Building geometry 10

[hoshang]

Hi all,
Please find the attached image.
Do you think the proposed geometry acceptable regarding center of mass and center of rigidity considerations? FYI the red circle is the center of mass.

 

[rb1957]

my "doomed" meant not a fire safety fate, but rather the inevitable unfulfillment of the design intent.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[BAretired]

Going out on a limb here to say this is probably relating to wall area, not floor plate area. I.e., 20% of the wall area must be open to the outside (for ventilation purposes...?). Total shot in the dark.

That is not the way the floor plans read. Look at the Typical Floor Plan for example.

My interpretation is that the crossed areas represent openings in the floor slab, apparently unoccupied floor space.

my "doomed" meant not a fire safety fate, but rather the inevitable unfulfillment of the design intent.

I interpreted your "doomed" correctly. There may be some design aspects which cannot be fulfilled, but that means the owner may have to be content with a lesser version of the building than he hoped for or expected.

 

[hoshang]

My interpretation is that the crossed areas represent openings in the floor slab, apparently unoccupied floor space.

Correct. The crossed areas are open to the sky.
What one can do to reduce the column sizes at the lower floors? My first thought is increasing f'c. Any other options?

 

[BAretired]

and BA[/color]]Correct. The crossed areas are open to the sky. A very unusual requirement. Why does the municipality require this?
What one can do to reduce the column sizes at the lower floors? My first thought is increasing f'c. Any other options? Increasing f'c, increasing the percentage of steel or reducing applied load are the only measures I know to reduce column sizes.

 

[hoshang]

Hi BAretired,
I'm planning on eliminating columns at left, right, and back sides. I'd rather use concrete walls at these locations. It would be beneficial for stair and lift. Any considerations?

 

[BAretired]

What prompted you to do this? Structurally it's okay, but seems like a lot of concrete. You should have another look at the floor to floor height to see if it can be reduced.

The back wall (Grid Line 1) has fresh air on both sides of it. It will not be braced by floor slabs, so you will still need lateral support for the wall at each floor level.

If the neighbors have built to the property line, you may not be able to strip the outer wall forms for the first two or three floors.

 

[hoshang]

What prompted you to do this?

Sorry for being too late to respond. Please look at the columns on gridlines D, E. They are as wide as the partitions (in my case the partitions are light-weight hollow concrete blocks) which are 200mm wide. How can I make these columns work in resisting gravity loads and a portion of seismic loads?

 

[hoshang]

Hi all,
any opinions would be highly appreciated.

 

[hoshang]

Any thoughts?

 

[BAretired]

My main thought is this: use walls only where it makes sense. Otherwise use columns. Cut down on story height where possible.

Grid Line 1: Use wall adjacent to the stair only. Otherwise, use columns. If Column D1 is not required, omit it. If you require a column at D1, make it the same as B1 and C1.

The left stair is going to expand in area in order to match the number of risers in the other stair. Wall may be used for length and width of stair.

Grid Line A: Use columns except adjacent to stair.

Grids D and E: Use wall full length to avoid projections.

 

[hoshang]

Hi BAretired,
do you mean these modifications?

 

[countervail]

Sorry but just so I understand the 20% requirement, is this fresh air? Where I do this we have a requirement for natural (not mechanical) ventilation with an opening area equal to 20% of the floor area.

 

[BAretired]

Hi BAretired,
do you mean these modifications?

Yes hoshang, that is precisely what I mean. But the left stair is going to be extended further North to provide for additional steps, so the wall on Grid Line A will also be extended. That will reduce the area of the open space next to Grid Line 2.

 

[BAretired]

Sorry but just so I understand the 20% requirement, is this fresh air? Where I do this we have a requirement for natural (not mechanical) ventilation with an opening area equal to 20% of the floor area.

I believe you nailed it. Natural ventilation is the intent, but it sacrifices a large area of floor space which could be used much more effectively for occupancy.

 

[hoshang]

Hi BAretired,
So, these walls will become part of vertical load resisting system. Which sections of the ACI code deal with these types of walls? Can they be considered as lateral load resisting system as some of them are located at building perimeter (in addition to their role in resisting vertical loads)?

 

[BAretired]

A wall can be both a bearing wall and a shear wall. I don't know all the ACI sections offhand; you will have to get a copy of the code and read it for yourself.

The plan below is the First Floor Plan. The left hand stair is not large enough to accommodate all of the steps in the right hand stair. I don't know whether it will expand to the North (dashed red line) or to the West (dashed green line). In either case, walls W1 and W2 should be extended as shown.

Walls W1, W3 and W4 may be adequate for North-South wind, but W2 is too far South to resist East-West wind. I have indicated two rigid frames, RF1 and RF2 which may be used for E-W wind, but rigid frame deflections are not the same as shear wall deflections, so there may be a torsion issue on the building as a whole unless you can add a rigid element in the wall parallel to Main Street. At the moment, that wall seems to be all windows.

Also, seismic events in your area should be considered; I cannot help you with that as the Province of Alberta, Canada is not in an active seismic zone. But other E-T members can probably help.

 

[hoshang]

but W2 is too far South to resist East-West wind. I have indicated two rigid frames, RF1 and RF2 which may be used for E-W wind, but rigid frame deflections are not the same as shear wall deflections, so there may be a torsion issue on the building as a whole unless you can add a rigid element in the wall parallel to Main Street. At the moment, that wall seems to be all windows.

So, are there other ways to deal with E-W wind (or lateral loads in general)?

 

[BAretired]

@ hoshang:

Typically, lateral loads are resisted by shear walls, cross bracing or rigid frames. There may be other ways of dealing with such loads, but those are the only ones which come to mind at this point in time.

You hail from a part of the world which I believe, is subject to severe seismic activity, and as I have mentioned, my experience with seismic design is virtually zero because my location has almost no seismic events. Hopefully, you will find someone on Eng-Tips or elsewhere who can offer better advice in that regard.

 

[hoshang]

Hi BAretired,
your W3 wall, doesn't it interfere with the door there?

 

[BAretired]

No, a door opening can be formed in W3. Perhaps I should have extended W3 to RF2, similar to your diagram.