

Frankly, I do nothing for setbacks and large openings at irregular diapgragms. Disclaimer, I'm usually 115mph wind and SDC B. My diaphragms are usually 30'-60' deep and shear walls are every 24'-36' or so.

I'm sure I could try and find a rational approach that reduces the loads and validates the assumptions, but I haven't seen issues at these conditions in my experience. And I would have a hard time getting some contractors to build a weird details there. A 3/4" plywood deck glued and screwed to trusses at 24" is pretty darn strong.

 

[Aesur]

I understand your frustration; we do a good bit of high end residential and the trick was finding that one client who understands structural (at least to an extent) and is willing to pay more for it. We have a few of these clients and get reasonable fees, although many times we still spend the whole fee at hourly rates on the project for high end. We let the other projects go to others who are willing to do it for actual costs, or even lose money on it.

Over time I have gotten quicker and quicker for lateral on these irregular structures, being conservative many times; however what is conservative when the diaphragm is still 8d at 6" o.c. nail spacing 75% of the time? Because of this, I tend to calculate the worst case diaphragms, and calculate each drag/chord as required, again you get a feel for grouping after doing many.

For details, I have a library I have been developing for years, it probably has nearly 1000 custom details, many of which honestly are not reused completely, but at least allow for quicker modification. The nature of custom residential is mostly custom details, or trying to convince your client that standard details suffice, but the contractor has to think a little.

I recently sat through a webinar with Malone and about a year or so ago had a conference call with him regarding the complexities of wood design; great guy and extremely knowledgeable. I agree, we cannot, and few do, design to the extents he shows; this is where engineering judgement comes in. From a teaching standpoint he shows how to do it, but leaves it up to the engineer to find methods to simplify the design. He has a webinar coming up in June I believe to go through his new method of force transfer around opening.

Basically, if the price isn't right, I walk, someone else can lose money on it.

 

[dhengr]

Flotsam7018:
Makes you kinda wonder, are you acting as the needed/essential Structural Engineer or the insurer of last resort, for when things go wrong. When you can’t get a reasonable fee for your effort to do it right; when the Arch. and the AHJ don’t back you up, in the need for a complete design/detail for someone else’s crazy conditions; when the builder already knows it all, his Grandpa showed him how years ago, and this is the way they’ve always done it, even though it’s not the same condition as they imagine; ... why would you even bother? Find a few good contractors/builders/Archs. who semi-understand what you are trying to do & say, and appreciate your input to keep their what-cha-ma-call-its out of the wringer, and work with them, it well be more rewarding and enjoyable. Alternatively, wait until they get in trouble with the AHJ, a red flag has to be corrected or redesigned for the real conditions, then they actually tend to appreciate you effort, to dig them out of their hole. When an owner, Arch., builder want crazy details and conditions, they should be willing to pay for their engineering design. We are not conjuring this crap up, just to make own life/work difficult, for the fun of it.

 

[Flotsam7018]

Thank you all for the replies, lots of good insight provided here.

@Just Some Nerd – true! I’ll start adding that to the proposals.

@dhengr – it is a salient point that we are not the drivers of this additional design time and detailing. Yet, it often ends up feeling like we are the ones that have to deal with the repercussions of the architectural choices. We are almost exclusively in a market of the builder knows everything already, and yet the practices they have learned were handed down from a time when structures were not so irregular – as I have seen others discuss in similar threads.

@milkshakelake – indeed it is much more time intensive to design and detail an irregular wood structure compared to steel. When a structure triggers a certain level of complexity we’ll often just frame out the most complicated sections in steel and strangely enough this seems to get less pushback from contractors than the more involved wood framed details that would otherwise be required. This obviously can greatly simplify the VLFRS, but I also have a lot more faith in the steel being fabricated and erected in accordance with the design than complex wood framed details, as you mentioned.

I suppose in the end (for those of us who have some clients/contractors who don’t understand why additional design time is required and why they need blocking/straps where they’ve never had them before, etc.) we can keep pushing for these additional details, hedging our bets where it is most logical, and slowly overtime architects and contractors will reset their expectations. It’s a process, hopefully.

 

[KootK]

I'm curious about other peoples approach towards complex residential structures and how you achieve a design that doesn't require an inordinate amount of time, doesn't make the contractors complain to everyone in town, and also makes you feel that you've done your job.

By the sound of it, I do what you do:

1) In terms of lateral design rigor, I try to be in the upper 1/3 of the engineering community in my market. And no higher than that. One does well to remember that, should legal trouble arise, the standard of care is that of your fellow engineers in your market. It's not what Terry Malone thinks is best. Perhaps it shouldn't be this way but, for now, it is.

2) I scheme up a structure that I feel will perform well for both lateral and vertical loads.

3) I pick my battles.

4) I don't sweat the Terry Malone stuff as being gospel. Everything that Malone proposes makes the simplification that sheathed diaphragms can resist no in-plane loads. No tension, no compression, no bending. There are some good reasons for that, including sheathing gapping but, in general, I feel that this assumption is where a lot of reserve capacity in these kinds of buildings winds up left on the table.

5) I long for the day when, like friends in Arizona and California, I can do a more rigorous design of the lateral design of the buildings. At the same time, I make peace with what I feel is fact: the only path to this happy outcome is enforced regulation. My market is just starting to get the regulation, we don't yet have the enforcement. I don't feel that it is reasonable to expect engineers to specify and enforce lateral load paths that are not industry palatable within their markets without the aid of enforced regulation. There's little room for heroism when you have to sing for your supper in a free market filled with competitors.

6) While some spaces are obviously better than others, I feel that a good structural engineer can make a go of things in any space so long as they calibrate their work appropriately. Moreover, I see serious problems in engineers fleeing spaces that they deem difficult. That road leads to the worst engineers doing the most difficult work which becomes an ethical problem from the perspective of public safety.