topic Re: AS4100 member design in Robot Structural Analysis Forum

AS4100 member design

Max-88 — Thu, 22 Feb 2018 00:53:07 GMT

Hi Artur,

Re. Steel member design to AS4100

Can you please have a look at the attached model (no results, please run) and confirm the design check for member 17 is correct.

The section of interest is the lateral buckling length of lower flange - whether this is the active length in this instance.

Screen capture also attached.

With thanks.

Max

Re: AS4100 member design

EduardoMeruvia — Thu, 22 Feb 2018 06:14:11 GMT

max,

It seems like your member is failing under in-plane bending other than buckling.

Having a look at your effective lengths, everything there seems as you set them up on the "Member Type" assigned and set up in the Code Parameters module.

From what I can see and they way you set up your member types, seems like your member verification results for member 17 is correct I.e. Member is failing.

Re: AS4100 member design

Max-88 — Thu, 22 Feb 2018 07:37:37 GMT

Hi Ed,

Thanks for taking the time and looking into this.

I suggest that the Mby value of 348.52kNm is also incorrect as it is based on calculation for an effective length of 6.812m - as I have highlighted in my original post. The correct length for lateral buckling should be calculated based on location of restraints at the critical flange - the upper flange - which are positioned at 2m centres.

The Mby being incorrect leads to Mcy being incorrect hence the fail under buckling.

I would suggest that the correct calculation is:

fi*Moy = 507.11kNm (5.6.1.1) obtained with an effective length for lateral buckling of 2m - that is, the critical flange is restrained at 2m centres

fi*Mcy = fi*Moy*(1-N_star/fi*Nc) = 493.98kNm (8.4.5.1 , 8.4.4.1) where fi*Nc is correctly calculated for a general buckling length of 6.812m

(My/(fi*Mcy))^1.4 + (Mz/(fi*Miz))^1.4 = 1.005 which is far different from the calculated 1.97

Max

Re: AS4100 member design

Artur.Kosakowski — Thu, 22 Feb 2018 08:50:22 GMT

Hi @Max-88

Let me refer to the 3rd Robot webinar:

'You may also assume that in most of the situations load is applied to the upper flange of a beam and adjoining elements stand for restrains against lateral buckling for this flange. For the lower flange (as lateral buckling can happen only when the flange is under compression) you may set the restrains at the locations where bending moment changes its sign.'

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Re: AS4100 member design

Max-88 — Thu, 22 Feb 2018 09:57:48 GMT

Thank you Artur.

I am not sure if I understand your post correctly.
I am not looking for ways to restrain the member in question but for Robot to correctly apply AS4100.

If I misunderstood, please provide specific advise to my query.

Max

Re: AS4100 member design

Artur.Kosakowski — Thu, 22 Feb 2018 10:16:42 GMT

Hi @Max-88

I'm not AS expert therefore I'm not sure if I understand what you considered as being done incorrectly. My understanding was that you expected the critical verification to be in a location where the top flange is under compression with LTB unrestrained length being 2 meters as limited by the positions of the entered restrains however you sort of 'disregarded' the fact that at the supports it is the bottom flange which is under compression instead. If so you defined the unrestrained LTB length as 7 meters and the verification at this location returned the grater ratio. as in reality not the whole 7 m bottom flange is under compression but only the part from the support to the position of the zero bending moment I provided this additional 'virtual restrain' in the way shown on the attached to my previous post picture. As this reduces the unrestrained LTB length for the bottom flange the critical verification as as you expected at the location where the top flanges under compression (in the middle of the beam).

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Re: AS4100 member design

Max-88 — Thu, 22 Feb 2018 10:48:33 GMT

Hi Artur,

From memory, for the load case under consideration the bending moment diagram does not suggest that the bottom flange is under compression at any location.

If that is true, the LTB restraints to the bottom flange are irrelevant to the member check as the bottom flange is in tension over the whole length of member.

Subsequently the value for fi*Moy should be as per my previous post?

Max

Re: AS4100 member design

Artur.Kosakowski — Thu, 22 Feb 2018 10:58:52 GMT

Hi @Max-88

From memory, for the load case under consideration the bending moment diagram does not suggest that the bottom flange is under compression at any location.

I'd say the evidence you provided is against you

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

Re: AS4100 member design

Max-88 — Thu, 22 Feb 2018 11:56:49 GMT

Hi Artur,

I see 🙂 It was the fine print (of the diagram) and thanks for pointing it out 😛

If the critical flange is the bottom flange, I suggest that the design bending moment in the segment should be the negative value.

If the above is not acceptable, please change the design code to Eurocode and try to explain the difference 🙂

Max

Re: AS4100 member design

Artur.Kosakowski — Thu, 22 Feb 2018 12:05:43 GMT

HI @Max-88

I'm not sure what you mean. The bending moment is negative for both codes (as it is the result ofthe static analysis it is code independent).

What are the differences you meant?

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

Re: AS4100 member design

Max-88 — Thu, 22 Feb 2018 12:16:28 GMT

Hi Artur,

The design ratio.

ACI, CAN, Euro some 0.89 PASS, AS4100 some 1.97 FAIL.

