Reinforcement of Concrete Beam using STAAD Pro

[nkclarke]

I'm currently trying to obtain STAAD output to obtain adequate reinforcement for a concrete beam. I've sort of been able to do this but unfortunately, it's not designing the rebar based on the predominant force that I've applied to the beam. My problem is that the predominant force is applied as a horizontal point load into the beam, whereas STAAD appears to be giving me rebar by only taking the vertical loading into account (only self-weight of the beam, in this case). I'm not sure if I have to guide STAAD by *telling* it that the main force is lateral to the beam. I would have thought that STAAD should automatically look at the complete set of forces about all 3 axis and design for the worst case loading but it doesn't appear to be the case. To further explain; STAAD is designing the rebar for the bending and shear forces about the x axis when the worst bending is being obtained about the z axis (vertical axis)

I'm currently trying to get it to work for a simple problem such as a single beam and then I'll develop the model for a more complicated structure when I manage to get it to work.

Could anyone offer me assistance in any of the following ways:
(a)Point me towards a good tutorial beyond those given in the example manual that STAAD provides
(b) Enlighten me on the finer points of using the keyword TRACK and if it has any further options that would enable me to direct STAAD on a more appropriate rebar design.

I can provide further information on my model to anyone who might be able to help me such as the input file etc.

Thanks

 

[edward1]

Model this as SPACE FRAME with your load in Y direction and use SELFWEIGHT Z -1.

 

[nkclarke]

Hi Edward. Thanks for that. I ended up doing it the way you suggested and it worked. Still, it would nice to know if there was a way to do it without having to "trick" STAAD into designing the correct reinforcement. If I have a more complicated structure such as a frame with horizontal loading, it wouldn't seem practical to have to do this. And if I had horizontal loading in different directions (along x and z axis)...?

So is the bottom line that STAAD is only capable of designing rebar for forces in the Y direction? If that's the case, then the concrete design function appears to be of fairly limited use.

 

[SethGuthrie]

A colleague of mine, Phil, gets the credit for the following:

Regretfully, in STAAD.pro beams are designed for strong axis bending (MZ) only. The weak axis bending MY is not considered. Section 3.6.1 of the Technical Reference Manual describes this:
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3.6.1 Design for Flexure
Reinforcement for positive and negative moments are calculated on the basis of the section properties provided by the user. If the section dimensions are inadequate to carry the applied load, that is if the required reinforcement is greater than the maximum allowable for the cross section, the program reports that beam fails in maximum reinforcement. Effective depth is chosen as Total depth - (Clear cover + diameter of stirrup + half the dia. of main reinforcement), and a trial value is obtained by adopting proper bar sizes for the stirrups and main reinforcements. The relevant clauses in Sections 10.2 to 10.6 of ACI 318 are utilized to obtain the actual amount of steel required as well as the maximum allowable and minimum required steel. These values are reported as ROW, ROWMX, and ROWMN in the output and can be printed using the parameter TRACK 1.0 (see Table 3.1). In addition, the maximum, minimum and actual bar spacing are also printed.

It is important to note that beams are designed for flexural moment MZ only. The moment MY is not considered in the flexural design.
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If the user has implemented the command SET Z UP to make the Z axis vertical instead of the Y axis, then the software also switches the local Z and Y axes. As a result, strong axis bending becomes MY and weak axis bending becomes MZ. However, in this scenario the concrete beam design still only considers strong axis bending MY.

Hope this helps.

 

[nkclarke]

Thanks SethGuthrie for your reply and thanks to your colleague too. It's a bit disappointing that STAAD has decided to limit rebar design in this way but I suppose it's better than no design at all. I suspected that this was the case but I hoped that I was wrong.