Plain concrete wall footings-4 story building
Plain concrete wall footings-4 story building
(OP)
4 story precast building w/ foot print of about 200 ftx70 ft
Seismic loads govern with SDC = B (project site is just outside Detroit so low seismic activity area). Allowable soil capacity = 4ksf.
The client has asked for an option of plain concrete footings. The typical D+L load on foundations range from 16 klf to 23 klf. Lateral loads do not result in any net uplift and the effect of downward force (distributed over a larger area based on the footing thickness) also do no result in forces larger then D+L.
The typ exterior footing size I am using is 3'Dx6'-6" wide. I am checking factored moment and shear at the center line of the footing (conservative) using chap 22 of ACI-318. Vc = 4/3xsqrt f'c and Mn = 5 x sqrt f'c x S. Phi = 0.55. I am neglecting 6" of footing thickness for my S calc.
The numbers seem to work for the given loads and even though the footing is designed as plain, I am going to place 3 #5s T&B and also #5 vert dowels @ 48" oc.
I haven't designed a plain footing for a structure this magnitude so I wanted to get a second opinion if I was missing anything. From what I can understand from IBC-06 & ACI 318-05, I can use a plain wall footing for a structure in SDC-B.
Seismic loads govern with SDC = B (project site is just outside Detroit so low seismic activity area). Allowable soil capacity = 4ksf.
The client has asked for an option of plain concrete footings. The typical D+L load on foundations range from 16 klf to 23 klf. Lateral loads do not result in any net uplift and the effect of downward force (distributed over a larger area based on the footing thickness) also do no result in forces larger then D+L.
The typ exterior footing size I am using is 3'Dx6'-6" wide. I am checking factored moment and shear at the center line of the footing (conservative) using chap 22 of ACI-318. Vc = 4/3xsqrt f'c and Mn = 5 x sqrt f'c x S. Phi = 0.55. I am neglecting 6" of footing thickness for my S calc.
The numbers seem to work for the given loads and even though the footing is designed as plain, I am going to place 3 #5s T&B and also #5 vert dowels @ 48" oc.
I haven't designed a plain footing for a structure this magnitude so I wanted to get a second opinion if I was missing anything. From what I can understand from IBC-06 & ACI 318-05, I can use a plain wall footing for a structure in SDC-B.






RE: Plain concrete wall footings-4 story building
RE: Plain concrete wall footings-4 story building
For a masonry wall or a poured in place wall system the wall becomes a part of the structural system because the masonry wall or concrete wall is bonded and creates a deep beam that is far stiffer than the footing.
Since you are dealing with precast walls, you have the traditional and recurring problem of connection and load distribution to the footings.
The important factor is if your connection between the wall and footing will allow the interaction that you get with site connections.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Plain concrete wall footings-4 story building
Thanks for that reminder. I actually was going to put #5 @ 16" oc horz only along the bottom of the footing.
RE: Plain concrete wall footings-4 story building
RE: Plain concrete wall footings-4 story building
RE: Plain concrete wall footings-4 story building
RE: Plain concrete wall footings-4 story building
Its a project funded by the state. Our understanding is that someone at the state is trying to justify their salary.
RE: Plain concrete wall footings-4 story building
RE: Plain concrete wall footings-4 story building
If you have a 36" deep footing, and the numbers work, transverse rebar isn't required - so why negate the advantage of a plain footing by adding rebar?
If you are going to add rebar to "feel good" about the design - design the thing as a reinforced footing and reduce the thickness per hokie66's suggestion above.
RE: Plain concrete wall footings-4 story building
Detroit? Hey maybe the Ambassador Bridge owners can help you out now that they are out of jail. They seem to have some political ties.
PS - please read this sarcastically.
RE: Plain concrete wall footings-4 story building
Thanks. The client has asked for Plain concrete and thats what we am going to provide. And indeed the rebar I mentioned is a feel-good hunch but the footing with this reinf is no where near a reinf design. Also, the only bar they'll have to set before the pour is the bottom rebar. I have seen them "muck in" the top rebar and dowels. Plus I think the cost of rebar that I am adding is minimal compared to the overall structure.
But now I have another question. ACI 318 section 1909.3 talks about contraction/isolation joints. Does this apply to trench footings too?
RE: Plain concrete wall footings-4 story building
They were give a preliminary design with plain and reif options. The opted for plain.
RE: Plain concrete wall footings-4 story building
One thing worse than someone spending their money foolishly is someone spending other people's money foolishly. I just had to spout off.
For a smaller footing, plain might make sense, but a 6'-6" wide strip with excavation costs for the added depth, I can't help but feel sorry for people in Michigan (even with their voting record).
RE: Plain concrete wall footings-4 story building
They are trying to save the steel for future cars they'll never build, with the same thinking.
I think you meant 'please read this as a bit of my sarcasm.' I tried to 'read it sarcastically' and all I could find was honesty and truth and sometimes that hurts those deserving of it.