Max

Re: AS4100 member design

Artur.Kosakowski — Thu, 22 Feb 2018 12:38:56 GMT

Hi @Max-88

I'm not going to compare the results for different design codes as they don't have to be similar. Mind that the safety ratios are the functions of load factors and rules for combinations for given codes which means that you usually cannot compare the results for the same load values. In addition the automatic conversion of the design parameters may not be 'exact' either.

On the other hand if you are able to indicate what is wrong and why for AS we will definitely focus on that.

Re: AS4100 member design

Max-88 — Thu, 22 Feb 2018 12:43:32 GMT

Hi Artur,

But I did: the design bending moment for the case when the critical flange is the bottom flange should be the negative value.

Max

Re: AS4100 member design

Artur.Kosakowski — Thu, 22 Feb 2018 13:05:34 GMT

Hi Max,

As I wrote I'm not an expert in AS so I'm struggling to understand what you mean. In general the negative value from x=1.0L is user for the section resistance check in this point whereas the maximal value of bending moment between the defined flange restrains is used for the stability check. Could you illustrate what you described with formulas and values from this element and combination so that it is easier for me to understand you correctly? Thank you.

Re: AS4100 member design

Max-88 — Mon, 26 Feb 2018 07:28:40 GMT

Hi Artur,

I suggest to go one step at a time.

I am referring to the attached showing LTB restraint locations and bending moment diagram for member 17.

I suggest that member (bending) design to AS4100 should be done as follow:

STEP 1

Identify all segments along the member that need to be checked for bending.

AS4100 page 11 defines Segment as:

"Segment (in a member subjected to bending)—the length between adjacent cross-sections which are fully or partially restrained, or the length between an unrestrained end and the adjacent cross-section which is fully or partially restrained."

In order to identify all segments, one need to know where the restrained cross sections are.

Start with the restraints defined by designer and in conjunction with the bending moment diagram - which shows the position of critical flange - assess whether these cross sections are fully, partly or unrestrained.

Assessment

Cross sections 1 to 12 are fully restrained as per clause 5.4.2

Comment: Cross section 4.5 (between 4 and 5) is unrestrained as per clause 5.4.2 as the critical flange is the top flange and there is no restraint to top flange.

Following that, the segments are 1to2, 2to3 etc, with a length (not effective length) of 2m or less

===

Do you agree so far?

Max

Re: AS4100 member design

Artur.Kosakowski — Mon, 26 Feb 2018 14:39:10 GMT

Hi Max,

It seems my brain was at the verge of its capacity at the end of the last week

Obviously for the check at x=1.0L the stability should be done for the maximal negative moment (-9.58) instead of larger value from the positive and negative ones (495.53) from the distance from the end support and the last restrain for the bottom flange.

Seems like 'missing' part of the programming which is done for other codes (as you correctly already noticed).

I have asked the dev team to have a closer look at this program behavior. The workaround is to treat the zero bending moment locations as restrains for the bottom flange which IMHO has got sense anyway.

Thank you for your patience with me.

Re: AS4100 member design

Max-88 — Mon, 26 Feb 2018 22:41:52 GMT

Hi Artur,

Re brain capacity, Robot support staff - yourself included - are doing a tremendous job by handling a large variety of topics, providing sound advice in a timely manner and with impeccable manners. Well done to you all and THANK YOU again.

====

Further on the AS4100 design check.

The length of the verification segment including the section at 1.0L is also incorrect. There is no "segment" of that length along the bar, where "segment" is as defined in AS4100.

I suggest that a "segment (in a member subject to bending)" is not necessarily the length of a bar in between two consecutive restraints of the same flange but between two consecutive sections that are fully or partly restrained or the length between an unrestrained end and the adjacent cross-section which is fully or partly restrained. Refer again at the definition given by AS4100 page 11.

In regards to considering a restraint at the location along the bar where the bending moment is zero, AS4100 does not explicitly accept it.

AS4100 deals with the shape of the bending moment distribution by using the moment modification factor alpha_m - such as 5.6.1.1 (a) (iii) for open sections with equal flanges.

In supporting this view is the table 5.6.1 where various bending moment distribution show a zero bending moment location within a "segment" .

If there was a restrained section at every zero bending moment location then some of the diagrams shown in table 5.6.1 would be obsolete.

Furthermore - I'm almost done - I suggest that the design bending moment in the segment under consideration should be the value at the "critical section" as defined in 5.3.3 and reproduced below:

5.3.3 Critical section The critical section in a segment shall be taken as the cross-section which
has the largest value of the ratio of the design bending moment (M*) to the nominal section capacity in
bending (Ms) (see Clause 5.2).

===

Please review and if agreed submit the above to the dev team for their consideration.

With thanks.

Max

Re: AS4100 member design

Max-88 — Fri, 13 Apr 2018 05:17:37 GMT

Hi Arthur,

Kindly advise please if any progress with AS4100 member design check?

Max

Re: AS4100 member design

Max-88 — Fri, 04 May 2018 04:56:17 GMT

Hi Artur,

Please advise if any progress with getting the member design to AS4100 right.

Thanks.

Max

Re: AS4100 member design

Max-88 — Tue, 08 May 2018 05:36:36 GMT

Hi Artur,

It seems there is no progress with fixing the member design to AS4100.

If there is no further comments I'll close the topic and take it up with Autodesk customer service in Australia.

Max