design review for 90000 psi steel

[am]

Hi

I am currently reviewing a structure and noticed that the Yield strength of some of the main steel members is 90000psi [620 MPa]. These members are subjected to very high compressive and/or tensile forces. I never dealt with such a high grade steel and wondering if standard formulae of ASD can be applied. Can any one please give suggestions on this topic?

Regards

AM

 

[ishvaaag]

I don't find a 90 ksi yield steel between those listed as permitted in ASD. Hence, I would say this steel is outside the LEGAL scope of applicability of ASD.

Other thing if the use of ASD as a technical resource, in absence of any other known to be more accurate makes sense. It is clear that for some technical aspects, the use must not be leading to the level of safey ASD wants warranted, or at least it sees it not being warranted for such steels. But in the absence of individualized expression of such, one can't but guess.

The first things that come to mind are if reserve strength beyond yield and elongation from yield to final rupture do not influence negatively the use of ASD in order to attain the implied targeted (by ASD) level of safety. As much as this, and respect fire risks, whether the high strength mechanical properties degrade in akin way to the steels accepted in ASD or has some sudden loss of strength, due to constitutive changes in the chemistry of the steel under fire.

Hence one may not be reassured is doing the right thing applying ASD to a steel not listed in such code. Even in the availability of accurate description of the mechanical properties etc by say a provider of the steel, one can't be reliant on that the suggested procedures attain the same safety and serviceability standards than using the proper steel and code, because the single source simply lacks the industry consensus meant by codes of general application, where the surmised peer review of what proposed is superior.

 

[Lutfi]

This steel may very well be ASTM 514 (T-1). However, I thought the yield is about 100 KSI. I have used this steel in many applications. It is tricky to weld! It is machinable.

Be careful with stresses in exotic steels. You must develop good and clear understanding of the steel and how it is being used and connected.

Good luck

 

[am]

ishvaag, Lufti

Thank you very much for your quick response.

The structure is in operation for last 30 years+ and in very good condition. It is a very tall guyed mast structure and (hopefully!) we can eliminate the fire hazard problem.

I will definitely look into connection details and any other aspects that will spring up during my investigation. But I am really looking for some literature that may /will reinforce me with more technical data.

Please let me know if any of you can give me a direction towards this literature.

Best regards

AM

 

[VoyageofDiscovery]

Hi am,

Is this guyed mast a tube stack or latticed?

VOD

 

[am]

Hi VOD

This is a latticed guyed mast, about 390m tall in a cyclonic zone. Very good combination - isn't it?

AM

 

[VoyageofDiscovery]

Hi am,

You must use a tower code if ASD or LRFD codes do not apply.

However, the tower code I know only provides design information of individual structural components within the structure while pointing out buckling stability of the mast. Thus the larger issue is the overall global buckling stability of the mast between guy elevations for the new loading you are considering. If my memory serves me correctly, towers taller than 250m require dynamic wind analysis as well.

There are specialist companies that can do the review for you.

Regards

VOD

 

[am]

Hi VOD

Thank you for your suggestion.

Yes, I am going through the tower code(s)[such as 222F, AS3995 - not referring to BS8100 but at this stage!]. We are in fact a specialist tower company. We are modelling it in MSTOWER and GUYMAST program. We are reviewing all the buckling mode.

I am feeling a bit awkward for very high strength steel and its ramification. Any literature on this subject will be helpful.

Regards

AM

 

[VoyageofDiscovery]

Hi am,

Not to cause you alarm, however, I have had different results when using GUYMAST and another Canadian tower software. That led me to believe that GUYMAST may not be conservative for these tower heights or under heavy icing.

Wrt. literature on high yield steels I don't have any, however the CSA-S37-01 has complete guidance for LSD and you can ask one of the steel commitee members a yes or no type question by email as to whether or not it covers your scenario.

You can order this Canadian standard at

http://www.csa.ca.

HTH

VOD

 

[ishvaaag]

It is also worth to mention the extreme discrepancies seen about the design of angle members, just in case they are used in the towers. I have had the opportunity to make worksheets for some and since so much things may affect the result, that only very careful consideration of the standing circumnstances may diminish such differences.

In any case, and returning to the essence of the initial question, it is important to note that as long as one stays well in the elastic-proportional realm for the applied forces the structure is for sure safe (well, I am forfeiting what at bolts etc happen). That was the rationale of working stress methods. Hence, maybe a suitable approach is experimentally determine by coupon tests the actual nature of the behaviour of the steel, and seeing the stress-strain diagrams decide on to what extent the ASD approach may be used or suitably modified to get a safe and servideable structure. The tests may be even forfeited as long one knows for sure what steel was installed and if the mechanical properties are available.

 

[VoyageofDiscovery]

Hi ishvaaag,

In Canada we have been designing towers to Limit States Design since the early 80's by using the CSA-S37. Much testing has been done in Canada on angles for towers to justify the Standard.

Regards

VOD

 

[ishvaaag]

I assume such may be the case of your code. Maybe others have not been as thorough, but in no way I imply they or others acted without thinking about, it is simply that first the target structures may be different (a building with continuous occupation surely has a higher life risk that one structure set alone in the barren) and also the perfection with which they dedided to portray the analyses of concern. Plus that as one stated "What to some is gruesome excess, to others is simply safe". But in any case the range is striking.