Existing Plate Girders 3

[LPPE]

I have a floor system consisting of concrete slab supported by old school steel plate girders. I've reverse engineered the girders, and have come up with rather large DL and LL capacities, almost like a factor of safety of 2. Am I missing something in my analysis? Is there a strength reduction for plate girders? I checked my section properties twice, and came up with the same results (thank god). I used 20 ksi for Fb.

 

Yes, there would be a reduction if either b/t or d/t exceed non compact requirements. Two reduction factors need to be considered Qa and Qs as defined in AISC Appendix B. The total reduction factor is Q= Qa Qs.

Additional reduction may be needed if it is hybrid girder, or the flange Fy differs from the web Fy.

Use WFbeam.exe at http:

http://www.lanxun.com/pce

to double check your work.

 

[redhead]

Not all engineers design "on the money". Your girders may have someone's own "factor of ignorance" built into them.

 

[JAE]

I would second ET62's comments. Be sure to fully review the AISC Section on plate girders and check the compact criteria for your shape. Also, be sure to check shear in your girder. I've found that engineers from the first half of the 20th century were not very adept at offering adequate shear capacity.

 

[dlew]

pylko,

How old are the plate girders? Do you know what type of steel was used in their fabrication? I think during the 1940's and maybe 1950's, steel had a yield strength of 33,000 psi, with a maximum allowable tensile stress of 18,000 psi. Also during that period welding electrodes were Series 60 (60,000 psi).

AEF

 

[LPPE]

THey are old. Exactly how old, I'm not sure. Somewhere around early part of 1900's is my best guess. Unfortunately, I dont have much info to go from. There are web stiffeners spaced on center as the girders are deep. There are flange coverplates that increase in number as you move towards the center of the girder. I'd imagine these coverplates are riveted, because I dont think welding was too popular back then. Even the connection info of beams to girders is sketchy. The columns for this building are also built-up sections. Actually, this building is in DaveViking's neck of the woods. Look out, Dave, I'm muscling in on your territory! Any thoughts, Dave? I assume you've done some work on old steel buildings in the city!
I also read a quick note in an old text about a rivet reduction factor of 15%. Can anyone clarify this?

 

[ishvaaag]

Then the factor of ignorance surely is there, not as much as now was known about these things in 1900. Foremost concern would be the yield strength of the steel, with which you may proceed to one present verification as per AISC.

Respect rivets, I have a spanish code on them, it would be easy to recalculate as long as the standing condition of the same would be determined. A 15% reduction on the shear capacity might be in relation with the fact of the rivets being upon cooling prestressed in an unknown amount (interaction) ...yet this should be directly accounted in any code values or procedure.

 

[jheidt2543]

Pylko:

A number of years ago I had to evaluate an existing built-up plate girder highway bridge on a small county road. I went to the local engineering library and found a series of structural books written in the 1920's by Hool and Kinne. They had worked examples of plate girder design and information on the types of steel in use at that time. The engineering calcs. are not the difficult, the real problem is to make some assesment of the material used in the the plate girder and its present condition. That is where your engineering judgement comes in.

Good luck!

 

[Dinosaur]

Your girder sounds like an old railroad girder. I believe the steel will be the old A7 steel with Fy = 33ksi and Fb = 18 ksi. The rebar in the deck is probably Grade 40 and the concrete is who knows what. Another item I would consider is that the engineer may have designed the girder as non-composite. This was done in this country for a very long time.

Good Luck.