50 KSI Beams, When? 2

[jchi]

I am checking an existing (25-year old) seed corn tower supporting (4) surge bins. The problem I have is that I cannot get the existing support beams to check out. They fail under flexure at 150% using 36ksi steel. If I check with 50ksi they just barley work 103%. However, I do not believe (no drawings) 50ksi was that readily available 25-years ago...

These beams also support the 4" thick concrete surge bin roofs/upper floor. I typically design these floors for 100 to 125psf (IBC - light manufacturing). They are typically only used for equipment access. We typically classify these seed toweres upper floors as equipment access to keep overall structure sq footage down trying to avoid sprinklers.

Does anyone else work on the these seed towers, if so what do you use for your upper floor live loads? 125psf seams heavy, 60psf seams light. I would prefer 100psf and use that for a minimum. However, the code has no good classification for seed towers.

Thanks,
Jeff

 

[civilperson]

50 Ksi was common in late seventies and 103% calculated would get a OK from me when checking someone else's work. 50 psf is a standard loading for walkways and access. What density are you using for corn volume loading?

 

[rb1957]

was "barley" a pun or a typo ?

 

[msquared48]

We used 50 ksi material in the late 70's when constructing a 26 story high rise. Yes, it was very available 25 years ago.

Mike McCann
MMC Engineering

 

[Den32]

Remember that 50 ksi steel beams are typically governed by deflection. Maybe deflection is not an issue in this application? I am not too familiar with this type of structure

 

[JedClampett]

As I remember my days in the nuclear business in the 1970's we used to get mill certs on 36 ksi steel all the time for record purposes. It almost always came in at over 48 ksi and many times it was over 50 ksi.

 

[abusementpark]

I've always wondered what field tests we have at our disposal to determine the in-situ yield stess of steel beams.

Is there anything reasonable??

 

[rb1957]

hardness testing would be appliciable as a field test

 

[RockEngineer]

I think you would have a hard time using hardness testing to determine between 36 ksi and 50 ksi steel. Hardness testing works much better on the high strength steels where there is a much greater difference in strengths. When I took the ASCE class on Condition Assessment of existing buildings, the instructor confirmed this statement and said the only way that you could usually tell the difference was cutting out a coupon and testing it. As has been pointed out, there is a lot of overlap between 36 ksi and 50 ksi steels. Many beams in that time period came with dual certs.

 

[lsmfse]

We had a client who bought an old railcar to be used for a bridge. Railcar was used by the Military during the 1940's. We had a sample cut out and testing lab determined that the steel would comply with 50 ksi steel.

 

[enginerding]

You can cut out a coupon of steel (say at the bottom flange of a beam right next to a simple shear connection) and have it tensile and metalurgically tested. They can then tell you what grade of steel the piece would comply with. The tests are cheap. Both test for under $200 around here (Chicago). Why mess around? Just have it tested.

I don't trust any of the in-situ testing methods yet. Hardness tests would not make me confident enough to use more than 36 ksi... I need to see better research on in-situ methods that proves they are reliable.

 

[Lutfi]

I would not use 50 ksi steel unless I have positive proof that it is indeed 50 KSI steel. I would ask the owner to do a test coupon, at minimum. One thing I can guarantee you, your fee is not worth the risk you are exposing yourself to.


civilperson, I know where you are coming from by stating 103% is OK by you. How will you answer lawyers if you are on the stand when they ask you if 103% is per code! I am not making this up. This is the trend my friends. No code will stand by you either in stating that 103% is OK. The code will say that you are overstressing the structural element more than allowable! I would make sure that my stress is right at 100% of the allowable.


Regards,
Lutfi

 

[jchi]

Thanks to all the posts! I did inform the owner, he wants to add posts and not hang off the existing. I still did inform them that based on 36 ksi steel this structure/beams are underdesigned.

I now have another structure (different location) with beams at 158% (36ksi) and 200% (50ksi) as it currently sits, before we touch it. Also over 1/2 of the bracing is missing... Am I just lucky, or are other structurals running into these inadequate existing facilities.

Jeff

 

[youngstructural]

Gentlemen:

Just a quick point to address Lufti's 103% comment... I will often accept stress checks that exceed up to 5%, however if I am accepting a solution which has shown 105% check, I only do this after a review of my input assumptions. Often you have more than 5% in your back pocket from your assumed weight of the floors, self weights, etc.

I strongly agree with Lufti: Lawyers will twist the rationale into the seemingly implausible and irresponsible, and then sell it to the judge/jury. The important thing is that your calcs NEVER say "overstress accepted/ok", but rather say "I have been conservative in my dead load assumptions, thus this is actually an artificially high check, and by verification show Demand/Capacity < 1.0"

Just my two thoughts... Otherwise an interesting conversation!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[s16]

use a brinell hardness tester to determine the strength of steel?

 

[s16]

http://www.geinspectiontechnologies.com/en/products/hardness_testers/dynapocket.html

 

[civilperson]

Lufti,
I have talked to the lawyers on the stand and when they are shown the variability and the significant figures upper and lower limits used in arithmetic they understand that loads and strengths do not have that much accuracy to state stresses to three significant figures.