Determine Design Loads;
Tributary area "AT" = 2.5' 20' = 50-ft2 (4.65m2)
( )
Dead Load = 13psf(2.5' +(estimated self wt. of 25plf)
)
= 57.5 plf (0.84KN/m)
Note: Due to the small tributary area, live load reduction (as per ANSI/ASCE 7-95) is not permitted.
Live Load = 2.5' 20psf)
(
= 50plf (0.73KN/m)
Strength requirements;
Note: Beam has continuous lateral support from attached roof deck
Wu = 1.2(57.5plf)+1.6(50plf) = 149plf (2.17KN/m)
Mu = wuL2/8 = 149plf(20' 2/8 = 7.45ft-kips (10.10KN-m)
)
Deflection requirements;
L
20' (12"/1' )
∆ allow =
=
= 1.0" (25.4mm)
(Per ANSI/ASCE 7-95 section B.1.1)
240
240
5w L L4
5(0.05klf )(20' ) 4 (12"/1' ) 3
= 6.2 - in4 (2.5X106 mm4)
I req'd ≥
=
384E∆ allow
384(29,000ksi)1.0"
By inspection, a W12X14 (W304.8mm X 0.20KN/m) will work due to the light loading condition
Choose W12X14 (W304.8mm X 0.20KN/m)
(4) Transverse Beams. Transverse beams are part of the moment frames. As a first approximation, it will
conservatively be assumed that the beams are simply supported. Over designing for gravity loads is not
expected to produce overly conservative results considering that drift due to seismic loads will probably
govern the final design. One design will be produced for the worst case gravity load and used throughout.
The worst case situation occurs at the low roof interior beam, which has the largest tributary area.
Determine Design Loads;
Tributary area "AT" = 20' 30' = 600-ft2 (55.7m2)
( )
Dead Load = 20psf (0.96KN/m2) (assumed)
Live Load Reduction per ANSI/ASCE 7-95;
AT = 600-ft2 ⇒ R1 = 0.6
Roof Slope < 4:12 ⇒ R2 = 1.0
∴ Reduced Live Load = 20psf(0.6)(1.0) = 12psf (0.57KN/m2)
wU = 1.2wD+1.6wL = [1.2(20psf)+1.6(12psf)]20'= 864plf (12.6KN/m)
Strength requirements; use Fy = 36ksi (248.2MPa)
w u L2
Mu
with M u =
Z req 'd ≥
φb Fy
8
864plf (30' )2 (12"/1' )
= 36 - in3 (589.9X103 mm3)
≥
8(0.9)36,000psi
w L 864plf (30' )
= 13k (57.8KN)
φv Vn ≥ u =
2
2
Deflection requirements;
5w L L4
L
30' (12"/1' )
≥
with ∆ allow =
=
= 1.5" (38.1mm)
I req'd
384E∆ allow
240
240
5(0.24klf )(30' ) 4 (12"/1' ) 3
= 101 - in4 (42.0X106 mm4)
≥
384(29,000ksi)(1.5" )
Compact section criteria (per AISC seismic provisions);
Try W14X26 (W355.6mm X 0.38KN/m);
H4-5
Previous Page
