COLUMN

Question 1

Column Design Steps

Answer 1

1) Design Actions
2) Load Combinations
3) Column Moments
4) Section Design
5) Code Checks
6) Transverse Reinforcement
7) Lapping

Question 2

1) Design actions

Answer 2

• Dead Load, Ng
• Live Load, Nq (apply area reduction factor)
• Earthquake Induced, Neo (resultant of beam overstrength shears either side of joint)

Question 3

2) Load Combinations

Answer 3

critical load combinations: max. compression; min. tension

1) N* = Ng + Nq + Neo (max compression)
2) N* = Ng - Neo (min tension)
3) N* = 1.2Ng + 1.5Nq (typically never governs)

Question 4

3) Column Moments

Answer 4

• determine overstrength factor
• magnify moments obtained by dynamic modification factor
• since moment gradient of column unknown, assume 60% of critical shear to act concurrently with design moment

Question 5

overstrength factor

Answer 5

phi0 = sum(Mo) / sum(Me)

• scales up column moments from analysis to ensure stronger than the overstrength moment of beams
• this is to: 1) avoid “soft storey” mechanism 2) have “weak beam” mechanism
• because a) column sections less ductile than beams b) ductile detailing for column hinges more elaborate

Question 6

dynamic modification factor

Answer 6

• factor allows for higher modes

- static analysis considers only the first mode

Question 7

4) Section Design

Answer 7

• use design charts or appropriate software to determine required long. reinforcement
• design charts underdesign by about 7%

Question 8

5) Code Checks

Answer 8

• determine max. bar size to avoid bond failure
• check min and max reinforcement ratios
• determine practical bar layout (must be symmetrical and satisfy code spacing)

Question 9

6) Transverse reinforcement

Answer 9

• determine PPHZ where ductile detailing required
• size of PPHZ dependent on axial load f(N / Agf’c)
• transverse steel area requirements for confinement, anti-buckling and shear
• check stirrup spacing against code

Question 10

7) Lapping

Answer 10

• below level 2 all lapping should be at mid-height of column (cannot lap at PPHZ due to likely loss of bond as columns elongate)
• above level 2 can lap just above joint (easier to construct and safer)