Plain concrete footing design
Plain concrete footing design
(OP)
Does anyone have an example of plain concrete wall footing design? I was going through ACI 318-05 and it doesn't seem that min. reinf. provision applies to footings. If my footing is 12" thick and 24" wide with 12" pier on top - shouldn't arch effect create compression and nearly eliminate tension at bottom allowing design for bending without reinforcement? I know I still have to put longitudinal reinf. bar at least at top of pier and bottom of footing. What about temperature and shrinkage reinf?
Thanks
Thanks






RE: Plain concrete footing design
RE: Plain concrete footing design
RE: Plain concrete footing design
RE: Plain concrete footing design
RE: Plain concrete footing design
RE: Plain concrete footing design
RE: Plain concrete footing design
RE: Plain concrete footing design
RE: Plain concrete footing design
I assume you mean 4.1 k-ft per ft of width, with 3000 psi concrete? Also, I know you don't mention it, but your calc bears out that you are ignoring the bottom (2) " of concrete per 22.4.8.
This seems like such a small moment. I know I learned this stuff in school, but has anyone ever really designed (and used) a plain concrete footing?
RE: Plain concrete footing design
WillisV, I do know some local codes have certain overrides (such as Chicago Building Code limits lintel deflections to L/600 or 0.3in, whichever is greater), but I couldn't find any in IBC 2003 (adopted code by the municipality with jurisdiction) regarding reinforcement in footings.
MYerges; yes, it is per foot of width and I reduced the section per 22.4.8. Moment of 4.1 kip-ft is the ultimate moment using 0.9 reduction factor for 4000 psi concrete, since tension in concrete governs. However, since I had room to play - I used 0.65 factor to obtain 3.4 kip-ft, because it could be argued that without reinforcement a brittle failure occurs, therefore requiring more reduction.
I think I will get an answer to this question, because I issued the drawings and calcs for city review. Final design was; plain concrete in footing (12" thick), but temperature and shrinkage reinf in foundation wall (13" thick). foundation bears @ 42" depth, so foundation wall is only 2.5 feet high. Thank you all for discussion, if i get comments from the city reviewer soon - I'll post them.
RE: Plain concrete footing design
RE: Plain concrete footing design
RE: Plain concrete footing design
kd = 2.53*SQRT(qs/SQRT(f'c))
and the thickness of the footing:
t=kd*overhang
The design manual also has a table of kd coefficients. The reference may predate 1971...
Dik
RE: Plain concrete footing design
RE: Plain concrete footing design
We've gotten a few calls from inspectors that are concerned we made a mistake. The design makes logical sense to me. I only check it if I have a project with bad soil and my wall footings need to be wide. We always reinforce spread footing however.
RE: Plain concrete footing design
Sometimes for taller wall (for basement) I still use no reinforcement in the footing because I assume the slab in the basement is going to restrick the bottom of the wall so that it will act like it was pinned at the bottom. So I design the basement wall like if it were pinned at the bottom (instead of fixed in most cases).
RE: Plain concrete footing design
If enough engineers throw reinforcing in the footings because it's cheap, then building inspectors and other engineers start believing it's a requirement.
RE: Plain concrete footing design
RE: Plain concrete footing design
Strut and tie model is a very rough approximation and is ONLY valid after the concrete cracks and stresses are redistributed. At low stresses (i.e. less than modulus of rupture) strain within the D-region (D-region is within the distance equal to height of member) such as brackets or footings where cantilevered part is equal or less to the footing thickness, St. Venant's principle is NOT valid, plains DO NOT remain plane and strain DOES NOT vary linearly with depth. Therefore, Final Element Analysis is required to find actual stresses. Assumption that strain function is linear is conservative, and, if stresses obtained by this linear analysis are less than allowable tensile stress in concrete - we do not need reinforcement.
Sorry for long reply. This is starting to turn into discussion on how much of theory, computational accuracy and computer aid we should use in everyday practice. We should start a new thread.
RE: Plain concrete footing design