Western Building Structural Mitigation Options
1. Introduction.
This document summarizes possible structural strengthening alternatives to mitigate the effects
of tunnel induced deformation on the Western Building, located at 619 Western Avenue in
Seattle, Washington. Three alternatives are presented with conceptual level design drawings to
illustrate each approach.
The preferred ground improvement method regardless of the structural strengthening approach
would be compensation grouting. Foundation strengthening schemes for both options would
also be essentially the same, except that the new pile and pile cap layout would be revised as
needed to support differing structural layouts. These ground improvement and foundation
strengthening approaches are described in Western Building Risk and Mitigation Technical
Memorandum – Revision 1 dated October 12, 2010.
2. Structural Strengthening Option A
a. Description.
The building would be stiffened in order to behave as a rigid body with full-height steel
bracing at the exterior walls. Certain damaged or weak existing elements would be
repaired or strengthened.
Stiffening of the exterior walls would consist of an extensive latticework of steel bracing
located just inside the building cladding on the east, south, and west elevations of the
building. A deep concrete grade beam as described above would be constructed below
this latticework. A network of concrete grade beams would interconnect the individual
column pile caps and the remaining wall foundations, thus minimizing the differential
movement. This latticework would be permanent.
The floor structure would be tied together with steel elements connected to the timber
floor structure and epoxy bolted to the concrete structure. The timber girders would be
tied to the interior timber columns.
The cracked concrete columns at the east façade would require epoxy injection and a
composite fiber wrap or concrete jacket. Horizontal channels would be added at each
level across the large wall cracks, epoxy bolted to the sound concrete on either side of
the crack.
Installation of the structural frame would likely require structural cribbing and/or
additional framing to be installed inside the building to provide support to the existing
structure during construction.
Construction time is approximately 12 months.
b. Concerns:
• Reduces usable space by 5% (based on existing condition)
• Bracing visible through windows impacts aesthetics
Public Disclosure Request 10-0123 for Elizabeth Campbell
Third installment
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• Construction time and impacts to adjacent tenants.
• Triggers substantial alteration code requirements
• High cost
3. Structural Strengthening Option B
a. Description.
The building would be stiffened to behave as a rigid body at the interior of the building
with steel bracing. The exterior walls would be stiffened by steel trusses at the upper two
floors which would span to the interior steel bracing. Certain damaged or weak existing
elements would also be strengthened or repaired.
The exterior walls would be reinforced by a steel truss just inside the building cladding
on the east, south, and west elevations of the building. The truss would be two stories in
depth, consisting of a bottom chord just below the fifth floor decking and a top chord just
below the roof decking with diagonal web members. At each column line vertical web
members of the truss would extend the full-height of the wall, continuously attached with
expansion bolts, thus hanging the wall from the truss. Each truss would be supported by
the steel bracing described below, as well as a two-story truss running north-to-south at
the center column line of the building.
The interior building framing will be stiffened by a latticework of steel bracing located on
the first interior column line of the building on all four sides. The steel bracing will extend
from the foundation to the underside of the roof framing, and will also support the steel
trusses described above. A network of concrete grade beams would interconnect the
individual column pile caps and the remaining wall foundations, thus minimizing the
differential movement. The latticework and trusses would be permanent.
The floor structure would be tied together with steel elements connected to the timber
floor structure and epoxy bolted to the concrete structure. The timber girders would be
tied to the interior timber columns.
The cracked concrete columns at the east façade would require epoxy injection and a
composite fiber wrap or concrete jacket. Horizontal channels would be added at each
level across the large wall cracks, epoxy bolted to the sound concrete on either side of
the crack.
Installation of the structural frame would likely require structural cribbing and/or
additional framing to be installed inside the building to provide support to the existing
structure during construction.
b. Concerns.
• Reduces usable space by 10% (based on existing condition)
• Reduces functionality due to extensive internal bracing
• Triggers substantial alteration code requirements
• Approximately 25% more costly than Option A
Public Disclosure Request 10-0123 for Elizabeth Campbell
Third installment
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4. Structural Strengthening Option C
a. Description.
The building would be stiffened to behave as a rigid body at the interior of the building
with steel bracing. The exterior concrete walls, and two interior concrete walls, would be
stiffened by a shotcrete wall epoxy doweled to the existing concrete. Certain damaged or
weak existing elements would also be strengthened or repaired.
The exterior walls would be strengthened and stiffened by a reinforced shotcrete wall
just inside the building cladding on each side of the building. The north concrete wall
shared with the Polson Building to the north and the interior concrete wall would also be
reinforced by a shotcrete wall on one side. The shotcrete walls would be full-height,
matching the current configuration of the existing wall openings and would be reinforced
with continuous vertical and horizontal reinforcement. Continuous attachment to the
existing concrete would be accomplished with epoxy dowels in a grid pattern.
The exterior walls with shotcrete would be supported by the interior braced frame
columns where they intersect. To tie the exterior walls to the interior braced frames, the
exterior shotcrete walls would also be supported by diagonal braces at each column line
(where braced frames do not occur) between the 2nd and 3rd level and between the 4th
and 5th level.
The interior building framing will be stiffened by a latticework of steel bracing located on
the first interior column line of the building on all four sides. The steel bracing will extend
from the foundation to the underside of the roof framing, and will also support the
exterior walls with shotcrete as described above. A network of concrete grade beams
would interconnect the individual column pile caps and the remaining wall foundations,
thus minimizing the differential movement. The latticework and shotcrete would be
permanent.
The floor structure would be tied together with steel elements connected to the timber
floor structure and epoxy bolted to the concrete structure. The timber girders would be
tied to the interior timber columns.
The cracked concrete columns at the east façade would require epoxy injection and a
composite fiber wrap or concrete jacket. Wall cracks would be fully grouted prior to
shotcrete wall installation.
Installation of the structural frame would likely require structural cribbing and/or
additional framing to be installed inside the building to provide support to the existing
structure during construction.
b. Concerns.
• Reduces usable space by 15% (based on existing condition)
• Reduces functionality because of more extensive internal bracing
• Triggers substantial alteration code requirements
• Approximately 50% more costly than Option A
Public Disclosure Request 10-0123 for Elizabeth Campbell
Third installment
Page #