Walls Flashcards
ductility formula
ultimate / yield
lp
equivalent length of plastic hinge
- lower bound approximation: lp = h/2
yield strain
= fy / Es
yield curvature
= 2 ey / lw
- function of the yield strain of the steel
- not a function of section geometry
- nothing to do with strength
estimate of wall neutral axis
k = 0.25 c = klw = lw / 4
Determining structural ductility factor
- longest wall yields and reaches max ductility first
- max drift targeting is limited by ductility of longest wall
- ductility demand on other walls decreases as length of wall decreases
- structural response determined by summing wall responses
benefit of lower stiffness
smaller loads
benefit of bigger stiffness
smaller drifts
columns of different height
- same hc = same yield curvature
- taller column = larger yield displacement
- shortest column = highest local ductility demand
- shortest column sets structural ductility factor
columns of different depths
- smaller columns = higher yield curvature and therefore displacement
- column with largest depth = greatest local ductility demand
- deepest column sets structural ductility factor
estimation of effective flange width
- width of wall flanges in tension or compression is reduced because of the effects of shear lag
- based on the number of rebars put into strain
- moment applied - web in shear - transferred into C/T in flanges
- as moment increases and so too the associated tension in the flange, the section of flange resisting increases
- for nominal bending tension flange: hw + bw (26.6 degrees or 1:2)
- reduced width in compression flange after loaded in tension - less contact further away from web
- can’t close cracks unless very high axial load
- compression: 0.3hw + bw
- for overstrength this is conservative, so consider 45 degrees for tension
coupling effect
typically provides 40-60% of overturning capacity
two types of coupling beams
- conventionally reinforced
- diagonally reinforced cage
advantage of diagonally reinforced cage
concrete not needed outside of diagonals -
can put service ducts there
basketing
to keep wedges of concrete formed in diagonal cage from falling out
simplifying assumption for wall design
- assume all steel yielding in tension or compression
- percentage difference less than 1%
ultimate curvature
0.003 / c
flexural overstrength factor
= Mo / Me
maximum credible earthquake
- structures are designed for the DBE but should be capable of resisting a larger earthquake
- NZ standards may design a maximum credible earthquake (MCE) in the future
- for now MCE taken as 2.0 x ULS actions
- not OK to fail after ULS
Comments on Singly Reinforced Walls
- should be avoided
- 150mm thick walls are TOO THIN
- cant get reinforcement in
- anti-buckling reinforcement should also be provided over central region for yielding walls
Thin walls with Drossbach
- need to have transfer stirrups like columns
- properly detailed end of wall that look like mini-columns
- anchor floor starters and other embedments properly
- anchor horizontal bars in boundary elements
Comments on Precast Panel Slices
- walls roughened to min of 5 mm to prevent slipping/sliding
- failure of lap splices observed where drossbach ducts were not confined
- drossbach ducts cause stress concentrations at panel joints
- Drossbach ducts should be fully confined by stirrups
- clamp bars to drossbach and ensure bond maintained
- panel splices must allow for de-bonding of reinforcement
Comments on concrete strength
- higher than in lab testing
- code equations based on lab testing (different behaviour to concrete in buildings)
- min. reinforcement requirements may be insufficient
- min steel is to ensure fan cracking: bar yields and elongates
- if real life concrete»_space;> stronger than lab testing : no fan cracking
- case studies showed failure of bars in walls and inelastic capacity of bars exhausted
order of most likely strained elements
- walls
- beams
- columns