Compression Steel in Tension

[BadgerPE]

I have a little problem that I would enjoy some guidance on. I am desinging a beam that will be cast against the ground. The beam dimensions are b=16" h=12" and the depth to compression steel is 3.75" and depth to tension steel is 8.25 in. The problem I am having is that when I set up my Whitney stress block, at equilibrium, my c value is less than the depth of my compression steel and therefore it is acting in tension. It takes a ridiculous amount of tension steel to move the equilibrium point to a depth greater than the compression steel. The reason for the compression steel is that the beam may be subjected to negative moment forces as the beam is continuously supported. If anyone has some insight, it would be greatly appriciated!

 

[ToadJones]

Is this essentially a grade beam?

 

[BadgerPE]

Yes it is a grade beam.

 

[ToadJones]

Are you designing it as doubly reinforced or just adding compression steel?>

 

[BadgerPE]

My intial thought was that I needed to add compression steel to react any negative moment forces acting on the continously supported beam. I'm not sure what you mean by doubly reinforced. Does that mean that the beam is desiged with symmetrical compression and tension reinforcing?

 

[ishvaaag]

Your concern is then that it have a fragile failure? Note then that when the moment grows towards the final moment strength the compression steel will be placed in compression, quite likely, or you can do so by increasing in what necessary the tension steel.

In any case even if I understand and agree on the general convenience of designing flexural members to have failures by yield of the steel in tension instead of concrete crushing, I see that enforcing so for every case may not be entirely logical. I mean, you can have covered six times the design moment with a beam to fail with the concrete in compression, and still have to add more steel to ensure that the failure will be by steel in tension. 1 vote no, for cases as extreme as this. This view was once sustained for footings where once you had covered 4/3 of the required design solicitations it didn't matter if the failure was going to be fragile; a minimum steel to ensure that the kind of failure for the footing would be soft was not then enforced.

 

[ToadJones]

what bars do you have for compression steel & tension steel?

 

[vandede427]

16" wide x 12" tall is not a grade beam. it's barely a strip footing.

 

[ToadJones]

I guess Vandede has a point here....once you get the bars in with coverage requirements there's not much room in there.
This "beam" is just slightly larger than a conventional footing for a house.

 

[BadgerPE]

Sorry for the confusion. I guess "grade beam" wasn't the best way to classify the structure. It is actually a continuous deadman used to tie a foundation wall section into. I have worked on the situation with my boss and I have rectified the problems I was having.....also I would just like to say thanks to all that provided their input. I am a recent graduate and often have problems wrapping my head around something and the help on this site is awesome! Thanks much!!!

 

[hokie66]

Glad you sorted it out. Just remember...just because you have steel in both faces, you don't necessarily have compression steel. Both layers can be tension steel for different load cases. In your case, you can just ignore the steel on the compression side.