I realized today that I have been missing a check in my composite beam design. My text book "Structural Steel Design - ASD Method" (Jack McCormac, 4th ed.) specifies the following design criteria:
before concrete hardens
fs = Md / Ss < Fb
after concrete hardens
fs = Md / Ss + Ml / Str-bot < 0.9 Fy
fc = Ml / n Str-top , 0.45 f'c
which is what I have essentially been using throughout my career.
The Green Book states in the specifications (Chapter I, section I2, part 2, pg 5-57) "...the composite section shall be proportioned to support all of the loads without exceeding the allowable stress prescribed in Sect. F1-1, even when the steel section is not shored during construction."
Section F1.1, as far as I can tell, boils down to Eq. F1-1, limiting Fb = 0.66Fy.
To summarize Example 15 (pg 2-249):
Md = 71.3 ft-k; Ml = 166 ft-k
Ss = 81.6 in3; Str-bot = 122 in3
Check 1 - Part C:
12*(71.3+166) / 122 = 23.3 ksi < 24 ksi
Check 2 - Part H:
12*71.3/81.6 + 12*166/122 = 26.8 < 0.9*36 = 32.4 ksi
According to the commentary, the second check only ensures that the beam remain elastic under service loads.
Etabs follows the code. RAM follows the code.
before concrete hardens
fs = Md / Ss < Fb
after concrete hardens
fs = Md / Ss + Ml / Str-bot < 0.9 Fy
fc = Ml / n Str-top , 0.45 f'c
which is what I have essentially been using throughout my career.
The Green Book states in the specifications (Chapter I, section I2, part 2, pg 5-57) "...the composite section shall be proportioned to support all of the loads without exceeding the allowable stress prescribed in Sect. F1-1, even when the steel section is not shored during construction."
Section F1.1, as far as I can tell, boils down to Eq. F1-1, limiting Fb = 0.66Fy.
To summarize Example 15 (pg 2-249):
Md = 71.3 ft-k; Ml = 166 ft-k
Ss = 81.6 in3; Str-bot = 122 in3
Check 1 - Part C:
12*(71.3+166) / 122 = 23.3 ksi < 24 ksi
Check 2 - Part H:
12*71.3/81.6 + 12*166/122 = 26.8 < 0.9*36 = 32.4 ksi
According to the commentary, the second check only ensures that the beam remain elastic under service loads.
Etabs follows the code. RAM follows the code.
