LRFD coefficients for load combinations

[3JC]

Dear friends from eng-tips,

I am performing the analysis of a steel construction of 50 years old. What I know the LRFD coefficients are based on this period of time. I have the next question: if I want to study the structure with new loads from equipements, and the new period of time is 30 years more, shall I have to use the same load combinations coeficients than on the case of a new structure, or I have to use higher. Regards,

JJJ

 

[wannabeSE]

Assuming you are in the USA:
50 years ago, they would not have used LRFD to design the steel. If new loads are being added to the structures, use the current building code requirements.

I don't understand "new period of time is 30 years." If the new loads are being added today, use today's building code.

 

[Lomarandil]

I think 3JC may be referring to design life/return period (particularly for wind/seismic loads).

I'm assuming you aren't referring to fatigue life due to cyclic equipment loads or fatigue sensitive connection details -- if so, let us know, that's a whole other consideration.

I'm also going to presume that the existing structure doesn't show any signs of deterioration due to it's history, or that you can account for that in your analysis by reducing section properties of rusted members, etc.

But if the building is in good shape, and your concern is only environmental loads, I would argue that using the standard LRFD load factors is appropriate. The probability of a 50 year storm isn't any higher in the next 50 years than it was in the last 50 just because your building is older.

 

[3JC]

Dear friends,

Thank you very much for your response. WannabeSE, you are right so on that date it was not possible to use LRFD... so it was not incorporated on code yet. And Lomarandil, you said what I refer: the structure is on good state, it has some elements that by visual testing show deterioration (a little corrosion and local buckling because the structure is emplaced on a seismic zone), but nothing else. On the other hand, according to the criteria expossed, coefficientes wouldn't be modified if a calculation and a verification of the structure is donde each 50 years? In my opinion not only the state of the structure, but also the microstructure of materials and phenomena not usually contempled such as creep, should be considered by employing higher coefficients. What do you think about it?
Regards,
3JC

 

[Lomarandil]

I'm no expert on creep, but it seems that if it is a concern, simply throwing a higher load factor at your dead load would be a serious oversimplification.

I'm also making the assumption that since you talked about local buckling and posted this in the AISC forum, we're talking about a steel structure.