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calculor (Structural) (OP)
8 Oct 02 19:59
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?
JedClampett (Structural)
9 Oct 02 1:14
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?
Helpful Member!  austim (Structural)
9 Oct 02 1:53
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]
Helpful Member!  trilinga (Civil/Environmental)
9 Oct 02 4:11
  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.
idly123 (Structural)
9 Oct 02 11:04

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

whyun (Structural)
9 Oct 02 12:27
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.
JimParks (Civil/Environmental)
9 Oct 02 13:31
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
whyun (Structural)
9 Oct 02 14:49
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.
DMWWEngr (Structural)
9 Oct 02 17:40
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

whyun (Structural)
9 Oct 02 20:26
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.
krd (Civil)
10 Oct 02 10:44
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.
whyun (Structural)
10 Oct 02 11:45
It sounds like a joke but true in practice.  On a big projects, how often do you see 100 diffetrent sizes of pad footings.  Many cases, you may see 5 different footings on the footing schedule.  Some will be close, some will be overdesigned by a bit.

I meant no disrespect to the contractors by any means.  I go by the old KISS rule.  Keep it Simple/Stupid.
JimParks (Civil/Environmental)
10 Oct 02 12:18
Whyun,

What I meant was that by using top steel, you can secure the anchor bolts in place by tying them to both mats of steel.  Whether the bolts are headed, J, or have an end plate retrained by a hex bolt, you still have to do something to keep them all in place and plumb during the concrete placement.  It takes just about as much effort (and makes finishing the top, if applicable, harder)to build a wood frame spanning the footing or some using other means to keep the bolts plumb.  Then you have to take the frame apart anyway.

Jim

whyun (Structural)
10 Oct 02 13:54
JimParks,

Now I understand what you meant.  Securing the "hanging" bolts to the top and bottom mat rebars so they stay put during concrete pour.  Yes, I do think that is prudent.  But engineers can not rely on those ties for resisting tension.

Thanks for the clarification.
DaveAtkins (Structural)
10 Oct 02 14:05
It is correct to consider wind uplift; in a building with a light weight roof it will control the size of the footing.  However, I never ask for top bars, because a plain (unreinforced) footing is generally adequate for the uplift case.
inspectorjeff (Structural)
10 Oct 02 19:09
Anchor bolts???  Do the bolts have to be hooked to steel and what are the increased values for doing so?
For example...1/2" bolts:: 2195 based on bond strength,
2105 based on steel strength.(Simpson)
now if the bolt was hooked to bond beam steel then I assume that the design loads will be determined by the mechanical bond between the anchor and rebar, and its subsequent shear cone.
Any ideas on the subject?

Inspector Jeff in Florida

whyun (Structural)
10 Oct 02 19:15
Hello Dave,

The type of conditions I had in mind were pad footings at the frame columns for braced frames (OCBF, SCBF, EBF, etc) or moment frames (SMRF typically).  99% of the time, lateral forces on a structure is controlled by seismic and often we encounter heavy uplift.

Regardless of the magnitude of uplift, top steel will experience much smaller tensile stress compared to the bottom steel.  We can control the thickness of the footing to avoid having any tension at all in which case top bar may be omitted altogether.

Common practice I see in engineers specifying same top and bottom steel perhaps is due to lazinedd on the engineer.  More often than not, engineers still tend to call out top and bottom steel for seismic columns, regardless of need.



austim (Structural)
10 Oct 02 22:03
Just another thought.  (Which might have been avoided if we had heard from calculor again to clarify the original post ?)

If calculor initially meant net compression in the column which would theoretically cause no negative bending in the footing, there could still be an argument for providing top steel (in addition to the good reasons proposed above).  

That is that when the 'nett compression' results from a combination of positive and negative column loads of similar values.  If our calcs say 100 down and 90 up, then simple engineering prudence would consider the effect of inaccuracies in our assumptions or analyses.  (How often can anyone tell precisely what the dead, seismic and wind loads in any single column will be ?). eg has foundation settlement settlement been included in the analyses, and how accurately can anyone really forecast that ?

Relatively minor variations in each of the loads could change the direction of the nett load, leaving you to rely on unreinforced concrete (possibly with shrinkage cracks) in tension.



whyun (Structural)
10 Oct 02 23:00
How often can anyone tell precisely what the dead, seismic and wind loads in any single column will be?

this is really great!  no one really knows for sure.  as engineers, all we can do is to determine our best prediction of what combined forces each members will experience in its lifetime and design everything strong enough to resist such forces.

often, i find engineers fall into the trap of believing that the analysis results are really accurate and i find myself chuckling.  5% overstress doesnt mean failure.  even 10% overstress (in my opinion) should be acceptable realistically (although i wouldnt design everything this way).

all types of building codes have load factors and strength reduction factors to justify this.
trainguy (Structural)
11 Oct 02 16:14
Your attitude, Whyun, is clearly that of an experienced engineer.  Have you noticed that as the years progress, esp those first 10 yrs as a structural engineer, that your tendency to over-design has progressively decreased?
You should see the absolute monsters I designed in the 1st 2 years of practice...
Needless to say, contractors/estimators didn't like me back then.  
whyun (Structural)
11 Oct 02 21:54
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 (Civil/Environmental)
11 Oct 02 23:04
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 (Structural)
12 Oct 02 5:04
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 (Structural)
12 Oct 02 9:35
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 (Structural)
13 Oct 02 7:02
Trilinga,

You may be right.  But how long do we have to wait to find out just what calculor really meant?
calculor (Structural) (OP)
14 Oct 02 0:13
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 (Civil/Environmental)
14 Oct 02 3:55
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 (Structural)
14 Oct 02 5:42
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.

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