topic Re: RC Beam Reinforcement - number of stirrup's legs in Robot Structural Analysis Forum

RC Beam Reinforcement - number of stirrup's legs

Anonymous — Tue, 31 Oct 2017 12:03:08 GMT

Hello everyone,

Why does the REQUIRED shear reinforcement area for the same beam with the same shear force is changing when I change the number of legs or the diameter of bars used for shear reinf. in the RC ELEMENT WINDOW, as shown in the picture below.

Thank you.

@Anonymous Artur Kosakowski edited the topic from "Shear reinforcement required" for better findability.

Re: RC Beam Reinforcement - number of stirrup's legs

Anonymous — Tue, 31 Oct 2017 12:03:32 GMT

Hey @Anonymous , could you attach your model? From what is shown the 4 legs is double the value (9.42) of the 2 legs (4.71) so I am assuming Robot is trying to fulfill your request of four legs while providing the minimum amount of steel area to resist shear which would be double of the 2 legs

Re: RC Beam Reinforcement - number of stirrup's legs

Artur.Kosakowski — Tue, 31 Oct 2017 12:03:55 GMT

Hi @Anonymous

Is it for ACI? The best would be as @Anonymous already asked to attach your model so that we could examine it.

Re: RC Beam Reinforcement - number of stirrup's legs

Anonymous — Tue, 31 Oct 2017 12:04:17 GMT

@Artur.Kosakowski@Anonymous

HELLO, THANK YOU FOR ANSWERING.

I am using ACI code,

this example is just a normal beam design using RC ELEMENT DESIGN WINDOW

The problem is that if I am changing either the diameter of shear bars or the number of legs then the required Av or Av min. is changing

It seem that RSA is taking into consideration the number of legs and their diameter that I want in order to calculate the required Av. However, when designing a beam manually, we calculate Av which is ={Vn-Vc/fyv*d}*spacing then we know which bars and how many number of legs we should use

So how can I know the exact area of reinforcement required regardless the number and diameter of bars provided

Re: RC Beam Reinforcement - number of stirrup's legs

Anonymous — Tue, 31 Oct 2017 12:04:41 GMT

Hi @Anonymous ,

Sadly I wasn't able to open your file. Something about an incorrect schema. Also you are using a higher version of Robot than me.I am not conversant with ACI as I only use BS codes but I feel Robot is providing you with the answer you are asking for. The formula I use is slightly similar to yours and so I personally feel that increasing the number of legs and/or increasing the diameter of shear bars will increase Av required. Why do I say this? Because you are asking Robot to give you the steel areas that will resist the stress and more importantly meet your minimum requirements (i.e. number of legs and shear bar diameters). Let's say you have X n/mm2 of shear stress with a minimum of 6 mm bars and 2 legs and Robot decided 6 mm is adequate, If you then ask Robot to design for 4 legs it will use you the 6 mm bars again (since its the smallest bar) but your Asv will be double now.

I feel you are simply getting the requirements you want. Atleast I think so, I may be missing something crucial but I feel that should be your answer. Hopefully Artur will correct any mistake I've made. I'm still learning like you as well

Re: RC Beam Reinforcement - number of stirrup's legs

Anonymous — Tue, 31 Oct 2017 12:05:20 GMT

@Anonymous

That may be the case but my problem is how can I know what Is the exact required reinforcement before choosing how many legs and diameter so I don't exaggerate in providing not necessary extra reinf.

Re: RC Beam Reinforcement - number of stirrup's legs

Anonymous — Tue, 31 Oct 2017 12:05:50 GMT

@Anonymous ,

This feels like déjà vu because my answer is it is up to the discretion of the engineer. For average beams 2 legs is the norm but sometimes (in the case of lets say the beam of a bridge) the shear forces are so high that providing 2 legs would require such large bars that even placement would be an issue so 4 or sometimes even 6 legs may be necessary. There are also cases where you need to 'break up' the concrete so it doesn't behave like a single mass so more legs may be necessary.

All in all though you could do a quick hand calculation and see if the 2 legs provided resist the shear well and if you are happy with the results then keep it.

Re: RC Beam Reinforcement - number of stirrup's legs

Artur.Kosakowski — Tue, 31 Oct 2017 12:06:09 GMT

Hi @Anonymous

I think that the difference in Av,min is due to this ACI condition:

however you don't have to decide if you have t use 2 leg or 45 leg stirrups as Robot will automatically generate either of them depending on the actual; shear force. As you can see when you allow both stirrup arrangements in the reinforcement pattern dialog 2 leg stirrups will be generated as 4 leg would be excessive:

On the other hand when you increase the load you will have 4 leg stirrups generated:

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

Re: RC Beam Reinforcement - number of stirrup's legs

Anonymous — Tue, 31 Oct 2017 12:06:34 GMT

Thank you @Artur.Kosakowski you are the best 🙂

I should have chosen tied AUTO instead of tied ALL and that was my problem I didn't recognize there was such option.