Common seismic requirements (Seismic: AISC 341)

Tekla Structural Designer
2021
Tekla Structural Designer

Common seismic requirements (Seismic: AISC 341)

Required strength

The required strength (including overstrength effects) for a member should be determined from:

The expected yield stress Ry x Fy

The expected tensile strength Rt x Fu

Grade Fy Ry Rt
A36 36 1.5 1.2
A53B 35 1.6 1.2
A500B 42 1.4 1.3
A500B 46 1.4 1.3
A500C 46 1.4 1.3
A500C 50 1.4 1.3
A501 36 1.4 1.3
A529 50 1.2 1.2
A529 55 1.1 1.2
A572 42 1.3 1.0[1]
A572 50 1.1 1.1
A572 55 1.1 1.1
A913 50 1.1 1.1
A913 60 1.1 1.1
A913 65 1.1 1.1
A992 50 1.1 1.1

[1]This value is 1.1 in AISC 341-05.

AISC 341-16 and AISC 341-10 seismic classification - all members

When required by the seismic checks, the classification of elements of the cross section for various member types is as follows.

Compiled from Table D1.1 of AISC 341-10

Section Element Width thickness ratio Application λhd – highly ductile λmd – moderately ductile
I (rolled) Flange bf/(2 * tf) Beams, Columns, Braces 0.30 * √(E/Fy) 0.38 * √(E/Fy)
Web h/tw Braces 1.49 * √(E/Fy) 1.49 * √(E/Fy)
Web h/tw Columns, Beams

LRFD - Ca = Pu/(ɸc * FyAg)

ɸc = 0.9

ASD - Ca = c * Pa/(Fy * Ag)

Ωc = 1.67

Ca ≤ 0.125

2.45 * √(E/Fy) * (1 - 0.93 * Ca) but for SMF only ≤ 2.45 * √(E/Fy)

Ca > 0.125

0.77 * √(E/Fy) x (2.93 - Ca) but ≥ 1.49 * √(E/Fy)

LRFD - Ca = Pu/(ɸc * FyAg)

ɸc = 0.9

ASD - Ca = c * Pa/(Fy * Ag)

Ωc = 1.67

Ca ≤ 0.125

3.76 * √(E/Fy) * (1 - 2.75 * Ca) but for IMF only ≤ 3.76 * √(E/Fy)

Ca > 0.125

1.12 * √(E/Fy) x (2.33 - Ca) but ≥ 1.49 * √(E/Fy)

RHS and SHS Walls (bf - 3 * t)/t and (d -3 * t)/t

Braces

0.55 * √(E/Fy) 0.64 * √(E/Fy)
RHS and SHS Walls (bf - 3 * t)/t and (d- 3 * t)/t Columns 0.55 * √(E/Fy) 1.12 * √(E/Fy)
CHS D/t Braces 0.038 * E/Fy 0.044 * E/Fy
CHS D/t Columns 0.038 * E/Fy 0.07 * E/Fy
C (rolled) Flange bf/tf Braces 0.30 * √(E/Fy) 0.38 * √(E/Fy)
Web h/tw Braces 1.49 * √(E/Fy) 1.49 * √(E/Fy)
Tees Flange bf/(2 * tf) Braces 0.30 * √(E/Fy) 0.38 * √(E/Fy)
Stem d/tw Braces 0.30 * √(E/Fy) 0.38 * √(E/Fy)
Angles Both legs L1/t and L2/t Braces 0.30 * √(E/Fy) 0.38 * √(E/Fy)
Double angles Outstand leg – legs in continuous contact L1/t (long leg B to B) or L2/t (short leg B to B) Braces 0.30 * √(E/Fy) 0.38 * √(E/Fy)
Double angles Both legs – legs separated L1/t and L2/t Braces 0.30 * √(E/Fy) 0.38 * √(E/Fy)

In the above table the terms have their usual meaning as follows:

