Stirrups in Foundation

[pattontom]

When the shear beam span to depth ratio is less than 2.5. It's supposed to be deep beams. Yet we still use stirrups in above column deep beams (RC). Okay. This is what perflexed me. In combined footings and even normal isolated spread footings. We don't use stirrups because we can increase the depths. But has anyone designed normal beams above columns where you don't use stirrups on the beams? What kinds of beams that don't use stirrups? I want to understand the differences and similarites between normal RC beams and foundation beams (Combined footings which function like beams too). Thanks.

 

[ishvaaag]

If you get a concrete handbook of the sixties and before it is likely that it will provide equations for the flexo-tensile design of concrete. You may even can find them today in CRC Structural Engineering Handbook and the ACI standards for plain concrete.

In those of the sixties service levels solicitations were checked against allowable flexo-tensile stresses in the concrete. Even then the danger of the fragile failure was acknowledged and so preferred solutions were reinforced to forfeit such fragility.

So in that era it was common to see mass concrete works in mass retaining walls, mass concrete dams, some relatively massive arch bridges or arches, and mass footings. Mass footings you can see designed even today sometimes, normally where significant depth is to be reached and a cheap concrete can still be used for the case.

The design books of the era made clear to which stresses the plain concrete footings could go; they were moderate and the resulting footings quite deep, what today (that one can still design plain concrete items such foundations) makes of them an implausible election, since concrete of standard quality is no longer cheap and less deep footings are preferable.

The difference between deep transfer beams and conventional deep footings is that the deep beams being thinner should show, comparatively, bigger tensile and shear stresses; so invariably a reinforced scheme was preferred I can't say from whence but surely after some chunks falling of unreinforced ones, from spalling under one so evidenced arch action.

Other than in foundations, some slabs on the ground, and dams, the realm of plain concrete is met today more than anything in prefabricated items of moderate size; where quality is required specialty mortars or concrete are used to ensure integrity against the tensile stresses. Other than that, most structural designers choose reinforced concrete.

Respect your final question it is only the ability to met the shear stresses and the width of the combined footings that may cause some differences in the reinforcements; for if stirrups can be shown unneeded they would be an unwelcome task to do for the width; whereas the width can require some transversal steel to contain transversal tensile stresses that for the narrower foundation beams the horizontal branches of stirrups or even the concrete itself can deal with.

 

[pattontom]

Hi, thanks for the comments. But one thing I'm thinking is the weight of the concrete itself. In beams above columns (near ceiling). Gravity or sole weight of the concrete can cause shear stress (by the downward force). In combined footings. There is no opposite gravity pulling the footing upward. So maybe this can cause less shear? Do you agree with this analysis? Or do you just ignore the gravity and weight factor of the concrete itself? And why?

 

[ishvaaag]

An expert designer may confidently disregard things that he knows won't cause problems. Even if that is true, to question things seeking how new viewpoints may evidence the need to change something may be interesting to those wanting to better the current procedures. However, these distractions maybe not precisely a plus for a practical designer that has little time for speculation.

So, investigation is always interesting, but it has a problem: cost. A cost that is not normally (reasonably) bearable by 1 person (nor may he have or master the necessary tools for that), and so one needs to gather the necessary amount of interest to gather the funds (or required dedication). Plus, wherever in Earth, the regulatory bodies around construction constitute all them a system respect what your question may become an immediate convenience or, momentarily at least (because scarcely practicable) something that they can entirely disregard.

Respect the particular question, so, of proper consideration of own weight, well, the answer is obvious; where it is; and if it at suspended slabs is a significant contributor to shear and not in mats or beams directly supported on the ground, one simply acknowledges the reality of the facts and proceeds.

But it is VERY important to understand that in construction we are not at a level of precision in which we can predict with extreme exactitude the final level of loading our structures can attain. So by just some fine tuning one can even within the code argue one has code compliance when before a grosser check showed a small defect in that.

You may elect your position in the field, but one north american colleague once stated that "What for some is a barbarity, for others it is just enough" and both extremes show case per case to be true, for the good, when much critiziced concepts perform well, or, for the bad, when something fails: it was not even enough, see the failure of the ceilings in tunnels being conversed about in other thread of this forum. So following sound expert advice, and condensed knowledge in books and codes is a more commendable way for most than just personal speculation.

 

[ukengineer58]

Stirrups are for shear. They also maintain the bending capacity of the section by stopping Propogation of diagonal tension cracks (shear) losing bond at the underside of the tension steel. This is caused by the tension steel pushing out on the concrete face and causing cracks. The stirrups stop these cracks propagating. Without stirrups you can actually lose bending resistance due to this effect. In a suspended beam lack of stirrups could lead to a sudden non ductile failure. I.e no warning. This Was a problem decades ago where there were failures. This effect is less likely on foundations on ground as if they fail they will still be on the ground and then undergo settlement. I.e similar to ductile failure.

 

[hokie66]

Not sure I understand the original question, but beams are beams, whether they are footing beams or suspended. But deep beams behave differently, in a non-flexural manner, than shallow beams. Deep beams are now generally designed using a truss analogy, i.e. strut and tie methods. Deep beams do have vertical reinforcement, but the design requires consideration of confinement of the struts and anchorage of the ties.

 

[hokie66]

Another thought relative to your question as to where stirrups are required...some codes, if not all, allow wide, shallow beams to be treated as slabs, thus with different shear provisions. Suggest you read your code to see if that applies.

 

[msquared48]

I always include at least nominal stirrups in all RC beams, if not for crack control as well as shear, also for providing a structural framework to tie any horizontal steel.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com