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.

 

[hokie66]

And just why is that? Whether required for shear or not, a nominal amount of stirrups are normally provided in a footing beam such as pictured here.

The top bars should have hooks or cogs.

 

[pattontom]

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?

 

[ishvaaag]

The most common solution is to use hat (or omega) shaped bars to which the superior bars are tied, in number enough to ensure that the cage won't collapse under the forces it will be subject to, that in mats uses to include walking over it (even if on some distributing platform like planks etc). Of course, this adds some "stirrup" weight.

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.

 

[ishvaaag]

It is worth to note that either the stirrups in the hat bar may be inclined or additional inclined bars are used to provide lateral stability to the cage. In the first case the segments of the bar touching the bottom reinforcement may be laid perpendicular in plan to the bar atop the hat, to get a stable seat.

 

[WillisV]

Google Standees

 

[pattontom]

ishvaagg, are you talking of the cage collapsing before putting concrete or after it is filled with concrete? If already filled with concrete, how can the cage collapse? The concrete to be used is 4000 psi (or 28 mpa)

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.

 

[dik]

Standees are normally used and tied to the bottom reinforcing. Also add a couple of diagonal bars to keep the top bars from collapsing (tie wire as tension ties also works). The use of closed stirrups @ 4' o/c or so is also not uncommon.

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

 

[ishvaaag]

The collapse I refer to is at construction time. It is typical when precaution is not used to walk over the top rebar, bending them and damaging what should be a proper cover and mechanical arm of the longitudinal reinforcement. In worse cases if some inclined rebars are not included in the depth the top mesh could plie over the lower.

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.

 

[SteelPE]

Never ever gave any though to how a contractor would support the reinforcing. Means and methods it their problem, I have enough problems trying to comply with the building code. However, Willis V and dik are correct, they use Standees to support top reinforcing.... at least that is what my shop drawings say.

 

[BAretired]

pattontom, on a side note, your images on thread507-334406 have been removed by ImageShack. I hope you are using a different method for posting diagrams in this thread.

BA

 

[pattontom]

ishvaaag and SteelPE. My only concern of standees connecting the top bars to the bottom bars is if there would be unwanted reactions. Can you see any reactions going on between them such as flexures being affected when standees iron held between them or some shear developing from the standees?

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.

 

[hokie66]

You cannot make a blanket statement that shear reinforcement is not required because the beam in question is a footing beam. Shear may or may not be an issue, just in any other beam, but you have to check it.

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.

 

[BAretired]

pattontom,

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

 

[rjenyoy12]

Use "chair bars" as they call there. These are made of reinforcing bars and can be customized in different shapes depending on depth of your footing. These are also used to support top bars for mat foundations and to prevent sagging of bars during concrete pouring. Ask some who are working in big construction firms in your country, they know about this.

 

[hokie66]

The last comment by BAretired is the most important post in this thread. You are trying your best, but you are not competent to design or supervise this structure. There are many stories about engineers who tried to be "heroes", and most ended very badly.

 

[pattontom]

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

 

[dik]

Hokie... or like the recent demolition of a building in Vegas, where they had experts involved...

pattontom... thanks for the reserved reply...

Dik

 

[pattontom]

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.

 

[paddingtongreen]

Scroll down here to the fourteenth picture. Actually, there is more there. In these pictures, they show the bottom bars on concrete pads, I always specify that they use chairs for this.

Michael.
Timing has a lot to do with the outcome of a rain dance.