AS4100 member design
Hi
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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Artur Kosakowski
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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Artur Kosakowski
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 
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Artur Kosakowski
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?
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Artur Kosakowski
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.
Artur Kosakowski
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.
Artur Kosakowski
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.
Artur Kosakowski
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.
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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).
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Please review and if agreed submit the above to the dev team for their consideration.
With thanks.
Max
