releky
Structural
- Oct 31, 2013
- 129
Hi, to those who have discussed with me before about missing seismic shear strength in the building I'm managing as engineer.. I'll update you with the situation concerning attempt for shear upgrade.
Based on the designer calculations which I have seen and discussed with them. Shear from concrete is totally ignored in seismic design. So you have to rely only on stirrups for resisting all factored dead and live load and all lateral movement.
We did actual tests on the stirrups and came up with 370 Mpa instead of just 275 Mpa. So shear capacity from stirrups is about 182 kN or exactly
Vs = 0.75 * Av . fy . d /s = 0.000000142 * 370 Mpa * 0.485 / 0.1 = 182.01 kN
V(g) or shear due to gravity = 1.2 LL + 1.6 DL = 170 KN
So the stirrups is only good for the factored dead and live load, note again the shear from
concrete capacity is totally ignored in seismic design so Vc is set to 0 ven though it has about
120 kN capacity (perhaps owing to concrete just breaking up in seismic plastic hinges so it is
ignored)?
Now see the following:
(see also
We lack shear capacity from the beam rotations occurring from the energy (and load) added by seismic movement.
What happened was the contractor didn't include middle stirrup legs so there are only 2 stirrup legs instead of 3.
Or from calculations of probable moment strength:
As (area of steel) =7x0.000314 = 2199.11
longitiudinal fy=414mpa
fc'=28mpa
b=0.300mm
a=1.25As.fy/0.85fc'.b = 0.159
Mpr 1.25 as.fy (d-a/2) = 435 kN
Mpr(-) + Mpr (+) / length = about 140 kN.
So we are missing about 140 kN just for the seismic rotations of the beam depicted above.
Since 1.2 DL and 1.6 LL is about 170 kN.. then the contribition from seismic shear capacity is like multiplying 1.2 DL + 1.6 LL by 1.8 times.
The designer still doesn't know to add I-beam below the beam for shear upgrade since this
has not been done before and composite action can't be guaranteed. They suggest metal plates in the beam but we are worried the metal plates bolts may crack the beam. And we
don't trust carbon fiber because of possible strain incompatibility. Also in seismic design,
the shear contribution from concrete is ignored, so shouldn't shear contribution from
external aid like carbon fiber or metal plates also ignored?
The building has no shear wall but rely only on frame action to resist seismic load. It is
2 storey with concrete roofdeck and has mat foundation (bit overdesign because it was
initially thought the soil is soft when it is stiff). The columns have adequate strength
for probable moment strength and shear and the mat foundation connecting all
the columns have rotational restrain.. but during seismic activity, thee floors may still
sway.. is there a way to brace concrete frames from sway (hence prevent the
beams from rotating at all)? Has anyone tried this?
Any tips (so I can discuss with the designer as they are running out of idea how to shear upgrade it)
and it will be very expensive or impossible to demolish the entire building and rebuilt from
scratch since the columns have no laps due to 14 meters being used connecting to the mat foundation
all the way to the top of the roofdeck above the second storey.
Thank you for any tips.
Based on the designer calculations which I have seen and discussed with them. Shear from concrete is totally ignored in seismic design. So you have to rely only on stirrups for resisting all factored dead and live load and all lateral movement.
We did actual tests on the stirrups and came up with 370 Mpa instead of just 275 Mpa. So shear capacity from stirrups is about 182 kN or exactly
Vs = 0.75 * Av . fy . d /s = 0.000000142 * 370 Mpa * 0.485 / 0.1 = 182.01 kN
V(g) or shear due to gravity = 1.2 LL + 1.6 DL = 170 KN
So the stirrups is only good for the factored dead and live load, note again the shear from
concrete capacity is totally ignored in seismic design so Vc is set to 0 ven though it has about
120 kN capacity (perhaps owing to concrete just breaking up in seismic plastic hinges so it is
ignored)?
Now see the following:
(see also
We lack shear capacity from the beam rotations occurring from the energy (and load) added by seismic movement.
What happened was the contractor didn't include middle stirrup legs so there are only 2 stirrup legs instead of 3.
Or from calculations of probable moment strength:
As (area of steel) =7x0.000314 = 2199.11
longitiudinal fy=414mpa
fc'=28mpa
b=0.300mm
a=1.25As.fy/0.85fc'.b = 0.159
Mpr 1.25 as.fy (d-a/2) = 435 kN
Mpr(-) + Mpr (+) / length = about 140 kN.
So we are missing about 140 kN just for the seismic rotations of the beam depicted above.
Since 1.2 DL and 1.6 LL is about 170 kN.. then the contribition from seismic shear capacity is like multiplying 1.2 DL + 1.6 LL by 1.8 times.
The designer still doesn't know to add I-beam below the beam for shear upgrade since this
has not been done before and composite action can't be guaranteed. They suggest metal plates in the beam but we are worried the metal plates bolts may crack the beam. And we
don't trust carbon fiber because of possible strain incompatibility. Also in seismic design,
the shear contribution from concrete is ignored, so shouldn't shear contribution from
external aid like carbon fiber or metal plates also ignored?
The building has no shear wall but rely only on frame action to resist seismic load. It is
2 storey with concrete roofdeck and has mat foundation (bit overdesign because it was
initially thought the soil is soft when it is stiff). The columns have adequate strength
for probable moment strength and shear and the mat foundation connecting all
the columns have rotational restrain.. but during seismic activity, thee floors may still
sway.. is there a way to brace concrete frames from sway (hence prevent the
beams from rotating at all)? Has anyone tried this?
Any tips (so I can discuss with the designer as they are running out of idea how to shear upgrade it)
and it will be very expensive or impossible to demolish the entire building and rebuilt from
scratch since the columns have no laps due to 14 meters being used connecting to the mat foundation
all the way to the top of the roofdeck above the second storey.
Thank you for any tips.
