BEAM

Question 1

Beam Steps

Answer 1

1) Loads
2) Demands
3) Possible Bar Layouts
4) Flexural Design Checks
5) Moment Redistribution
6) Curtailment
7) Phi Mn Envelope
8) Transverse Reinforcement

Question 2

1) Loads

Answer 2

• accurately determine loads based on trib area
• dead load
• live load
• apply area reduction factor

Question 3

2) Demands

Answer 3

• use code analysis to determine EQ demand
• important to consider EQ in both directions
• can use software (SAP2000) to analyse gravity loads

Question 4

3) Possible Bar Layouts

Answer 4

• determine phiMn for a number of possible bar layouts

- layouts must respect min. cover, and assume a maximum bar size

Question 5

4) Flexural Design Checks

Answer 5

• check max bar diameter to avoid bond failure
• check min and max reinforcement ratio
• check hook development length

Question 6

5) Moment Redistribution

Answer 6

• plastic behaviour of beam allows for redistribution
• target common M*
• should reduce absolute max moment (usually -ve) and compensate by increasing moments in non-crit regions (usually +ve)
• controls axial force induced in columns; more even distribution of long. reinforcement; standardise cages for ease of fabrication
• equalises moments for beam sections on opposite sides of interior columns
• reinforcement need not be anchored or terminated at beam-column joints

Question 7

Rules for moment redistribution

Answer 7

1. limit of 30% reduction of -ve moments
   - ensure no premature PHZ formation
2. no limit to increasing +ve moment
3. must retain storey equilibrium of centreline moments
4. use column face moments for beam flexural reinforcement design
   - must be equal before and after redistribution to maintain equilibrium

Question 8

6) Curtailment

Answer 8

• undertaken when either or both PHZ have reached overstrength
• plot Mn-Mo diagrams and capacities of sections
• curtailment lengths are functions of Ld and 0.3d (0.3 d accounts for cracking shallower than 45 degrees)
• important to use Tjd shifted diagrams

Question 9

Rules for Curtailment

Answer 9

• at least 2 bars > 16mm full length for stirrups
• at least 1/4 max positive tension rebar must extend to supports
• typically lap at midspan of beams

Question 10

7) Phi Mn Envelope

Answer 10

• plot capacity envelope
• check M*1.2G+1.5Q load case fits within envelope (can redistr. and drop BMD by up to 30% for -ve moment)
• also plot 70% M+1.2G+1.5Q (represents SLS - can not redistr.)
• capacity envelope must satisfy these two plots
• can add additional +ve steel on bottom if needed (curtail d from column face with vertical hook to ensure plastic hinge forms at desired location)
• capacity moment diagram should touch tenion shifted demand and at d from column face to ensure plastic hinge triggered under design EQ

Question 11

8) Transverse reinforcement

Answer 11

3 reasons

1. stop compression bars buckling
2. confine PHZ core concrete
3. carry shear
   - calculate shear forces associated with beam flexural overstrength
   - must determine PHZ (detailing depends on this)
   - maximum shear occurs at d from column face