  • bf = width of flange and for RHS width of shorter side
  • tf = thickness of flange of I/H, channel or Tee
  • h = height of web inside flanges (d – 2 * tf) of I/H or channel
  • tw = thickness of web
  • d = depth of SHS and for RHS depth of longer side
  • t = thickness of hollow section RHS, SHS, CHS
  • D = diameter of CHS
  • L1 = Short leg (from root to toe) of single angle
  • L2 = Long leg (from root to toe) of single angle
  • E = modulus of elasticity of steel – 29000 ksi
  • Fy = minimum yield stress
  • Pu = required axial strength using LRFD (seismic) combinations
  • Pa = required axial strength using ASD (seismic) combinations
  • Ag = gross area of section

AISC 341-05 seismic classification - all members

When required by the seismic checks, the classification of elements of the cross section for various member types is as follows.

Compiled from I-8-1 of AISC 341-05 and Table B4.1 of AISC 360-05.

Section Element Width thickness ratio Application λps – seismically compact λps – conventionally compact
I (rolled) Flange bf/(2 * tf) Beams, Columns[1], Braces 0.30 * √(E/Fy) 0.38 * √(E/Fy)
Web h/tw Columns[2], Beams, Braces

LRFD - Ca = Pu/(ɸc * FyAg)

ɸc = 0.9

ASD - Ca = c * Pa/(Fy * Ag)

Ωc = 1.67

Ca ≤ 0.125

3.14 * √(E/Fy) * (1 - 1.54 * Ca) but for SMF only ≤ 2.45 * √(E/Fy)

Ca > 0.125

1.12 * √(E/Fy) * (2.33 - Ca) but ≥ 1.49 * √(E/Fy)

3.76 * √(E/Fy)
RHS and SHS Walls (bf-3t)/t and (d-3t)/t Columns, Braces 0.64 * √(E/Fy) 1.12 * √(E/Fy)
CHS D/t Columns, Braces 0.044 * E/Fy 0.070 * E/Fy
C (rolled) Flange bf/tf Braces 0.30 * √(E/Fy) N/A
Web h/tw Braces

LRFD - Ca = Pu/(ɸc * FyAg)

ɸc = 0.9

ASD - Ca = c * Pa/(Fy * Ag)

Ωc = 1.67

Ca ≤ 0.125

3.14 * √(E/Fy) * (1 - 1.54 * Ca)

Ca > 0.125

1.12 * √(E/Fy) * (2.33 - Ca) but ≥ 1.49 * √(E/Fy)

N/A
Tees Flange bf/(2 * tf) Braces 0.30 * √(E/Fy) N/A
Stem d/tw Braces 0.30 * √(E/Fy) N/A
Angles Both legs L1/t and L2/t Braces 0.30 * √(E/Fy) N/A
Double angles Outstand leg – legs in continuous contact L1/t (long leg B to B) or L2/t (short leg B to B) Braces 0.30 * √(E/Fy) N/A
Double angles Both legs – legs separated L1/t and L2/t Braces 0.30 * √(E/Fy) N/A

Note 1: The relaxation on the compactness limit for columns in SMF as per note “b” to Table I-8-1 is not taken into account.

Note 2: These limits are not modified by Note [j] to Table I-8-1 i.e. this dispensation is not taken into account.

In the above table the terms have their usual meaning as follows:

  • bf = width of flange and for RHS width of shorter side
  • tf = thickness of flange of I/H, channel or Tee
  • h = height of web inside flanges (d – 2 * tf) of I/H or channel
  • tw = thickness of web
  • d = depth of SHS and for RHS depth of longer side
  • t = thickness of hollow section RHS, SHS, CHS
  • D = diameter of CHS
  • L1 = Short leg (from root to toe) of single angle
  • L2 = Long leg (from root to toe) of single angle
  • E = modulus of elasticity of steel – 29000 ksi
  • Fy = minimum yield stress
  • Pu = required axial strength using LRFD (seismic) combinations
  • Pa = required axial strength using ASD (seismic) combinations
  • Ag = gross area of section
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