Approach to Historical Steel Shape Analysis 2

[Simba13]

Afternoon all,

So I wanted to get your opinions on using the 1st edition of the AISC manual. We have a historical building that we need to analyze, I've been told that the yield strength for the steel is 30 ksi, also, I see on the general notes that the steel is ASTM A7-58T (can't find the specifics of this online). After perusing the 1st edition manual (which I believe based on the time of construction would've used this edition), I see that the allowable bending stress is 18 ksi, they also give tables of maximum moment based on the bracing conditions. My question is this: what is the Fy assumption here? I may have missed it, but giving an allowable stress means that there is a safety factor built into the 18 ksi but it doesn't really go into specifics that I'm seeing, it just basically says you can use an allowable stress of 18 ksi. If I were to use the contemporary ASD safety factor of 1.67 on my 30 ksi yield stress I would get almost exactly 18 ksi (it's like 17.96), I want to make sure I understand their assumptions before I use their maximum moments in the tables.

Any tips would be appreciated, Thanks!

 

[structSU10]

It is appropriate to use the modern codes to evaluate the structure if you know the yield stress of the material. It may offer some economy in some places, or consider a limit state that was unknown at original design and can be corrected as required.

The only part to be careful on is the connections and how they may limit the capacity. Modern 'plastic' design (using Z instead of S for bending) where you find significantly greater capacity in the member may overload the connection and not be a true strength increase that can be counted on without modifications to the connections.

 

[phamENG]

I agree with everything structSU10 said. I'll also add that you need to be careful with these stress values and spec's from the early steel manuals. Back then, these were more voluntary product standards than building code requirements. Unless you know for certain that the mill that produced the steel was conforming to the specification, those values are only guides for preliminary design. The actual yield stress should be determined either during design or confirmed once construction starts if the loading is going to change significantly. Also welding can be a challenge. If you need to weld to these early structural steels, a metallurgy review of coupons for inclusions and defects may be in order.

Search AISC's webinar database for this. There were a couple decent presentations in the 'Flash Steel Conference' last month that covered this topic. Also check out AISC Design Guide 15.

 

[Simba13]

Gotcha, Thanks strucSU10 and phamENG

I'll ask my PM how we came to the 30 ksi conclusion, it looks like the timeline is fairly long so I imagine that testing either has been done or will be done to confirm that.

strucSU10 Yeah, the connections are definitely going to be a little weird, on top of the beams being old, they're encased in concrete.

Thanks again both of you for your help, I understand what direction to go in now.

 

[SlideRuleEra]

1) I've been told that the yield strength for the steel is 30 ksi...I'll ask my PM how we came to the 30 ksi conclusion...

2) ...I see on the general notes that the steel is ASTM A7-58T (can't find the specifics of this online).

3) After perusing the 1st edition manual (which I believe based on the time of construction would've used this edition)...

4) ...I see that the allowable bending stress is 18 ksi...,

1) Follow through on your inquiry, because...

2) ASTM A7-58T was published in January 1958, so the building must be from that time period. That spec is available for purchase here.

3) First edition dates from the the 1920's. For a building from 1958, use the Fifth edition.

4) In ASTM A7-58T, yield strength was specified as 33 ksi. Allowable bending stress was 20 ksi. Testing will likely reveal that the steel is a good as A36 (yield strength of 36 ksi) which became available in the early 1960s.

The steel would have been designed using Allowable Stress Design (ASD) which is not the same thing as modern Allowable Strength Design (ASD).

For general guidelines on evaluation of older steel structures, see AISC's "Evaluation of Existing Structures"

http://www.SlideRuleEra.net

 

[kipfoot]

a general discussion from Modern Steel Construction:

https://www.aisc.org/globalassets/modern-steel/archives/2007/02/2007v02_steelwise.pdf

aside:
phamENG: can you recommend a local (petersburg/hopewell) firm that does the coupon testing? I've had a testing company tell me that they send coupons out of state.

 

[phamENG]

I don't have one - the few times it's been critical it was excluded from the design budget and left to the contractor to complete and I don't remember who they used - those records are at my old firm. I'm in the middle of a turn-of-the-century steel frame remodel right now, and I'll be determining whether or not coupons will be required this afternoon. So I might be doing that same research next week.

 

[Spanky7]

If you know Fy and Fu, then using the current steel design provisions would be appropraite. There is a good discussion of this by Larry Muir in the Steel Interchange section (attached) of the December 2021 Modern Steel Construction.