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