Basics of high rise design 3

[milkshakelake]

I'm trying to get into "high rise" design. For my purposes, it's 10 stories and above. I've done a few before but they were under the instruction of seniors, who treated them the same as other buildings.

I'm thinking of what the difference would be from low to mid rise buildings. I take some things for granted in mid rises, which might become important with high rises. I came up with this:
1. Consider P-delta effect
2. Need to calculate gust effect factor for wind (ASCE 7) due to flexibility of building
3. Check if special seismic provisions are triggered (I don't have much experience in this but I'm in New York so I don't think it's a huge factor)
4. Make sure footings/piles have similar pressure/forces to avoid differential settlement, moreso than in a typical building
5. Consider backstay effect at lowest floor
6. Consider diaphragm strength at lowest floor and transfer floors, including collectors and chords
7. Exactly and iteratively calculate which shear walls are cracked based on modulus of rupture
8. Consider alternative forms of lateral systems such as tubular design and outriggers
9. Check sliding at foundations

Also would appreciate business advice on securing this kind of work, but that's probably for a different section of these forums. I guess it would boil down to "be super good" and "be in the business for a thousand years."

 

[dik]

I don't consider them as high rise... have only done one about 30 storeys, a bunch about 20 storeys and whole bunch less than 6 storeys. No studies... just worked into it. My first was one down the street about 5 or 6 storeys... did the arch and structural and drafting for it (you could do that back then-50 years back, and just a rookie). No fancy lateral, all were shearwall using the elevator and stair cores for lateral... all still standing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[ggcdn]

One of the bigger differences from a seismic perspective is the need to ditch the equivalent lateral force procedure and use a dynamic analysis (most likely a response spectrum analysis). This opens up all sorts of questions both theoretical and software-specific that may be glossed over when using the ELF.

-JA
try [link calcs.app]Calcs.app[/url] and let me know what you think

 

[dik]

None were seismic... as far as the one mentioned, it's uglier than a hedge fence. I told the developer that my aesthetics were terrible and that he should hire an architect... he was one of the cheapest developers I ever did work for.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[milkshakelake]

@Sonofatkins I'll look into what triggers dynamic analysis in my region.

@dik I don't know if I could just design it as a taller version of smaller buildings, especially with lateral.

 

[dik]

That's what I did... they weren't tall enough to introduce secondary issues, IMHO. As I noted, seismic was not a concern.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[KootK]

In a low seismic area like NY, and with your pre-existing experience, I bet you'd be in pretty good shape for your typical, mixed use concrete tower up to maybe 40 stories. In that range:

1) There will be a bunch of concrete columns that will mostly stack.

2) You'll make a go of lateral with, probably, just the core wall systems. Coupling beams but no serious use of outriggers yet I suspect. Basically, if you've got ETABS, you're likely to be as good as the next gal.

3) You'll need to do a couple of high load transfer slabs which you probably already know how to do.

4) You'll need to produce an efficient floor slab design for your repeating floors which you probably already know how to do.

5) NYC basements don't go too deep so probably nothing terribly new there for you.

6) Shear wall overturning is probably resisted by some manner of high capacity pile into rock. You'll delegate much of that to the Geotech so, on your end, it's just understanding the technology and the cap detailing.

7) You'll need to plan out your floors for a quick casting cycle. Given your background with construction engineering, you're surely better versed in that than most EORs.

8) Yes to the backstay effect. Here's a decent article about it written by one of your NYC cohorts:

https://www.structuremag.org/wp-content/uploads/C-StructPractices-Tocci-June121.pdf.

Mostly it comes down to softening the first floors below grade so that you don't draw a ridiculous amount of shear into to your core wall systems there.

9) One thing that tends to make taller buildings simpler than shorter ones, in some respects, is that folks tend to stack the structure well owing to the massive costs of failing to do so. You don't see as much of the silly transfer -> transfer -> transfer stuff that you get in low rise buildings.

10) If the buildings will be predominantly concrete, a I expect, you can basically just recycle the same 30+ concrete details that pretty much everybody uses. In my opinion, concrete buildings are the easiest to detail and create plans for by far. And that seems to be why engineers often prefer these buildings from a profit generation perspective.

Also would appreciate business advice on securing this kind of work, but that's probably for a different section of these forums. I guess it would boil down to "be super good" and "be in the business for a thousand years."

You're clearly aware of the primary difference from a business development perspective. Without a firm resume littered with high-rise work, you'll have to somehow instill enough confidence in your potential clients that they'll give you a shot at, potentially, screwing up a big $$$ project for them. Additionally, your business development efforts will probably need to shift somewhat to a greater focus on developers rather than your usual client profile.

 

[KootK]

4. Make sure footings/piles have similar pressure/forces to avoid differential settlement, moreso than in a typical building

I doubt this is too big of a deal since you'll usually be on rock. Of a related note is the issue of column shortening in general. You gotta get pretty damn tall before this kicks in seriously though.

8. Consider alternative forms of lateral systems such as tubular design and outriggers

I doubt it. No offense but that stuff kicks in at a level above even the level that you hoping to get into. You're a machine, I'm sure. But you're no Fazlur Khan. Not yet at least.

7. Exactly and iteratively calculate which shear walls are cracked based on modulus of rupture

1) It's never exact or any where close to exact.

2) I don't feel that anybody is doing anything iterative in NYC in the 10-40 story range. It's mostly just the ballpark, effective EI's from ACI.

I say this primarily because I don't want to see you putting a bunch of pressure on yourself with this stuff that doesn't need to be there. If you wanna get fancy, get fancy. It'll be going above and beyond the level of rigor of most of your competitors though.

 

[271828]

Add wind-induced lateral vibrations to the list, although that's not much of an issue at 10 stories.

See the AISC Design Guide 3. Griffis "Serviceability Limit States Under Wind Load," AISC Engineering Journal first quarter 1993 is a good place to start.

 

[milkshakelake]

@KootK Thanks, that clarifies a lot and eases my concerns.

2) You'll make a go of lateral with, probably, just the core wall systems.

Thanks, that's the main thing I was concerned about. I read some stuff about high rises and thought that tubes or outriggers were the norm, but it's good to know that what I already know will generally work.

You don't see as much of the silly transfer -> transfer -> transfer stuff that you get in low rise buildings.

Haha I thought I was one of the only unfortunate people doing that, or just got unlucky with low rises where every floor is different. It's probably more widespread than I thought thanks to modeling software able to handle things like that.

Regarding the business development and targeting clients, that's the thing I'm most interested in but visit other sites/forums to learn about. I think I'm a perfectly adequate/average engineer, just good enough to do 99% of the work I need to. My previous boss was, to put it mildly, a subpar engineer. But he's an excellent salesman so he's doing high rises. I'm going to get there eventually.

@271828 Thanks, I'll definitely look into that.