why provide negative reinforcement in footings

huangyhg

why provide negative reinforcement in footings?
i'm curious what other engineers do when designing footings.  in our office the senior engineers usually always specify reinforcement in the top and bottom of footings when considering to uplift. regardless if the net effect is actually compression.i usually only specify bottom reinforcement if the net effect is compression.  is this reasonable or should i specify both top and bottom?
two reasons.  one is to guarantee that you have enough temperature and shrinkage steel.  the other is to remove any confusion of the contractor whether he should center the one layer of steel or place it on the bottom.  even an inept contractor knows that with two layers, one goes near the bottom and the other near the top.
anyone else have any?
calculor,
could you clarify just what you mean by "if the nett effect is actually compression"
if you are simply saying that you have a column base where the column is subject to uplift, but the total column_plus_ overburden load is downward, then the footing will be subject to negative bending, and your seniors are correct.  [for that case, the nett upward pressure under the base (although positive) would be less than the downwards overburden pressure - hence top surface tension]
  austim
you are right about the case of column uplift balanced by the foudation weight and overburden. negative reinforcement is essential in such case.
however, i feel that calculor does not mean that. we check for the gross bearing pressure (to be less than sbc) due to the total load at the footing base including the overburden and the footing weight. however, for design of the footing cross section, we calculate the bm and sf due to the net pressure which is gross pressure minus the pressure due to uniformly applied loads such as overburden and footing weight. (am i right calculor?)
if this net pressure is compressive, we get bm causing bottom tension only. we do not require top reinforcement for bending.  however, if the section is thicker(>750mm.), we provide nominal mesh of reinforcemet at the top to take care of temperature and shrinkage stresses.
for less thickness of footing, there is no need of top reinforcement.
hi,
there is also an added adavantage of having a top reinforcement for a footing of reasonable depth in that the concrete is confined between top and bottom mats, and side face reinforcments(if provided) and its resistance to compression and (shear??!!) is increased not from structural view point but from performance point of view. in terms of it being more confined as well as ductile.
thus when u detail a reinforcement for an isolated footing especially if column loads are relatively high u provide it in form a cage!!!...a specific typical case....may be something like a pile cap detailing....
but bottom line is enhanced compression resistance and ductility
hope this argument should also make some sense!!
may be ...iam not sure...i would appreciate if some one has any comments in this regard
hope it helps
regds
raj
calculor may correct me if i misinterpretted his original question.  i believe he is asking whether engineers specify top and bottom rebars for seismic column footings regardless of whether there is a net uplift or not.
more often than not, the downward force on a seismic footing due to proper load combination is larger than the maximum uplift on the same footing, thereby requiring, technically, less rebar at the top (sometiems none).
but it is common practice here in california to design the bottom bars and call out same number of bars at the top as well.  this leaves less room for contractor to make a mistake.  and it makes it obvious which footings are seismic and which are gravity only.
i think everyone has had good points so far.  to add another from the practical standpoint, ya gotta tie the anchor bolts to something. it might as well be steel that you leave in place.
jim
jimparks,
do you mean the anchors from the columns be tied to the bottom bars with a standard hook?
in california (state projects and perhaps some local jurisdictions), the use of j bolts is prohibited.  they recommend a headed bolt.  sometimes, i elect to use uplift plates.
any comments on requirements from other parts of the us will be appreciated.
whyun,
do you know what the reasoning is for not allowing j bolts??
this isn't an area i work in often but i'm curious to hear the reasoning!!
---
andrew
i suppose they do not want a bolt heavily loaded in tension to "straighten-out".  as it straightens, the concrete will crush near the inner radius of the bend.
i am not into testing or research so i am not sure whether this is a valid claim, but state of california thinks so.
with a headed bolt, supposedly, you can more reliably expect a cleaner shear cone.
for multiple bolts in a seismic column, often i tie the bolts together by using an uplift plate at the bottom using double nuts.
jedclampett, whyun,
ha ha that's a good one - that you detail top and bottom mats because contractors wouldn't know where to put a single mat.