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BC-11 The horizontal beam column shown below is subjected to service live loads as shown. The member is laterally braced at its ends. Check its capacity to LRFD code ? Solution:- Example BC-3 20 k 5'5' 5'5' 10 ' W8 35 A36 Column Action:- R x = 3.51 in, r y = 2.03 in, A g = 10.3 in 2
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BC-12 c F cr = 27 ksi (Table 4 - 22 page 4 – 319). c P n = c F cr A g = 27 10.3 = 278.1 k Use Equation (H1 – 1b) interaction formula. Beam Action:- L b = 10 ft, C b = 1.32, Z x = 34.7 in 4 I xx = 127 in 3 S x = 31.2 in 3. M p = Z x F y = 104.1 k·ft.
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BC-13 M n = M p = 104.1 k·ft b M n = 0.9 104.1 = 93.7 k·ft
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BC-14 Moment Magnification:- C m = 1.0 (transverse loads and ends unrestrained). M u = B i M nt = 1.013 80.52 = 81.55 k·ft Interaction Formula:- Section is adequate !
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BC-15 The beam column shown below is part of a braced frame. The factored loads and moments are given as shown. Bending is on strong axis. K x L x = K y L y = 14 ft. The W 14 61 section is made of A572 Gr 50 steel. Is this member safe ? Example BC – 4 A)Determine interaction formula, and Column Action :- Solution:-
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BC-16 B) Beam Behavior:- From the Beam Design Charts with C b = 1.0, L b = 14 b M n = 344 kips·ft. (page 3.121) For actual (C b ) 1 see the moment diagram of the column: b M n = 364.6 kips·ft. b M p = 383 k·ft (page 3 – 17 – Z x tables). (also confirmed on page 3 – 121).
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BC-17 C) Interaction Equation:- The factored load moments :- M nt = 82 k·ft, M lt = 0 M u = B 1 M nt + B 2 M lt = 1.0066 82 + 0 = 82.5 k·ft
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BC-18 Example BC – 5:- Determine the service load w(40% DL + 60% LL) that can be applied to the member shown below, Ends are simply supported. Lateral bracing at ends only A 572 Gr 50 steel, strong axis bending. Solution:- P u = 1.2 20 + 1.6 40 = 88 kips. Let w u = factored ultimate load (k/ft) = ? M ux = B 1 M nt + B 2 M lt = B 1 M nt P P w k/ft 20 ft DL = 20 k LL = 40 k w = 12 96
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BC-19 Column Action:- c F cr = 28.9 ksi c P n = c F cr A g = 28.9 28.2 = 815.0 k· Beam Action:- For l b = 20 ft & C b = 1, b M nx = 498 k·ft (page 3 – 119 LRFD). Actual C b = 1.14 Actual b M nx = 1.14 498 =567.7k·ft Beam-Column Action:-
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BC-20 1.2 DL + 1.6 LL = w u 1.2 (0.4 w) + 1.6 (0.6 w) = 10.5 1.44 w = 10.5 w =7.30 k/ft DL =3.0 k/ft LL = 4.30 k/ft
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