Maximum Shear Stress

[ahmedhegazi]

ACI limits the maximum shear on any section to 10 Sqrt(f`c).b.d
In my case I am having relatively high strength concrete of f`c=64MPa . I think Allowable shear value in the section can be increased as I read in many paper and researches, also I am having small span to depth ration(less than 2)

Does anyone have a reference according to ACI that give another equation for maximum shear strength?

 

[Lion06]

I assume you are getting the 10*Sqrt(f'c) from adding 2 for the concrete and 8 for the steel. I have not seen anything that allowed this to be increased based on a higher f'c.

 

[shin25]

You may find lot of things in the research papers, but if they are not in the code then you cannot use them.

I assume you are proceeding with a deep beam design concept? Your span to depth ratio suggests so.

 

[civilperson]

Use strut-and-tie method of analysis for deep beams and portions of the structure where shear can be taken in compression. The higher limits of shear strength noted in the original posting are from "Shear Strength of Deep Reinforced Concrete Continuous Beams", Struct. Eng. Report No.110, Univ. of Alberta, Nov. 1983.(Rogowsky and MacGregor) Use of (11-5) from ACI has upper limit of shear strength of concrete of 3.5*SqRt(f'c) where shear and flexure only exist. Equation 11-6 and 11-7 can increase the shear strength when compression exists. The upper limit on shear is a combination of steel and concrete sttrengths as StructuralEIT has stated.

 

[ahmedhegazi]

StructuralEIT
Yes I got this from adding conc and steel term.

but actually I think that f`c=64MPa is considered high strength concrete and in papers and searches they got higher shear capacity than lower grades

I am designing a coupling beam between two shear walls

I do not know if this upper limit can be waived or no

 

[Lion06]

I would probably say not. Even if you can find the research, ACI does not recognize it yet. Maybe this will apply to future editions of ACI, but it doesn't yet.

 

[ahmedhegazi]

i agree with you StructuralEIT , actually my problem is that i want to overcome the upper limit 10 Sqrt(f`c).b.d

I am thinking if i can provide steel section or steel plate inside my beam and design it to carry the total shearing force according to AISC, as i see in AISC there is no upper limit for steel sections encased in concrete

can this solution work fine?

 

[shin25]

The main reason for providing a maximum limit on shear capacity of a concrete section is to prohibit the premature failure of compression strut (as if you are designing by strut and tie method) before the section reaches to its capacity.

From this perspective,you cannot allow more shear on a concrete section than the current code equation predicts.

 

[Lion06]

I think if you provide a WF inside the concrete beam, that may be a solution. At that point I imagine you can use the shear strength of the WF section. The AISC manual has a section on this - I haven't read through it too thoroughly though.
Check out the AISC manual.

 

[csd72]

ahmed,

I dont see why that wouldnt work.

The tricky part will be transferring the shear into the steel and then from the steel into the support without having local concrete crushing.

Look up precast literature as they often do this.

csd

 

[Tomfh]

10 sqrt(f'c) bd is fairly low compared to other concrete codes around the world.

You must follow your own code of course.

 

[NewImhotep]

Ahmed

you should not design the coupling beams with a high shear value. cuz with lateral forces CB will not remain elastic while shear walls will be elastic so the CB shear will distribute to the walls.
this is an interesting paper for coupled walls system

http://www.ohiolink.edu/etd/view.cgi?acc_num=ucin1133636553

If you don't have a big drift issue try to reduce stiffness of the CB to have less shear .

composite sections with steel plate or I section may work but you need to check the embedding length in the walls to be sure the stresses from the steel section can be taken by the concrete wall

I don't advice u to use the steel sections, usually when you have high shear & steel section in the CB you will need a long embeding length which is not much practicable, in this case hybrid system may used by have steel section in the wall ends + the steel section which is costly and not easy to do.

 

[hokie66]

AhmedR has given you very good advice. I haven't read the paper he recommended, but core coupling beams are now in wide use and there are a lot of references. Don't depend too much on the concrete contribution to shear. Make sure you have plenty of well anchored stirrups. And solve the detailing issues where the heavy coupling beam reinforcement enters the walls. Just curiosity, how high is your building?