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# I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

## I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

(OP)
My designs will obviously be checked by persons who have done this before but I am looking to impress.

Some details/questions:
• We might be doing a wind tunnel model since the building has an unusual shape.
• The building will be founded on bedrock but is also within 50m of the sea.
• It will have 2 underground levels, (so crack width for basement levels?)
• Corrosion protection if the client/architect doesn't want galvanised?
• What are some considerations that I need to keep in mind regarding mechanical/HVAC/etc.?

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

"within 50 m of the sea" - wave impact and a Tsunami safe room or a quick way up to the upper floors?
Steel corrosion - stainless. 
Saltwater intrusion into basement.
Column shortening.
Story drift under high wind/seismic loads.
Isolation of Mechanical equipment.
Floor vibration.

Where is this structure?

Mike McCann
MMC Engineering

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

(OP)
Hi Mike,

The building is in Port Elizabeth, South Africa.

"within 50 m of the sea" - wave impact and a Tsunami safe room or a quick way up to the upper floors?
I don't want to sound neglegent but if I even mention this to the client/architect I will get laughed out of the room. We aren't close to any seismic zones.
Steel corrosion - stainless. 
I like galvanising because it lasts for ages, we sometimes paint but then we tell the client that they'll have to re-paint in 5-10 years. I don't have much experience with stainless steel to be honest.
Saltwater intrusion into basement.
Yes that is a big worry of mine. Not really sure what to do about that since I haven't designed a structure so close to the sea before.
Column shortening.
I went to a conference where we had a famous Australian tall structure engineer as a speaker and he mentioned that column shortening only becomes an issue when one goes above 50 stories. What is your experience?
Story drift under high wind/seismic loads.
I think a more experienced engineer will be handling that but we will be able to have several large shear walls as well as a lift shaft core. So the structure will be fairly stiff. PE is not in a seismic area as well.
Isolation of Mechanical equipment.
Not sure what you mean by that.
Floor vibration.
Are you referring to the harmonic frequency of the building?

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

Column Shortening... Well at 26 stories, we used steel columns and many of the lower ones had to be increased in area to control this. May not be an issue at 14 stories depending on your loads, but I would still check it. Eccentric loading could occur.

Induced vibrations in the floor structure or building frame due to resonant loading from Mechanical equipment, rooftop or otherwise.

And yes, harmonic frequency.

Mike McCann
MMC Engineering

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

If multiple stories are the same I'm often surprised at how few pages the structural portion of a tall building can be represented by. I've seen 6 pages for a 6 story building and 16 pages for a 28 storey building, if memory serves. If that proves to be the case don't be too surprised. On the other hand, if the building has unusual features then it can take many more pages, of course. In one case I've seen over 50 structural drawings for a building that was 8 storeys, or so. But it is ok to show identical stories on the same drawing.

Sorry, no technical guidance, just relaying what I've seen for page counts.

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

I grew up on the coast where it was very flat. If you were 150m from the water, you could dig down about 2' and hit water, so any basement would become a barge in those circumstances. A major design issue there was storm surge from hurricanes.

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

HermanLJ

Wind tunnel test for 14 story building is uncommon, but if the shape of the building is calling for wind tunnel investigation it could also be susceptible to wind induced dynamic acceleration.

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

Good supply of quality concrete, concrete cover, low w:c ratio, 25% or so flyash, good concrete strength 35 MPa or so. See what is common to the area... maybe HDG rebar would be reasonable.

14 storeys is too short to worry about column shortening unless proximity to very stiff elements and short spans.

See if you can model it without windtunnel testing even if you are conservative with your loading. It still may be cost effective. 14 storeys is not a height where dynamics and lateral drift would normally be an issue.

Bedrock is a good base, even for 14 storeys... and I don't know what seismic issues you may have. You can do a fairly conservative design of the roof and main floor areas, even if the footprint of the main floor extends beyond the main upper floor areas. You may have 13 or so floors where they are nearly identical and you can do a tight design for these floors due to the high repetition. Select your multiple floor design loads carefully.

You may have access to a local contractor and try two or three framing methods to choose the most economical one. The economy of design is generally in the system selected. You may try a couple of different preliminary designs to ensure you have proper quantities.

Determine floor to floor heights. On one project I was involved with the mechanical ducting, light fixtures, ceiling, etc. was mocked up to determine the minimum height... 4" was saved per storey for 30 floors. Adds up...

There are probably a few other things...

Dik

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

If your basement is tanked and below the water table, you will have considerable uplift loads to consider on the basement structure before the structure comes afterwards to hold it all down.

Consider the staging of the construction, especially the in ground works.

Consider the proximity of adjacent buildings and effect on your excavation and the effect of excavation on their buildings. Remember water flowing into your site will cause settlement of the ground around the site.

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

Don't write off seismic design because you're "not in a high seismic region." Keep it in mind.

"We shape our buildings, thereafter they shape us." -WSC

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

OK. No basement. Fine.

Then the "first floor" becomes - MUST BECOME - a "wet floor" or "open parking area' where the storm waves can pass UNIMPEDED under the rest of the structure for the maximum expected storm wave + tidal surge.

What? Storms don't come ashore in FL, SC, NC, VA, Maryland, Delaware, NJ, NY, CT, or Mass? Storms don't across the Illinois? Michigan? Ohio? Wisconsin? shores?
What shore are you against where there are "no seismic" and "no storms"?

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

He stated the location above: Port Elizabeth, South Africa

Maine EIT, Civil/Structural.

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

I just stumbled across this study entitled, "Structural Design of Tall Buildings Knowledge Acquisition Study Report". In case it's of use, see link...

http://repository.cmu.edu/cgi/viewcontent.cgi?arti...

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

What is the local geology of the bedrock?? Granite, weathered shale, limestone etc??? Depending on the answer your geotech may require bedrock grouting just to keep basement infiltration down to an acceptab;le level.

### RE: I will be designing my first large building (14 stories) shortly. Which issues should I be aware of?

1.) You will have increased wind speeds due to proximity to open water. Even more so if the shape of your building is weird (vortex shedding, etc.) A wind study is a good idea so that you can reduce cladding loads and optimize your lateral force resisting system.
2.) Design for seismic. There is no excuse to discount this. For a 14 story building, seismic may control (though probably not since you're on bedrock).
3.) 50m next to the sea? Your groundwater level is at sea level. If you're 2 stories below the ground... then be prepared to design a pressure slab at the foundation level. Also, design your 1st floor columns for impact loads due to flood debris. At a minimum, design for vehicle impact load.
4.) Your substructure should be concrete. Add corrosion inhibitor into the concrete design mix. Maybe use epoxy coated bars as well if you're really concerned... but it likely isn't a concern if you waterproof correctly. If you're talking about floods and stuff? That's likely a 50year event. I wouldn't design specifically against it unless it was requested or required.
5.) I don't think column shortening is an issue for such a short building. But it never hurts to keep the stress in the columns approximately the same so that you minimize differential axial shortening.

Above all else... figure out your lateral system first. Make sure it works and satisfies drift tolerances. After that, figure out your gravity load path. Don't waste your time laying out beams and columns until you know your lateral system is sufficient.

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