2
$\begingroup$

This question might look silly. I am not knowing the correct engineering terms to use in this question. I am trying to explain using the words what I know in English.

We have been hired some construction workers for a work in our backyard. I have some doubt on reinforcing beam for slabs. I came through following 2 models

.

1. Model one

Insert the steel bar for slab through the steel bear created for the beam

.

2. Model two

Put the steel bar for slab above the bar made for beam.

According to my perception, I believe the second one is much better in terms of strength. Because the beam is supporting the weight of slab. But my friend suggested me the first one is the right way. But I am not aware why the first one is better than the second. Could any one please help me out in this case to identify the better one.

$\endgroup$
4
$\begingroup$

Model 1 is always better, but may need some modifications

Whenever you have two reinforced concrete elements, you always need to facilitate the transfer of internal stresses between them. This is done by "mixing" their reinforcement.

For instance, model 2 will have no steel between the slab steel and the beam steel. This means that the connection between them will be very fragile and weak to horizontal shear, meaning that there's a risk of a horizontal crack like this:

enter image description here

With Model 1, this isn't a risk because the vertical steel from the beam ("stirrups") will resist these forces.

Model 1 also has an advantage in that it makes the beam itself stronger, since it allows the beam to behave as a much taller beam (including the height of the slab). Indeed, it even allows the beam to behave not as a rectangular beam, but as a stronger T-shape beam, using some of the slab to resist some of the beam's internal forces (how much depends on your country's standards and codes).

If the beam and the slab will be poured simultaneously, then that's it, and you can stop reading this answer.

However, if there's a chance that the beam will be built first and then the slab will be poured over it, then that means that the beam's stirrups and negative reinforcement (the longitudinal bars at the top of the beam) will be outside of the concrete when the beam is initially built but the slab hasn't yet been poured. This means that the beam's weight and that of the freshly-poured slab (before it gains enough stiffness to do anything) will have to be resisted by the "short" beam alone (without the added height from the slab), so some additional steel will be necessary:

enter image description here

You are basically reinforcing the same beam twice: once considering it as the short beam (without the slab), and once as the tall beam (with the slab). To avoid confusion as to how to define how much reinforcement goes into each position, here's a list:

  • positive reinforcement (longitudinal steel at the bottom of the beam): calculate the necessary reinforcement for the short beam to resist the beam's and the slab's self-weight, and then calculate the reinforcement for the tall beam to resist any additional loads (dead loads, live loads, etc). Add these two numbers and you have the total necessary positive reinforcement.
  • negative reinforcement (longitudinal steel at the top of the beam) and the shear reinforcement (stirrups) for the short beam: calculate the necessary reinforcement for the short beam to resist the beam's and the slab's self-weight. You may need to just put the minimum reinforcement.
  • negative reinforcement (longitudinal steel at the top of the beam) and the shear reinforcement (stirrups) for the tall beam: calculate the reinforcement for the tall beam to resist any additional loads (dead loads, live loads, etc). You may need to just put the minimum reinforcement.
$\endgroup$
  • $\begingroup$ appreciated. Clear and understood. $\endgroup$ – BlueBird Jul 23 '17 at 13:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.