Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

RAM vs RAPT for cantilever deflections to AS3600

RAM vs RAPT for cantilever deflections to AS3600

RAM vs RAPT for cantilever deflections to AS3600

Hi All,

Just looking for some high-level comments or guidelines for modelling large(ish) cantilever in band beam floorplates (in RAM - RAPT is fine).

The floorplate is a 4m cantilever, 10.5m back span with a 9m tributary; a 400dp, 2.4m PT band beam with a 180thk PT slab structure has been adopted. Typical office loading of 1.5kPa SDL, 3kPa IL and 2.9kn/m for facade. Adopting the same concrete grades and PT profiles between RAPT and RAM results in the cantilever deflections that greatly differ:

RAPT: Circa 8-9mm
RAM: Circa 16-20mm

I am traditionally a RAPT user so I may be doing something wrong in with my RAM modelling.

Thanks in advance for any help (i'm sure that the "RAM vs RAPT" question has been asked loads of time)!

RE: RAM vs RAPT for cantilever deflections to AS3600

Gonna preface this by saying I don't have much experience in PT slabs....

IIRC RAPT doesn't do beams for 2-way PT? I did also die a little inside when i noticed that you've set up your latitude and longitude tendons the wrong way around. I can't seem to find the facade loading in the RAM model either (though I think that would make the deflections even worse if they're located on the slab edge?). The main thing I can think of is that because the main span in RAM is properly 2-way in terms of load distribution and tendons, rather than 1-way like rapt, you're getting substantially less deflection there, which could actually be advantageous in terms of rotation at the column helping with the cantilever.


Why yes, I do in fact have no idea what I'm talking about

RE: RAM vs RAPT for cantilever deflections to AS3600

Thanks for the response JSN - its appreciated!

As a system, it should be relativity one way as it only two points of support. You could argue that the continuous support (away from the column) would pull load from the back-span but after rerunning the RAM model with a support under the beam only it only minimal change to the deflection cantilever. I also re-ran the RAPT run with the tributary reduced to simulate a yield line extent on the beam (accounting for the continuous support and the column) – again within minimal change to the cantilever deflection.
Drilling down on the calcs – it looks like RAM cracks the cantilever section to 0.6 to 0.8 igross at the PT transfer stage – which seems a little weird as the equivalent stress would be minimal?

fyi - the facade loading should be in the RAM model under a separate load case.

RE: RAM vs RAPT for cantilever deflections to AS3600

You can do it as a 2way beam in RAPT but you have to define the load distribution. Most designers take the conservative option for the band design and assume a 1way distribution of load to the band. It does not make a lot of difference and a 2 way is not correct anyway due to the relative stiffnessof the band direction relative to the slab direction.

For a 10.5m single span with a single 4m cantilever, I would expect the span deflection to be more than the cantilever deflection, which is what RAPT is showing.

RAM appears to be showing much less deflection in the span than the cantilever, which I find hard to believe, but ti would depend on the full loading that I do not know.

To say more I would need to see the RAPT model.

You can always send the RAPT model to me if you are a RAPT user.

RE: RAM vs RAPT for cantilever deflections to AS3600

Multitude of variables here that will dictate the differences between the two softwares.

Instead of trying to match the deflections, how about you first try and match the moment and shear distributions in the bands and slabs (or atleast explain the differences).

Then you can start unpacking all of the load history inputs...

RE: RAM vs RAPT for cantilever deflections to AS3600

Thanks for the response RAPT and Trenno,

Rapt run attached for reference (with the LLR at 1 for comparison to RAM).

This all came about as I completed a concept layout in RAPT and got my Grad to finalise the design who picked up deflection issues in RAM – which doesn’t make sense to me as it’s a relatively balanced system. I did reach out to RAM and they thought it was peculiar that the back-span was deflecting so much in RAPT – which I disagree with – it should and balance the cantilever.

@trenno – BMD and reactions have been reviewed (image attached) and are comparable. This is where we drilled down to the Ig/Ieff values in the LT deflection calcs in RAM. At transfer RAM has a negative deflection of circa 4mm (vs <1mm in RAPT) and the Icr values adopted are <50% (vs uncracked in RAPT) – The RAPT results appear to make a lot more sense to me but now I’m half way down the “RAPT vs RA” rabbit hole and need closure – I’m sure we have done something silly and I’m just not seeing it.


RE: RAM vs RAPT for cantilever deflections to AS3600

The back span effect is even more pronounced because the section is cracked in the back span at the bottom (about 4MPa tension) while the support region and the cantilever are uncracked.

At transfer, it is fully in compression (minimum about 1.3MPa compression). Hard to understand why you would use a cracked inertia!

I can get those sorts of relative deflections between cantilever and back span if you put a column at the end of the back span! But not with the arrangement you have.

Run it in a frame program without any prestress and check the relative deflections you get just for the applied loads. No prestress, cracking, long term effects etc. 2D or 3D, the relative effects should not change that much.

RE: RAM vs RAPT for cantilever deflections to AS3600

Hi GTD_18, it looks like you have a similar workflow to me for design of PT slabs...
I don't have time at the moment for a deep-dive into either of your models, but a quick look highlights some differences between the models that haven't been mentioned yet.

You are comparing a load history analysis result from RAM to RAPT's results - for the same loading conditions I'd expect these to give very similar results.
BUT, you are not looking the same loads. Your RAM model load history has the facade load applied with no load on the backspan for a month prior to the application of loads on the backspan (your temp construction load case is empty).

Also, the anchorage for the beam PT in the RAM model is at 200mm above the soffit (half the beam depth) whereas in RAPT it is at 220mm (geometric centroid of the T-section). An anchorge below the centroid in a T-section is going to be increasing your cantilever deflections as well.

I hope this helps with resolving your inconsistency,

RE: RAM vs RAPT for cantilever deflections to AS3600

Thanks Dominator,
Apologies for the delay in response - i have been off ill so I didn't have a chance to review the forum in a while.
I updated the comments that you made but with no change to the RAM results. I think i will continue to use RAPT (as i always did) :)

Going back to RAPTs point - a simple 2D frame check illustrates that the back-span balances the cantilever:

Thanks again

RE: RAM vs RAPT for cantilever deflections to AS3600

That is about what I get in RAPT if there are no concentrated loads on the cantilever and all pin supports. Cantilever up about 1/3 of the downward deflection in the span!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close