Two small questions please HELP!!!

[5laura5]

Hi, Just a few questions:

1. For a singly reinforced concrete beam, why is a minimum percentage area (100As/bh) of tensile reinforcement required?

2. Why is the maximum value of the lever arm (la = z/d) limited to 0.95?

Thanks.

 

[JedClampett]

For 60 ksi reinforcing the minimum rho without a strength reduction is .00333.
I'm not sure where you are getting the limit for la. Is this a working stress design?

 

[hokie66]

Minimum steel requirements in bending elements are intended to ensure that the ultimate moment is significantly greater than the cracking moment, thus avoiding sudden collapse upon cracking.

I am unfamiliar with the .95d limitation, but it could be related to the same thing, as most well designed beams will have enough steel so that the lever arm is closer to .85d.

 

[5laura5]

Thanks, i thought it was along those lines.

Regarding the lever arm depth, i seem to have found something that suggests it is because of two reasons;

1. As this is very close to the suface of the beam, steel is likely to fail as it will have very high strain

2. Concrete is weaker here due to higher water content from travelling off (Laitance Curing Effect)??

Could any of these be a valid reason?

 

[hokie66]

Maybe, but I'm just not familiar with the .95d limitation. Where did you find that?

 

[Ussuri]

This could be a guess, but the OP could be referring to BS8110, clause 3.4.4.4.

Lever arm z=d(0.5+[√](0.25-(K/0.9)) but not greater than 0.95d.

 

[5laura5]

Yeah its all referring to BS8110...sorry i should have mentioned that. Just wondering the reasons for that limitation?