Why provide negative reinforcement in footings? 2

[calculor]

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?

 

[whyun]

thank you for your compliments. i havent heard that in a long time.

trainguy. have you experienced as a young engineer when you find someone else's mistake (and it can be quite significant mistake) made by older contractor, architect, other engineer or a cad designer (draftsman in the days), they ACTUALLY respond by claiming they have been doing it that way for 20 years and had no problems with it. I heard that all too often and in my opinion, an educated person should be able to back up his position with logic. if you can't, give the young one the benefit of the doubt. if you're wrong, so be it. thank the young one for the lesson.

this field of ours is too vast, no one knows everything. we only remember what we've seen from previous projects. this is not rocket science or computer science.

worst thing about it is, we cant have disclaimer about not being liable from any losses resulting from the use of our product, like the computer software (including all structural analysis softwares).

ok... gotta go home now, have a nice weekend folks.

 

[trilinga]

Austim

I agree that the exact estimation of the wind/seismic loads is not possible. But I am sure that if you have not compromised in considering the site information/codal provisions to obtain the loads and their possible critical combinations during design, the minor change in their values during the lifetime of the structure or foundation will not cause havac.

I am also sure that we do not provide top reinforcement in footings for confinement of concrete or tieing the bolts. It is provided either to take top tension occuring during uplift or to cater for temperature stresses in thicker concrete sections.We can design the normal footings without top reinforcement. I think calculor's post also referes to a normal footing and not any special or exceptionally loaded footings.

Are we all in agreement?

 

[idly123]

well said that it was just a Normal footing. But do u think u have uplift pressures acting n normal footing. The problem of Uplift can it not also be given a special place.

The bottom line is the top reinforcemnt is provided from
1)temperature and shrinkage view point,
2)Such a detailing helps -In holding the column bars during concreng of the footing. It is really diificult to tie the column bars ( of length almost storey height +2m) just to bottom reinf. thus the top reinf helps in holding them
3)Such a provision provides enhanced dimensional stability with added confinement.

Uplift pressure...i doubt them...u dont always get to design reinforcement for tension at top fiblre in "NORMAL FOOTINGS"
Raj

 

[RiBeneke]

No-one seems to have mentioned bending moments on the footing (unless the seismic requirements imply bending moments - I am not familiar with the US seismic codes)

Most real-life footings have some degree of bending moment applied to them, even if the design assumption is a pinned connection to the structure above. When the bending moments applied to a footing exceed some moderate value they will cause a moment reversal in the footing slab and tension in the top face. This is another good reason to use top steel.

Regarding the use of J-bolts as holding-down bolts, I have read that they can suffer a progressive debonding and then pullout under conditions of cyclic fatigue loading. Therefore they are not recommended for anchoring rotating machinery etc. I dont know whether this issue has been adequately researched.

 

[austim]

Trilinga,

You may be right. But how long do we have to wait to find out just what calculor really meant?

 

[calculor]

Sorry took so long to get back. What I meant was exactly what trilinga stated on Oct 9 post. However I would also like to add this point....if a column was subjected to an uplift of say 50kips and the size of footing,overburden and slab etc perfectly balanced the uplift, I would not provide neg reinforcement. My reason is that most (not all) footings generally require minimum bottom reinforcement (.002xgross area), which a code requirement. Now if your footing is 500mm deep, two layers of reinforcement is ridiculous. The only time I think a top layer ( also minimum) is required when either the footing is > 750mm deep or the column is subject to some unusual loading or for seismic reasons.

 

[oijammu]

1)Suppose there is an accidental lateral knock on a stub column by heavy equipment,the top reinforcement should help in the restraining.
2)Shrinkage of concrete in thick footings should also be taken into consideration.How many times has any consultant taken the effort to dig out a footing just to see if there is any discripancy in his design.

 

[austim]

Calculor, welcome back.

Now another question - are you designing for ASD (Allowable stress design or Working stress design) for your design loads, or are you using LFD or LRFD?

If you are using LFD or LRFD, then I withdraw my comment about allowing for variation in the applied loads, since essentially the concerns that I would have should be met by a typical load combination of (for example) 0.9*Dead + 1.25 * wind.

However, if you are still designing on a 'working stress' basis, then I would still argue that you may not have sufficient allowance for variation of loads, since an increase in your uplift column load combined with a decrease in the column downward load could create significant negative bending (even without any coincident applied bending).

Note, the condition that I am thinking of is not the safety of the nominal 'working' condition, but the requirement to provide adequate reserve against an 'ultimate' load combination (either strength or stability), which may not be satisfied by simply using a factor of safety on nominal working stresses.

Adding appropriate top reinforcement could be needed to meet an ultimate load condition for strength in your footing, but of course would not have any effect on the stability factor.