designing a RC beam partly integrated in a slab 1

[mats12]

I am often dealing with beams that are partly integrated in a slab. Usually I design them as rectangular cross section. But I am wondering if this is really the best way to go since I think its pretty conservative assumption (that way we dont consider part of the slab that also helps with bending).
This way is the quickest and most simple.

If we design said beam as T seaction with effective width of a slab (beff) we get much wider compression area of the beam so we need less tension reinforcement. I think this option is slower (to determine beff) but more accurate?

Which way do you go?
Also how do you determine beff?

 

[WARose]

It's going to try to act like a T-beam whether you want it to or not.

As far as beff goes.....it is (according to ACI 318):

8.12.2 — Width of slab effective as a T-beam flange
shall not exceed one-quarter of the span length of the
beam, and the effective overhanging flange width on
each side of the web shall not exceed:
(a) Eight times the slab thickness; and
(b) One-half the clear distance to the next web.

8.12.3 — For beams with a slab on one side only, the
effective overhanging flange width shall not exceed:
(a) One-twelfth the span length of the beam;
(b) Six times the slab thickness; and
(c) One-half the clear distance to the next web.

 

[Tommy385]

As far as beff goes.....it is (according to EC2), similar to ACI:

 

[PaulG93]

Attached picture should explain it really well. This is taken from Reinforced Concrete Mechanics and Design by Wight and MacGregor.

I always found ACI's wording a little confusing. ACI 318 sections 8.12.2 and .3 define the overhang so don't forget to add the width of the stem to determine the effective flange width.

 

[Celt83]

Just an fyi ACI 13.2.4 provides secondary requirements for beams in two-way slab systems, see this criteria get missed a lot.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[mats12]

Thank you for help.
I have also found this if - its the same as WARose posted but in picture.

I found Eurocode's explanation confusing in a case when we are dealing with only one beam integated in RC slab. I like ACI's better and more clear.

So do you guys choose the faster and more safer way when dealing with this (rectangular cross section) or an actual T-beam?

 

[WARose]

So do you guys choose the faster and more safer way when dealing with this (rectangular cross section) or an actual T-beam?

About the only time I ignore T-beam action is when I am worried about openings in the slab. (At the risk of opening that can of worms.)

 

[mats12]

I have looked up what Celt83 posted.

I dont really know what is meant by projection - can someone explain?

 

[mats12]

found it:

 

[JAE]

We ALWAYS use the flange (T-beam action).
It makes very little difference in design time unless the compression block extends below the bottom of the slab - otherwise it is really just the same as a rectangular beam design....where everything below the compression block is assumed cracked and neglected whether it is a rectangular beam shape or a "T" beam shape -

So your design cross section is simply a rectangular compression block at the top and a transformed area of reinforcement at the bottom.
Plus our spreadsheets, software simply does it for us after we input the beff.

Heck - I even had my old HP programmed to do T-beams.

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[rapt]

"get much wider compression area of the beam so we need less tension reinforcement."

Including the effective flange will help deflections a lot, but it will only have a small effect on the amount of reinforcement required for Ultimate Strength!

I would always include it for the deflection benefits and also ductility at positive moment regions,.