AS3600 vs AS5100.5 Longitudinal Reinforcement 1

[Rstructural]

Hi All,

I've just been looking at some designs with concurrent bending, axial, shear and torsion to AS5100.5

Clause 8.2.8.2 of AS3600 very clearly states that the area of reinforcement on the flexural tension side "need not be more than that required at the section with the maximum tension force demand for flexure, axial force and torsion". I do note that this was added with Ammendment 2 in 2021 so is more up to date than AS5100, however, I don't recall seeing similar in the public draft ammendment for AS5100.5.

Clause 8.2.9.1 of AS5100.5 states "At every section, the longitudinal reinforcement and tendons shall be designed to resist the flexural design force....axial design force...additional longitudinal forces caused by shear and torsion as specified in Clause 8.2.7, Clause 8.2.8 and Figure 8.2.9.1"

It appears to depart from AS3600 and requires all design actions to be considered, or perhaps I'm just not interpreting it the same because it's not as explictly written in the text and should be inferred from Figure 8.2.9.1. This is despite there being similar general wording around extending the flexural tension reinforcement Docot(thetaV) for sections without direction tension or torsion, and Figure 8.2.8 (AS3600) and Figure 8.2.9.1 (AS5100.5) being the exact same.

Does anyone know if it is intended that you must design the longitudinal reinforcement for ALL design actions at ANY section per AS5100.5, or is it an inconsistency/ommision of wording in that particular code and it should be similar in principle to AS3600 where you just need to satisty Bending+Axial+Torsion at the maximum bending location?

Hopefully that all makes sense, happy to clarify the question if need be.

 

[rapt]

In a moment region (-ve or *ve) the requirement for flexure plus longitudinal shear reinforcement longitudinal tension plus should not be higher than the peak flexural longitudinal tension plus requirement at any point in that moment zone.

Then torsion needs to be added.

So in a normal case without section changes, the longitudinal force for flexure longitudinal tension plus at the point with highest moment in a zone will provide the limit for the flexure + longitidinal shear reinforcement longitudinal tension plus .

As shown in AS3600 Fig 8.2.8.

Varying depth or effective depth may change this if it moves the peak flexure + axial tension force requirement from the peak moment point.

 

[Rstructural]

Thanks for the explanation rapt.

Just to confirm was that first sentence meant to read "the requirement for flexure plus longitudinal shear reinforcement longitudinal tension plus axial should not be higher than the peak flexural longitudinal tension plus axial requirement at any point in that moment zone."?

If so, what you're saying makes sense and I'm on the same page. The intent of AS5100 and AS3600 is the same, it's just worded more clearly in AS3600.

Is it correct to say that in Figure 8.2.8 that Delta Ftd = Delta Ftds, as the intent of the diagram is to show a no torsion scenario (i.e Ftdt = 0)? Then as you say, if you do have torsion, it needs to be added.

 

[rapt]

Yes, missed a word.

Yes, we tried to improve on the 5100 wording to try to satisfy the code lawyers who want every case mentioned. I would prefer to leave it to common sense but as soon as you say something you have to define every different scenario these days and that then makes the whole code cumbersome and open to missing something.

It is a problem I have always had with ACI which tries to define every different scenario rather than have a general logic defined and allow intelligent engineers to apply it in all scenarios.

Yes, it shows a no torsion scenario.

 

[human909]

A slight aside...

I have always had with ACI which tries to define every different scenario rather than have a general logic defined and allow intelligent engineers to apply it in all scenarios.

Do you considered AS codes as 'better' than ACI? For this or other reasons? Is there generally more flexibility in AS codes?

I suppose I am just keen on understanding where AS3600 sits in respect to other codes. I'd be happy to hear about AS4100 too if you have comments there though as I understand it you are probably more of a concrete person.

EDIT:
I found this reference for my own education which seems reasonable enough, at least for those like myself coming from a fairly ignorant position:
The Comparison between AS3600:2018 and ACI318M-14 for ULS Design of Reinforcement Concrete

Of course 'better' is completely subjective and also such a difficult term to judge when it comes to comparing codes. I suppose as somebody who only know AS3600 and even then not particularly well, I just want to know how it compares with foreign codes.

 

[Rstructural]

Thanks rapt for the follow up and answering all my questions.

I lurk around these forums on occasion and I always find your posts quite helpful; both from a technical perspective but also the insights you provide into the code.

Cheers.

 

[rapt]

Better is not necessarily the right word.

The problem arises mainly when codes try to provide "simplified" solutions. That normally means that the solution is only correct for certain cases that match the conditions of the simplification.

Then you either have a lot of cases not covered or you provide further simplified solutions to cover any cases not covered by the original one.

