I am working on an existing building with wide flange steel beam joists at the roof (simple span configuration). They are adding a significant amount of load to the roof. To increase the capacity of the beams, I am adding a steel plate (flat) to the bottom of the beams. Using Enercalc to find the properties of the modified beam I get two values for the section modulus (S). Speaking with another engineer I was told to use the higher value as the added steel will increase the tension capacity of the beam. Looking at my old text book it says that 'c' is the distance from the neutral surface to the extreme fiber and thus would always result in the lower value of 'S'. Please verify which of these is correct and if the second is correct is there ever a situation where you would use the higher value of 'S'.

Using the current code you would calculate the new plastic modulus "Z" instead of the section modulus.  The plastic modulus will only have one value, but if you are adding a plate and the wide flanges are 50 ksi yield than the calculation would have to incorporate the different yield strengths (assuming 36 ksi plate).

And you can have any Stop and an Sbottom, usually the "c" used is where yield will first occur.  

Also, adding the flange plate will not help with deflection unless the beams are jacked up first.  It does not matter for strength.

Also, the code has has a section for singly symmetric shapes bent about their major axis - F4 in AISC 360-05

ash060, thank you for the information. The existing beams are 36 ksi, to the best of my knowledge. The as-built drawings reference the 5th edition of the AISC from 1947, but give no other clues to the steel strength.

Deflection does not seem to be an issue, but I will double check this.

I will certainly utilize section F-4. It is interesting that in that section it utilizes Sxc for compression and Sxt for tension.

SamB35:

On page 327 of my 5th Edition (Ninth Printing) of the AISC code, under section 9(a) for tensile properties, it shows the minimum yield point to be not less than 33 ksi.  

I think I would be conservative here, unless 36ksi is specifically stated on the drawings you have, and use 33 ksi material.

Mike McCann
MMC Engineering

 

Mike:

The drawings specify the fourth printing, but I doubt that makes any difference. I will use 33 ksi for the existing steel. Is the 33 ksi minimum for steel angles as well as steel beams? I ask as there are some steel angles that I need to review as well.

Thanks,


Sam

The only exceptions mentioned in this section are Bolts, Rolled Base Plates, plates 3/16" in thickness, shapes less than 1 Sq. In. in cross section, and bars, other than flats, less than 1/2" in thickness or diameter.

So, I would say that angles are included too.

In all likelihood, you are dealing with a minimum of A7 steel.

Mike McCann
MMC Engineering

 

Not sure the original question was ever answered.  The two values of elastic section modulus are for the bottom and top of the beam.  One is in compression, one in tension, so you need to check both against the allowables.  The plate helps both, the bottom due to the added steel, and the top due to the increased "c".