Jake,
Thanks for the advice – I did think
about this. If the City who owns the building allows me to treat it like a URM,
then I would be able to design the moment frame to the capacity of the
diaphragm rather than potentially higher loads from the second story structure
above. In other words, as UCBC Appendix Chapter One works, the building will
fail at the weakest shear transfer material. Normally it is a roof structure of
1x straight or diagonal sheathing, however, this building had been upgraded
sometime after the early 30’s. The diaphragm might have been improved
(see Roger Turks comments) and this needs to be evaluated.
I think that the City might consider the
building to be a potential Hazard and allow me to design to provide sufficient
secondary support and shear at the soft story (and maybe interior cross-walls
if necessary) to protect the occupants until they can safely leave the
building.
The problem is that this building is
within 2 Km of the San Andreas Fault and we do need to consider that this area has been determined by
Cal-Tech to be 50-years beyond a major seismic event.
I have a copy of FEMA 356 and will look
though it as you suggest.
Thanks
Dennis
-----Original Message-----
From: Jake Watson
[mailto:jwatson(--nospam--at)utahisp.com]
Sent: Thursday, November 20, 2003
4:37 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: 1930's Poured in
Place Concrete Building
Remember to consider relative rigidities of materials. If you
put a moment frame on the front, how far will it move before it does resists
much force? Will the masonry along that line fail before the frame takes
over? If the masonry fails in shear, will it still carry gravity loads or
is there an alternate gravity load path?
Also, I second Gerard's comments about change in occupancy. I
would review the new occupancy with the municipality before proceeding too
far. If you are increasing the "public hazard" significantly,
you may need to be fully code compliant.
As for the concrete, you might look a FEMA 356. It might give
you a better procedure to deal with the concrete.
Jake Watson, P.E.
Salt Lake City, UT
-----Original
Message-----
From: Dennis Wish
[mailto:dennis.wish(--nospam--at)verizon.net]
Sent: Wednesday, November 19, 2003
11:52 PM
To: seaint(--nospam--at)seaint.org
Subject: 1930's Poured in Place
Concrete Building
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:
1.
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?
2.
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?
3.
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.
