How to support top bars in combined footings?
How to support top bars in combined footings?
(OP)

Today I was discussing with the contractor engineers of how to support the top bars (longitudinal) in the combined footings like shown above. They haven't encountered anything like it in their years of constructions. This is because in normal footings.. only the bottom bars exist. Now for the combined footings like the above. How do you suspend the top bars? What methods or techniques do you use? Please describe what is the standard use in such because we will construct it next week and we want to be sure the way the tops bars are suspended is optimum. Remember combined footings don't have any stirrups like in beams where you can tie the top bars with small wires. Thanks.






RE: How to support top bars in combined footings?
The top bars should have hooks or cogs.
RE: How to support top bars in combined footings?
Beams have many stirrups at certain distances to avoid shear failure like in following.
But in combined footings.. There are no stirrups like the above. Unless you are saying then that it needs stirrups like above? anyone has actually designed combined footings that use stirrups like above?
RE: How to support top bars in combined footings?
By the way at the ends of your longitudinal bars either a hook or spliced C shaped rebar seems to be lacking. In some situations it may be critical, and it is always something good to have.
RE: How to support top bars in combined footings?
RE: How to support top bars in combined footings?
RE: How to support top bars in combined footings?
do you know the formula when non-linear settlement occurs or the threshold. The theory is that when footing is light.. the soil influence would only be shallow.. when the footing becomes massive, the soil influence would go much deeper and there would be greater settlements. Do you consider a combined footing size of 3 meters width by 12 meters length and 0.7 meters depth big footing or just normal size or still manageable? Our experience is only dealing with 3 meter by 3 meter spread footing. We haven't used combined footings before so it looks monstrous to us.
RE: How to support top bars in combined footings?
With the large bar sizes, you may want to look at anchorage and possibly use bars hooked one end and alternate them... you may be able to shorten them a tad (even the amount of the hook).
Dik
RE: How to support top bars in combined footings?
Respect the depth of influence that I remember the lesser width is usually considered for that of importance. Of course there will be some difference, but respect the action of competent foundations I learnt by making some worksheets to expound the effects of a variety of loads on elastic half spaces that what is important is (for average soils) the magnitude and position of the loads, far more than the foundation type and geometrical disposition itself. This is mainly because the average effect of the loads once passed to the ground is as an average the same, and so the settlements, except significants soil irregularities, or upper structural stiffness make the things become different.
RE: How to support top bars in combined footings?
RE: How to support top bars in combined footings?
BA
RE: How to support top bars in combined footings?
Btw... in footings.. shear stirrups not needed because the only loads acting is from the soil and from the column. There is no upside down gravity attracting the combined footing beam from above. So no need for regular stirrups. If there is sufficient depth, the shear is taken care of. Therefore the function of standees is simply to make the top bars stand, right.
RE: How to support top bars in combined footings?
I have no idea what you are talking about with "unwanted reactions" or "shear developing from the standees". That is nonsense, as the standees are just bar supports until the concrete hardens.
What you should be aware of in casting an element like this is the possibility/probability of plactic settlement cracking along the bars. This can be addressed by revibration after an appropriate waiting time.
RE: How to support top bars in combined footings?
The nature of your comments and questions in this thread and others suggests very strongly to me that you need to discuss the design of this building with a local structural engineer experienced in this type of work before proceeding with foundation construction as early as next week. To do otherwise is just plain foolhardy.
It is far better to delay the schedule a week or two than to start off on the wrong foot with a bad foundation. Please, get help...you need it.
BA
RE: How to support top bars in combined footings?
RE: How to support top bars in combined footings?
RE: How to support top bars in combined footings?
hokie66 and others,
As I have mentioned before. I didn't do the project and I experts handle it. I'm just middleman between them and client. They have designed over a dozen 30 storey buildings which were actually constructed and they are experts in mat foundation. Unfortunately. They rely entirely on software and forgot how to manually calculate (as they don't have time to manually calculate each of the 30 storey thousand members). My task is to look at their combined footing design at another side and see where they could have missed. Just double checking. Also because they are big time engineers. I can't communicate to them directly. But I asked a while ago. They suggested chair support and they said it's the contractor problem. I'm the connection between the contractor and client. I'll ask more details from them tomorrow. Thanks for the bit of help you guys have shared. Also I found the following article very useful (where the above picture comes from).
http://www.nehrp.gov/pdf/nistgcr12-917-22.pdf
RE: How to support top bars in combined footings?
pattontom... thanks for the reserved reply...
Dik
RE: How to support top bars in combined footings?
Please check out the above combined footings rebars details (slow to load). I was asking the engineer in charged how much was the axial load in each column. He didn't know because it is the program called SAFE that produces all details which are imported from ETABS and they have about 10 engineers verifying it. Anyway. The above is located at rear of lot. I'm estimating the axial loads of the side columns are only about 500 Kn and the center only about 700 Kn because it is only 3-storey. Actually 2.5 meter by 2.5 meter footing is more than enough for each. But because they are located eccentrically and for maximum seismic resistance. The combined footings were made larger a bit. No problem with that. Now my concern is, has anyone encountered combined footings like these before? The top has both longitudinal and transverse bars. Normally in combined footing, the top only has longitudinal bar (the first image in this thread is just example from articles I saw.. I didn't say it is ours). Now they have top tranverse bars. Do you think these are overkill or just basic that you'd also do? For those who don't understand the drawings. The blue are bars, the red are extent of its reach or location.
RE: How to support top bars in combined footings?
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: How to support top bars in combined footings?
RE: How to support top bars in combined footings?
I think we had the mistaken idea that you were designing the building, along with some other engineers in your office. If you are just trying to build this structure, which has been designed by "big time engineers" who are not adequately involved in the construction, then this is indeed a recipe for disaster.
