Question about Australian deflection limits

[Tomfh]

Hi,

I am a recent Australian structural graduate. I am about to start working and am reading up on the various codes. I am currently wondering about the deflection criteria.

The deflection limits listed in AS1170.0 (general principles) and AS4100 (steel structures) are called "suggested limits". The deflection limits in AS3600 (concrete structures) are written more strongly, they are "limits", not "suggested limits". Why the difference? Why are the former limits written more softly?

I know an architect who recently fought with an engineer because a steel beam deflected double the limit suggested by the code. The engineer was responsible for sizing the beam. The engineer appealed to the fact that the deflection limits are merely “suggested”. The engineer is obviously admitting no fault for indeminity reasons, but the whole thing got me wondering...

What exactly does "suggested" mean, and why is it included in some codes but not others?


Tom

 

[dbuzz]

One would need a good reason to deviate from the standard to an allowable deflection greater than the suggested limit.

Unless the Engineer, in your example, had a signed-off Basis of Design for the project, specifying the allowable deflection, he would have a difficult time justifying the deviation from the standard in front of a Mediator of Magistrate -- if it went that far.

 

[FalsePrecision]

Here in the USA, steel deflection limits are also foggy. They are also borderline unconservative. You would be reckless to exceed them. I have defined my own deflection criteria, based on the situation. It is really strange that the AISC makes such a rocket science about allowable stress and load factors, etc, while virtually ignoring deflection issues.

 

[UcfSE]

Typically deflection limits are judgment calls, based on studies on existing structures. Sometimes they are called out more specifically. Here in Florida our Florida building code has a table that gives us deflection limits we must follow. Obviously more stringent is acceptable as well. The masonry code we have here, the ACI 530, also has some limits set on beams supporting masonry. It just depends on the code and the wording, but there are cases of suggested and cases of required. Stronger limits for concrete make sense in order to alleviate cracking that will inevitably occur. Steel is more ductile and can handle more strain if everything it supports can also handle it.

I agree with dbuzz on the deviation. I think that would be called not providing the "standard of care" that should be expected, meaning doing something very different from what the norm is or what another typical engineer would do. Basically if it says suggested I would do it unless you have good reason to do otherwise. Remember your rear can be on the line too.

 

[rapt]

Apparently the people who control the direction of the Australian structural design codes do not think they should provide rigid limits on deflection and that it should be left up to each client to decide what limits he is willing to accept in his building and to require his buildings designers to adhere to those limits.

For this reason, the Australian Loading code is only providing sugested limits based on state of the the art at this time. It is then up to the client to decide if he wants to abide by these limits or to use more stringent limits or more relaxed limits.

This is happening because engineers are losing the control of the direction and content of the structural design codes and are being forced to adhere to rules made by non-engineering bodies.

This would possibly work ok if the client was always the end owner but this is not normally the case. The eventual result of this will be fought out in litigation in the courts when eventual owners of buildings will sue because the building they have purchased is sub standard because the developer built the cheapest option, well outside the "suggested" limits.

AS3600 concrete design code has not as yet adopted this type of approach and its limits are still of the old strict type.

 

[GregLocock]

Isn't it more that deflection limits are a shorthand way of encapsulating a whole bunch of assumptions, and are not, in their own right, specifically meaningful?

If the customer actually cared about deflection, per se, then the limit would be independent of the type of structure underneath.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[pba]

On the issue of concrete deflection versus steel.

Here in the UK we have a span/depth ratio for concrete rather than an actual deflection check. The wording around the code clause makes the engineer responsible for doing a proper deflection check for circumstances where that is appropriate. For the more 'normal' condition the span/depth ratio is sufficient.

I assume that the Australian code is similar to the UK. Its much easier to be firm with a span/depth ratio than with an actual deflection limit.

I agree with the other posts that there is a need for more formalised advice on what deflection limits are appropriate

 

[dbuzz]

I think the move to suggested deflection limits are part of the evolution of structures standards to being (i) less prescriptive and (ii) focused of fitness for purpose outcomes.

A strict deflection limit for a particular structural elements may be too conservative for some application, resulting in wastage of materials, or unconservative in other applications, where an Engineer may blindly follow the limit, resulting in serviceability issues.

The onus is back on the Engineers to have a thnk about the what serviceability criteria, inlcuding deflection, is appropriate for the structure.

Good practice is to prepare a Basis of Design document for each project nominating design loads and combinations, defelection limits, etc. and get the Client's review and approval.

 

[rapt]

dbuzz,

Yes, that is what the Australian codes are now trying to achieve. Unfortunately, many designers would not know how to design to this type of design criteria.

Their understanding of deflections and how to limit them has always been as a limit for a building/loading type. Now they can set any limit depending on "fitness of purpose". So, if a developer says he is willing to accept L/100 deflections, the engineer provides it. Does the Real Estate Agent selling the units in the building then advertise them as being designed for that amount of deflection? How does the end customer know what he is getting for his money?

The codes have to get back to defined minimum limits and allow tighther limits to be imposed to cater for more severe requirements for "fitness of purpose".

pba
The Australian codes do have some less accurate methods but they are far more accurate than your BS8110 span/depth ratios which were thrown out of the Australian codes in 1974 because they were too unconservative and unpredictable. I find that many designers using BS8110 blindly use the span/depth ratios for all conditions irrespective of code comments.

 

[apsix]

I'm sure there have been some serviceability failures, but there must be an overwhelming number of successful projects designed to BS8110 in the 20 years since it was published.
The method may be unpredictable but I would have thought over-conservative results are more likely.

 

[rapt]

apsix,

Deflection serviceability failures are often not reported. It is not as if it falls down and someone gets killed. People put up with it, and who are they going to complain to 5 years after the building has been purchased from someone else.

It would be nice to think the BS8110 Part 1 span/depth ratios would be conservative.
Unfortunately, in many if not all cases BS8110 Part 2 deflection calculations (the real ones) are far more conservative. As the Part 2 methodology is reasonably logical, we can only assume that the Part 1 ratios are unconservative and therefre someone messed up in determining them.

 

[apsix]

rapt,

My wild assumption is that you are associated with RAPT software, and therefore your statements are probably correct. However, during the mid-late 90's I worked under an engineer who was both on the BS8110 and EC2 code committees. The issue that the Pt 1 method may be non-conservative never came up.

Possibly a lot of slabs/beams are not subjected to the serviceability loads used.

 

[rapt]

apsix

That was a hard one to guess.

Agreed the normal service loads in parking structures, offices etc will normally be much less than the design values. That is why they are reduced for serviceability calculations in many codes.

However, the permanent live load defined in BS8110 is relatively small at 25% LL and probably more correct than the 40% in Eurocode and Australian code. This partly offsets the use of the full short term load at service in BS8110.
The factor which hurts the most is the assumed tensile strength of the concrete at which the cross-section is assumed to be cracked. This is very low in BS8110 and basically results in relatively high deflections for lightly loaded/stressed members compared to the Deemed to Comply in part 1. I discussed this with Andrew Beeby several years ago but he was happy with Part 2 as it is.

The other problem with the Part 1 rules is the hidden assumptions especially regarding end moments. Simplely Supported does not mean pin supported in BS8110, it assumes a large amount of fixity. Many are misled by this. Also, many forget about the span length limitations and the rotations for cantilevers.