But you forget to mention some of the rules governing the simplifications, or users do not really understand the limitations and you end up with a design code which is 500 pages long and does not cover everything but you are not sure what is not covered and is impossible to follow. Or designers interpret it how they feel like anyway so no matter how you word it, someone will twist it around to suit they way they want it to read.

So you rewrite it to give direction on how to follow it all and you end up with a code that early users used to understand and now no one knows where anything is and everything is multiple cross-referenced to find a path through it all which increases it by another 100 pages to satisfy new users who do not understand it in the first place and find it too complicated to follow because it is not laid out in order to match the design process! Just like the last sentence!

AS3600 was lucky in that it did a complete re-write and re-organisation from scratch in the early 1980's and one intelligent person tried to order it in the same order as you would approach design (approximately), so we are a long way behind on making is unreadable, but we are getting there. And made it a Concrete Code as distinct from a RC code and a PT code at the same time.

I was told by a very clever professor a lifetime ago that a design code should not be more than about 6-10 pages. It is a Code providing LIMITS, not a reference text book or a Design Guide. Unfortunately they grow!

 

[IDS]

Further comment on the wording of the two codes:

It is true that AS 5100.5 does not have a clause the same as AS 3600 Cl. 8.2.8.2: "the total longitudinal tension force at every section ... need not be more than that required at the section with the maximum tension force demand for flexure, axial force and torsion", but the wording in 8.2.9.1 effectively says the same thing (at least for sections not subject to "significant" torsion): "For members not subjected to significant direct tension or torsion, these requirements may be satisfied by extending the flexural tension reinforcement and tendons to develop the flexural tensile force beyond the location required by flexure alone ... " as shown in Fig 8.2.9.1.

There are however differences in the wording of the two codes:
AS 360o says the simplified approach may be used for members that are “not designed for axial tension or torsion” but AS 5100.5 says “not subjected to significant direct tension or torsion”.

AS 3600 has a further requirement that “there are no sudden changes in the calculated tension force”.

Does a point load cause a "sudden changes in the calculated tension force", and if so, how should that be handled?

Presumably the simplified method in AS 5100.5 is applicable to bridge beams subject to vehicle wheel loads, which will have sudden steps in the design shear force, and hence the design longitudinal force, but that is not seen as a problem. Why is it a problem for building structures?


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

Doug,

"Sudden Changes" was to cover possible increases in the tension force requirement in the direction of the extension of the reinforcement (direction of reducing moment).

For downward point loads in negative moment regions Point loads would not cause that. In positive moment regions they would as V* would increase in the direction of reducing moment.

Reverse for upward point loads.

Maybe "sudden changes" should be changed to "increases in the tension force requirement in the direction of numerically reducing moment"!

These changes could be caused by changes to V* or changes in section shape etc or the presence of termination prestressing tendons which is why they were left out of the simplified method in AS3600.

It should not be in AS5100 either, especially when considering the most prevalent PT bridge members are pre-tensioned beams with multiple strand termination points, strands relying on bonded transmission lengths reducing PT force to 0 over a significant distance and point loads all of which would cause the simplified method to give unconsertively incorrect results.

 

[rapt]

Doug,

I will review what I said above,

AS3600 wording is correct.

Total tension force at a point is made up of flexure (based on M/Z) and longitudinal shear (based on V cot theta).

A point load will increase V cot theta in the cases I mentioned but M/Z will reduce.

The peak tension force requirement in a moment zone (-ve or +ve) is the peak M/Z requirement where V cot theta is 0. The total at any point in the zone is never more than this.

So a point load at any location will not increase the total longitudinal force requirement, only the shear component.

AS3600 says the changes in the calculated tension force, which is the sum of the 2 effects, not the longitudinal tension force from shear only.

Regarding application of the simplified offset method,

- If it is done by offsetting the reinforcement by d cot theta, all reinforcement contributing of the flexural strength needs to be offset. So terminating prestressing tendons or other reinforcement also need to be offset, and the points they terminate at are normally fixed by other factors, so offsetting does not work.

Not thinking, designers used to offset the reinforcement but not think about the prestress. This can lead to serious problems in heavily prestressed members with light passive reinforcement

- if it is done by offsetting the Moment Diagram, the old method in AS3600 for PT because offsetting reinforcement does not work with PT, variations in effective depth give weird and incorrect results that can be conservative or unconservative. So Steps in the section depth or profiled PT tendons give erroneous results. And terminating tendons and development lengths can lead to seriously stupid reinforcement requirements

We tried this in RAPT many years ago and it simply does not work for PT or sections with changing depth.

RAPT has been using a version of the current approach based on the Eurocode logic for about 15 years because of the limitations of the 2 old code offset approaches. They were replaced as the main method and limited in AS3600 in 2018 based on these problems.

It is the old problem that simplifications have limited applicability, and in earlier codes these were not understood or explained and the simplifications were incorrectly applied.