A local empowerment zone
redevelopment agency is leasing a 1930’s concrete wall building from a local
city. The building was a high bay fire station and since the original
construction, a second story wood frame structure has been added. The walls
were cored and while the cores were tested for compression, the reinforcement
was found to be the old square rebar used in that period of time.
A firm from outside the area was
brought in by an Architectural firm representing a bank that wishes to
sub-lease the building. I was recommended by the local testing agency as a
second opinion based on my knowledge of old Unreinforced Masonry
buildings.
Even if there is minimum rebar,
the walls that pose the least threat are the side walls which are long and
solid. The shear transferred from the wood frame addition above is not a
threat to these walls. There ends of the building pose another problem. The
front is where two large doors were placed to allow fire trucks in and out.
The rear wall is inaccessible at the time the testing lab inspected the
building, but I would assume that the wall which the Architect shows on his
plan is concrete, but too short and possibly too weak to be used as an
appropriate shearwall – especially due to the shear from the second story wood
frame addition. This rear wall is concealed by a wood one story addition added
sometime after the building was constructed. This portion of the building will
be removed by the bank and a new structure will be built in its
place.
For those in California, here are
my questions:
- I don’t know of any Hazard
Mitigation program in California that
would require retrofit of the existing roof (floor of the second story) to
the concrete walls. UCBC Appendix Chapter One does not address reinforced
concrete buildings. Is this building exempt from retrofit rules in
California?
- The concrete is not the best –
some good cores, others that have too much aggregate and local desert sand
in the mix. Still, the length of the walls and the minimum reinforcement
would resist lateral loads. I would plan on a steel moment frame in the
front of the building connected by welding to the steel lintel above the
doors. The new design pops the front out two or three feet which makes the
frame an ideal solution as there is enough room for the grade beam and
erection pads in proximity to the property line. The rear wall can be
strengthened with Gunnite for shear and additional foundations added or
replaced as needed. What choices do I have to reduce further deterioration
of the concrete on the exterior face of the sides (long sides) of the
building?
- A mezzanine is to be added by
the bank. I believe this mezzanine (which will induce additional shear) can
be supported on a wood stud wall and the floor joists (TJI) supported by
these walls. The mezzanine can be used to brace the walls from buckling, The
bearing stud walls below can also be used to run utilities with minimum loss
to leasable space. Does this sound like a reasonable plan?
The tough part is that the
Empowerment zone has limited funds and I have no idea how to calculate the
retrofit or upgrade portion of the project. If the mezzanine is used to brace
the walls then the cost is absorbed by the bank as an improvement and does not
come from the Empowerment Zone. This leaves me only the moment frame, a
Gunnite wall at the rear and any coating or covering of the concrete that can
improve or reduce further deterioration of the walls. The building is probably
about 2000 square feet and the walls are 8-inch thick with joints at 8-feet
horizontal as the this was the height of the lifts.
Another firm figured the
reconstruction to bring the building into a safe zone would be close to
$90,000.00 which seems very high to me. It is a prevailing wage project but I
would think that at the worst case scenario, we would be talking between $50K
and $60K.
I could use some advice as how I
might approach this type of structure. My experience, besides wood, is URM and
Structural Clay (Unreinforced). I’ve also done some Adobe but have not worked
on this type of Concrete structure. Finally, the building is not on a state
historic registry and not protected except by the allowances of the
city Building And
Safety division.
Please let me know what you
think.
TIA
Dennis S. Wish,
PE
PS. We have not done a Pachometer
test yet and I am waiting for options before recommending spending any more of
their money on this building. I was not involved in the original testing or
preliminary plan development – I was brought in to offer a second opinion to
another engineers report. While his report appears valid, his approach to
solving the problem is to attempt to bring the building up to compliance. My
approach would be similar to URM buildings – identifying their weakness and
designing to the failure of the weakest element but providing sufficient
secondary support to get people out of the building. After that there would be
no guarantees.
