How to support top bars in combined footings? 1

[pattontom]

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.

 

[JacksPanic]

In south africa we have standard shape codes for rebar. The one we use to keep top rebar up is called a stool and is shape code 83. Do you have national standards which you must adhere to in your country? I suggest you study these as it seems that from your recent posts you have a lot of studying to do. I wish you good luck!

 

[hokie66]

pattontom, yes, there would "normally" be transverse bars in the top, if only to tie the longitudinal bars together and aid in support. You don't want to have to chair each individual bar.

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.

 

[pattontom]

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.

 

[hokie66]

In my 40+ year career as a structural engineer, I never designed a structure, and then had no further say in the construction. If location is an issue, then a local office of the same firm, or else a vetted office of a peer firm, should be engaged for the construction oversight. When this is not done, and everything is left to the builder, disaster awaits. If the "big time engineers" on your project say they are not experienced in construction, someone is lying to you.

Combined footings are not complicated or a new idea...they are just beams cast on the ground.

 

[pattontom]

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.

 

[dhengr]

Pattontom:
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.

 

[dik]

Hokie... correct... I thought it was under the block in July... still standing until at least 2014... or beyond...

Notwithstanding, it was designed, inspected and constructed by 'experts'...

Dik

 

[hokie66]

Dik,
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.

 

[pattontom]

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?

 

[dhengr]

“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?”

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.

 

[BAretired]

The cage must be strong enough to support the concrete crew stomping around on it like a herd of elephants, so I would expect, in addition to what is shown, a number of standees would be required to support the top bars.

BA

 

[TXStructural]

The industry practice is to use standees and carrier bars as needed. It is not typical for the design engineer to be involved in such decisions except in very large footings, such as those in deep mat foundations. Increasingly, many rebar cages are being tied on site and lifted into place by crane. In all cases, the placers and construction engineers need to be aware that tied cages behave more like a loose bundle than connected bars. If not braced sufficiently, they will shift and bend, and people can be seriously injured if they are not handled and supported properly. Once in the hole, side-form spacers will be used to maintain side cover and prevent lateral movement.

This is where the value of using experienced detailers, fabricators, and placing contractors really come out.

 

[pattontom]

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?

 

[fcu45]

Dear pattontom this is pretty simple and basic thing so don't complicate your life.
It doesn't need all this descussion.
Good luck

 

[pattontom]

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.