Dik, the Harmon Hotel Tower has yet to be demolished. Still tied up in the courts and arguments over whether or not it needs to come down.
RE: How to support top bars in combined footings?
hokie66, why.. structural engineers are not connected with the actual constructions. Are you for each of your project? We make and sign and seal plans. It is up to the contractors to build them. I'm coordinating and asking with the engineers who designed it the best way it can be build. But they are not experienced in construction. Your idea of tying up the top transverse and longitudinal bars is a good idea. I will talk with the more senior construction engineers who have experienced with mat foundation. It's just that for some engineers who only have experienced with spread footings. Combined footings are new idea. So I need to let them change construction engineers who have experienced in the latter.
RE: How to support top bars in combined footings?
Combined footings are not complicated or a new idea...they are just beams cast on the ground.
RE: How to support top bars in combined footings?
Oh yes. They will of course still have further say on the contruction. It's just that they won't be working full time on the project. And the thing is combined footings are new to me and my assigned construction engineer.. so I'll just let them change construction engineer. Remember I approved the combined footing idea because of our previous discussions in other threads where you guys stated that eccentric columns at edge need combined footings to work property instead of isolated spread footings. Now I'm just getting familiarized with beams on grounds. There are just many engineers who have never worked on this even after 20 years on the job. So this is new to many of them and some of us. Anyway. I'm very familiar with special moments frame on beams and columns. But how many of us have heard of special moment frames on foundations and stuff. We are familiar with the concept of strong columns, weak beams and ductility. But how many of us have heard of strong foundation, weak column concept and foundation ductility. We who have mostly spent time on spread footings are just not familiar with such.
RE: How to support top bars in combined footings?
BA, Hokie and others have really been very patient with you and they have given you good sound advice, and good explanations of what they are trying to tell you. They are smart Structural Engineers, very good at their profession, and at explaining structural concepts, so reread what they say several times, and think about it, so you get the full meaning out of it. Another lesson you might take from your various posts is that it rarely benefits anyone when they are not forthcoming, right from the start, about their true position on the design and construction team. If you explain your real educational background, knowledge and experience level and what you are actually doing, we all know much better how to interact with you, and at what level to start our explanations. I think what perplexes most of us, is the difficulty you seem to be having with the concept of the combined footings. It is not a new or novel concept, and despite your protestations to the contrary, this may be an indicator of a serious lack of design and construction experience on your part. If you have selected a qualified contractor to do that building, he should not have any trouble building these footings.
Except for their proportions, the combined footings that we have been talking about with three columns, are exactly the same as a continuous beam on top of the columns, just tipped up-side-down. A couple other exceptions; the loading (upward from the soil) varies slightly along its length being a max. under the columns, look up beams on elastic foundations; and because of the continuous footing the two end columns do not have to carry as much moment to overcome the eccentricity that you first had with the eccentric simple spread footings. You will very likely need some shear reinforcing in these combined (continuous) footings, you have to check that and treat it just like you would in your upper moment frame beams. Except, you don’t use “U” shaped stirrups in this case, you use closed ties. These serve two purposes, they are your shear reinforcing, and they support your top longitudinal rebars, which you have been asking about.
RE: How to support top bars in combined footings?
Notwithstanding, it was designed, inspected and constructed by 'experts'...
Dik
RE: How to support top bars in combined footings?
It seems to have been designed by a well regarded, very large consulting firm, albeit perhaps by a satellite office of that firm. But apparently, the inspection during construction was assigned primarily to people with little experience in major structures, and who were not employees of or contracted to the design firm. The builder is large and should have 'experts' of its own in interpreting and implementing construction documents, but very important structural elements got built incorrectly, for many floors. The problem was only discovered on a "walk-through" visit by a senior engineer of the design firm, and by then the structure was well advanced, to about half the original designed height.
RE: How to support top bars in combined footings?
I talked to the contractor team at length today. I asked how the top bars would be connected to the bottom bars. They said what they do is to extend each transverse and longitudinal bars to the sides and tie up each one of them to create super strong cage for the combined footings (they drew it above). Any comment of it? How come no one amongst you adviced it. Is it because it's more costly or do you think it can cause some unwanted reactions?
RE: How to support top bars in combined footings?
That sketch is essentially what we have all been proposing and/or assuming would be done. Again, you just didn’t read carefully enough, or with enough engineering experience, to read between the lines. It isn’t particularly more costly, it is what has to be done to do it right. The exact rebar sizes and spacings in each direction and at each significant beam/footing location must be designed by the Structural Engineer.
RE: How to support top bars in combined footings?
BA
RE: How to support top bars in combined footings?
This is where the value of using experienced detailers, fabricators, and placing contractors really come out.
RE: How to support top bars in combined footings?
I wonder if I can let the contractors weld all the sides so they would be act like 3 meter width full fledge stirrups. The suggestion was just to tie up all the sides with small wires to hold the top bars. Remember in combined footings transverse bars on top was put to take care of moments in the transverse direction... not really as shear reinforcement (which was not necessary because of the deep depth in this particular design). Anyone know what would happen if welding the traverse bars up and down turn them into giant stirrups?? Any unexpected reactions anyone can think of?
RE: How to support top bars in combined footings?
It doesn't need all this descussion.
Good luck
RE: How to support top bars in combined footings?
97% of structural engineers I met haven't experienced doing any combined footings more than 3 meters and most use eccentric footing without making the column stiff enough to handle it so they are not really designed for seismic performance. The reason is cost. If they won't make the eccentric footing and small column. No architects would hire them. And combined footings to compensate for eccentric column is much more expensive because it is designed like big beams so this is rarely used here. I only convinced the client and structural team to go for it after being convinced (and acquired confidence in it) by you guys that it is the way to go. So thanks for all the help.