huangyhg

how to place minimum steel in a rectangular footing?
i have got a question about the placement of minimum temperature and shrinkage steel, say for a 35" thick rectangular footing 14 feet long and 5 feet wide. let's say, that all i need for my footing is minimum temperature and shrinkage steel.
hence, ast = 0.0018bh = 0.0018 * (14*12) in * 35 in = 10.58 in2.
say, i am using #7 bars, thus, i will need 10.58 in2/0.60 in2 = 18 # 7 bars.
method 1: or, say i place bars in the top and bottom layers. for my bottom layer, do i need to place 14 #7 parallel to the short side, and 4 # 7 parallel to the long side. and do the same for the top layer.
or, method 2: i still place bars in both the top and bottom layer. for the reinforcment parallel to the do i just need to place a total of 18 #7 in both the top and bottom layers, say 9#7 in the bottom layer, and 9#7 in the top layer.
for method 2, the ast required parallel to the long side will be 0.0018 * (5*12) in * 35 in = 3.78 in2, or 7 # 7 bars. do i place a total of 7#7 bars in both the top and bottom layers, say 4#7 in the bottom layer, and 3#7 in the top layer.
method 3. not to confuse matters, but can i place reinforcement in the bottom layer alone, say a total of 18# 7 bars, say 14#7 parallel to the short side, and 4#7 parallel to the long side.
please advise whether method 1 or method 2 is more appropriate. and is method 3 practical?
for a two-way rectangular footing, you need to put more steel under the column - see aci 318-05 14.4.4.
also, there is some difference of opinion on this matter, but i'll give you mine and let others chime in as they see fit.
aci states that any section in which reinforcement is required by analysis (i.e. the plain concrete section doesn't work), tensile reinforcement not less than 0.0018bh shall be provided. tensile reinforcement, to me, means at the tension face, not distributed between the two faces. that is also in each direction, not to be split between the two directions.
i don't think either method is appropriate.
also, a 35" footing is pretty thick. i'm guessing there is uplift on the column or maybe the column base is fixed and there is moment being taken into the footing. i don't believe that i would use a single layer for a 35" thick footing (even without uplift or moment), and i would use 0.0018bh on each face in each direction while maintaining the appropriate distribution per 15.4.4.
structuraleit, thank you so much for your prompt response, as always.
btw, there is not section 14.4.4 in aci 318-05. section 14.4 is on page 239. did you mean section 14.3.4?
my 35" footing is for illustrative purpose for this question only.
what would be your best solution for the question posed above?
minimum in tension face, i.e. all in bottom or all in top (or both to keep it simple). shrinkage and thermal in both faces as required. all in both directions.
thats how i see it.
my apologies, i meant 15.4.4 (i noted it right at the end of the post and botched it at the beginning).
i would provide (18) #7 in the short direction on the top and bottom face. be sure to check development length of the bars, they may need to be hooked or use a smaller bar. 2' or so isn't much to develop a #7. i would probably get away from the top t&s steel (as long as it isn't required by analysis) for anything less than, say 24" thick.
for the long direction bars, i would provide (7)#7 on the top and bottom face. again, check development, though it's likely not a problem for this direction. again, i would probably get away from the top steel (unless required by analysis) for a thickness less than, say 24".
i would also meet the distribution requirements in 15.4.4.
method 1 is incorrect. minimum steel should not be divided into perpendicular directions.
i would use method 3 unless there is some reason you need top steel.
daveatkins
dave-
method 3 is splitting the total in the two perpendicular directions, just doing it for one face only.
just out of curiosity - where do you draw the line as to where you want a top layer even if it's not needed for strength?
as per the current spanish code cte, 0, since one still can make mass footings. cte clearly states that structural concrete can be mass concrete; provides tensile and flexural design strengths, and vc contribution to shear (or if not you can state atechnical correct one); no other element in a structure can be more easily targeted to be mass concrete, and there are still current examples out there giving example of this kind of structure. one must conclude the minimum flexural steel in this type of structure is 0.
this said, i have never made one without reinforcement in 32 years of practice. but i may, who knows.
structuraleit,
you are correct--my mistake. you need minimum steel in each direction.
i might ask for top steel in a very thick pour.
daveatkins
good, and interesting, thread. cudos to structuraleit for a very good treatment of the subject...
i'll just add my personal view on the addition of top steel / covering steel when not needed for strength: i do this routinely for extreeme exposures. ie: if potential cracking will lead to ingress of harmful liquids (salts, corrosives, etc) or if the surface is to be visible (mass concrete retaining wall for a hospital i did comes to mind), i include a minimum of 0.00233 steel at the exposure standard stipulated cover (typically 30, 25, or 40mm for new zealand).
normally welded reinforcing matt is sufficient for this purpose, and not too difficult to place. you do get some complaints from the contractors, but usually pointing out that in a structure for an aggresive environment the choices are a little more steel or increased inspections, the contractor ops for the steel. oh, and one more thing: i often ask for galv, or passively protected systems, when the environment is really aggressive. just my thouths/sop.
cheers,
ys
b.eng (carleton), p.eng (ontario), mipenz (structural-new zealand)
working in canada, and missing my adoptive new zealand family... at least i brought the little kiwi with me!
i agree that if minimum steel controls, you need 7-#7 in the long direction, 18-#7 in the short direction. if this is a column which only sees gravity load and not uplift, i would only use bottom steel. i wouldn't use top steel in a footing unless required by analysis. when you do use top steel in a deep footing, the concrete usually requires revibration to prevent plastic settlement cracking.
it is unfortunate that codes continue to refer to t&s minimums when minimum flexural steel is required.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]