old beam strength 1

[ick2]

i have an old steel building built in 1906 in New York City. I have to check the existing beams for some new loading. I have tested the beams and found that they are A36 steel. What allowable bending stress i am allowed to use? do i have to find the steel code of that era or can i use the .66Fy of the current code?

 

[CSEllc]

I find it hard to believe the steel is A36 since the building is from 1906, but if you had it
tested and found it to conform to A36 steel, use the values for A36 steel.


For additional info, refer to AISC design guide #15, AISC Rehabilitation and Retrofit Guide. It includes the historic shapes and specifications.

 

[LPPE]

I dont believe A36 in 1906 either, but if thats what the tests say...
NYC 1948 lists allowable bending stress of structural rolled steel as 20ksi. I certainly wouldnt use anything more than that for your steel sections.

 

[jheidt2543]

Here are a few more thoughts to consider:

1. I agree with CSEllc, it is VERY doubtful that the steel is A36, although it COULD meet A36 specifications - there is a difference. (A36 steel replaced the commonly used A7 steel back when the steel manual changed from the dark blue color to the light blue color - hows that for a scientific description?)

2. I would suggest finding a 1906 era structures text and reveiw design methods and values from that time to get a feel for what MIGHT have been done at that time for design and code issues. Computing those values will give you some good insights to the building structure.

3. Check the building against the current codes. The differeces from what you find in #2 will tell you where you may need to reinforce the structure.

4. A reference like "Structural Analysis of Historic Buildings" by J. Stanley Rabun, John Wiley & Sons, 2000, would also be a good buy.

Good luck!

 

[ronster]

Allowable working stresses for buildings as published by Bethelem Steel for the years 1907-1911 is 16,000psi. I would use that value unless you can find a code reference that says otherwise.

 

[JAE]

No No - you do not use old codes to check old buildings other than to verify the original assumptions.

When checking an old building, the current code is what governs. IBC even has an "existing buildings" chapter.

The current, "best" provisions of the adopted code are what apply to any building in a governing agency's area. That doesn't mean that you have to bring every element of the structure up to the code. Just that you have to use the current code for new work.

 

[ronster]

Obviously you use the current code to analyze the building but I do not think you can apply the .66Fy to the old steel even if you were to determine the actual yield strength. The steel was limited to 16ksi based on steel mill practices at the time and in court you would have a hard time proving that it got stronger over time.

 

[JAE]

ronster - your point is well taken - the difference between yield and ultimate in the steel of the beam itself may not meet A36, or A992, etc. - those steels listed in Chapter A of the AISC spec. If so, you would simply look at the reserve ratio (Fu/Fy) and adjust your levels of safety accordingly.

But if he gets 36 ksi for the yield, there is nothing inherently wrong with using 0.66Fy for that beam and material. The old 16ksi "allowable" limits were due to lack of knowledge back then - not a limit of the steel properties.

 

[ick2]

I would like to mention that the results from the testing of the steel gave yield stress of 38300psi and tensile stress of 64300psi. The bending stress from the new loads is 21000psi. i do not see how this steel could have an allowable of only 16 or 18 ksi. I will tend to agree with JAE unless someone else convinces us otherwise. Please let me know if anyone has any other thoughts on this. thanks

 

[SlideRuleEra]

Before proceeding perhaps you could post in the "Material engineering other topics" forum. I just made a rare visit over there - looks like you might have a different audience for the most part.

 

[AlohaBob]

To me the 16ksi seems reasonable. You could try some back engineering to see what design stress did they use on the existing structure.

I think you have to keep in mind, the quality control of the steel itself was likely not as good as current manufactured steel.

 

[jheidt2543]

From "Modern Framed Structures" by Johnson, Bryan and Turneaure, John Wiley & Sons, 9th Edition, 1916:

P. 14 & 15
Working stress for tension members is usually placed at from 16,000 to 20,000 psi. "It should be understood that this discussion relates to ELASTIC strength and not ULTIMATE strength..."

P. 33
"In compression members the yield point (practically the ultimate strength)is reached with a load of 22,000 psi..."

From "Steel and Timber Structures" by Hool and Kinne, McGraw-Hill, 2nd Edition, 1942, Table 3 page 5:

Unit Stress Allowed on Steel by Various Cities (ksi)
Chicago Dallas Denver Detroit New York San Fran
Tension 18 18 18 18 18 20
Compres. 18 18 18 15 18 18

These valuses are from the noted cities building codes adopted between 1929-1941.

It is interesting to compare them to today's values and also interesting to see the values for San Franciso.

Just some more food for thought.