Capacitiy of Existing Concrete Joist System
Capacitiy of Existing Concrete Joist System
(OP)
I've been asked to look at an existing one-way concrete joist system. It was built in 1953.
I have a good set of the original construction documents. The f'c is listed to be 3,000 psi minimum and the reinforcing steel is indicated to be intermediate billet with an allowable tensile stress of fs = 20,000 psi.
I was just thinking about something here.....if the nominal moment of resistance was governed by the following equation:
Mn = (As)(Fy)(d - a/2).....and I know Fy for intermediate billet steel is 40,000 psi.......I'm thinking to myself if 20,000 psi should be used for Fy instead of 40,000.
My thinking is that this Mn equation is defining the internal couple inside a beam, with d-a/2 being the lever arm. If the allowable steel tensile stress is only 20,000 psi, shouldn't that be in the equation instead of Fy at 40,000 psi?
I have a good set of the original construction documents. The f'c is listed to be 3,000 psi minimum and the reinforcing steel is indicated to be intermediate billet with an allowable tensile stress of fs = 20,000 psi.
I was just thinking about something here.....if the nominal moment of resistance was governed by the following equation:
Mn = (As)(Fy)(d - a/2).....and I know Fy for intermediate billet steel is 40,000 psi.......I'm thinking to myself if 20,000 psi should be used for Fy instead of 40,000.
My thinking is that this Mn equation is defining the internal couple inside a beam, with d-a/2 being the lever arm. If the allowable steel tensile stress is only 20,000 psi, shouldn't that be in the equation instead of Fy at 40,000 psi?






RE: Capacitiy of Existing Concrete Joist System
Most people would use LRFD, except maybe old-timers. In which case you'd use 40ksi and multiply by Phi. Then back off your load factors when you report the load capacity to your client.
RE: Capacitiy of Existing Concrete Joist System
To check the original design, you could factor the loads and use USD with Fy = 40 ksi.
BA
RE: Capacitiy of Existing Concrete Joist System
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Capacitiy of Existing Concrete Joist System
Thanks BA.....I know that the factor needs to be added when I do my analysis.....I was just trying to break this down to the moment arm and the force.
I do wonder, in my mind, when the fs = 20,000 would come into play? When determining bending capacity (using factored loads, etc) we're using yield strengths of steel in the equations (Fy and not fs).....and not going back and considering that the allowable tensile in the steel is 20,000psi.
That allowable tensile stress is bothering me.
RE: Capacitiy of Existing Concrete Joist System
RE: Capacitiy of Existing Concrete Joist System
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask