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Subject: Phase One - Seismic Resistance Study
Saratoga Fire District Headquarters
This report has been prepared to present the results of our Phase One site inspection and conceptual evaluation of the current seismic resistance of the Fire District Headquarters building. Based on our non-destructive observations, experience, knowledge of code requirements and limited con crete test data, we have developed conclusions and concep tual repair recommendations for your consideration.
Introduction
The Saratoga Fire District Headquarters was established in 1923.
The existing structure was built sometime in the 1920's, and
served as an auto repair garage and gas station for some years.
During the 1960's, the Fire District took over the entire facility
at 13850 Saratoga Avenue, having rented some space in the building
prior to that time. Reduced copies of the 1/4-inch equals one
foot scale drawings developed to show the existing floor and roof
plans are presented in the back of this report as Figures One
and Two. The wall grid system presented in these figures is referenced
throughout this report. The front of the struc ture is assumed
to face north. Some representative photo ture is assumed to face
north. Some representative photo ture is assumed to face north.
Some representative photo a short description.
The building is constructed on top of a four foot high concrete retaining wall, except at the front wall from the dispatch office past the fourth truck bay. This retaining wall tapers down to grade along the grid line A side of the structure. The floor is slab-on-grade concrete. The Line D and D' walls are six-inch thick concrete, with a half-inch thick stucco overlay. The short wall facing the street intersection on grid line E is built from concrete masonry units. The remainder of the walls are wood framed. All of the exterior surfaces are stucco, with wood trim elements at windows and doors. The roof is comprised of two truss bays, with flat sections sloped towards the drains along the north wall. The flat portions are covered with cap sheet roofing, while the sloped portions are covered with asphalt shingles. The parapets are stucco covered on the outside, but cap sheet or shingle-covered on the roof side.
A tile-roofed and stucco finished bell tower was added at the center of the north wall in 1970. There are architectural tile accent elements at the north wall corners, and a low height tile roof on the southerly side of the building.
Observations - Exterior
The low height retaining wall that supports three sides of the
structure appears to have been made from river run aggregate.
If the spalling concrete cap along grid line 1a outside the medical
office is representative of its condition, then the wall has only
a limited life span left. The evidence of water intrusion on
the interior at various locations indicates that the wall does
not have a waterproof barrier on it. While asphalt ditches and
paving can minimize the amount of seasonal water intrusion, they
cannot completely eliminate it. The exposed retaining wall portion
in the work shop area along line D' is relatively crack-free.
The footing for this wall appears to be adequate, given the absence
of signs of foundation settlement.
The north side raised planters do not seem to have an irrigation system in them, which is probably best, given the questionable nature of the lower wall waterproofing system.
The short concrete masonry block wall on grid line E is covered with unreinforced stucco. A rebar detector picked up indications of vertical steel reinforcing within the block cells at an average of 24-inches on center, with horizontal steel at four foot on center. More horizontal rebar was found near the intersection of the grid line D' concrete wall, suggesting that a proper lapping of the concrete wall rebar and block work was done.
The concrete wall along grid line D and line D' was cored in order to obtain some compression samples. The lab results of the three cores taken gave ultimate compressive strengths of 2510 psi, 2910 psi and 2920 psi. These values are not bad, considering the age of the concrete.
The outside concrete wall surface is covered with a half-inch thickness of stucco. With no wire mesh or other mechanical bonding, the stucco easily broke away from the concrete cores. In a significant seismic event, it can be assumed that large chucks of this stucco exterior may fall off the wall. This area is not near the sidewalk, however.
The concrete wall reinforcing steel was detected at a grid of 24-inches on center, both horizontal and vertical. One vertical bar was uncovered, revealing a corroded 0.5-inch square steel bar positioned 2-inches in from the outer sur face of the six-inch thick concrete wall. The tie wire around the bar has some corrosion, however.
There is a significant amount of cracking in the stucco covering on the line D walls, located approximately where the wood framed parapet starts on top of the walls. With no stucco control joint at this obvious point of flexure, the cracking is not unexpected. The cracking has probably been exacerbated by the condition of the roof top parapet bracing, discussed later. Similar cracking can be observed at the parapet along grid line A.
The stucco covering on the southerly grid line 4 and 5 walls is in remarkable good shape, considering that it is somehow attached to corrugated metal siding backed by an irregular post and beam wood framing system. Calculations may well show these walls to be inadequate for out-of-plane seismic forces. It is good that the parapet is only two foot high along these walls, and simply follows the gable line of the roof trusses. The stucco thickness seems to vary widely, when compared to the constant width of the overhanging para pet wood cap.
The antenna tower mounted to the line 4 wall appears to be well anchored, but the wall construction itself is sus pect. However, the antenna does not appear to have enough mass to break free of the wall. It is more likely that por tions of the wall could break free of the roof and pull the tower down.
The wall parapet cap is generally galvanized and/or painted metal, and is in relatively good condition. The cap portion along grid line 4 and adjacent to it is made from wood that is highly weathered, and has reached the end of its useful life.
The wood parapets are braced with a twin 2x4 system attached with varying success to the roof framing. The 2x4 members are highly weathered, locally split, or missing at some locations. The adequacy of this bracing system is sus pect, due to one or more deficiencies at each brace. One section of the wood parapet along grid line 1 is not braced. It is not clear if the internal framing is sufficient to support this section of parapet as a cantilever. If not, then sections could fall off during a major seismic event, blocking the truck exit doors. The roof side of the grid line 1a parapet is shingled, but some of the shingles are sliding off, since shingles are not usually installed on a vertical surface (see photo 7).
Roof leaks have been occurring over the dispatch office and over the office area. The permanently attached bucket system does not lend itself to long term protection of the wood sheathing. Mold growth will destroy the local strength of the sheathing in a relatively short period of time (see photo 21). The bare asphalt edges of the shingles on the easterly sloped roof (photo 12) indicates that it is time to consider replacing this roofing surface, and repairing any decayed areas below.
Broken pieces of tile can be observed on several sides of the bell tower. The front broken piece looks like it was caused by the flag rope. Additional tile pieces are broken on the architectural overhang at the controllers office. Generally, tiles installed at this steep an angle do not perform well in a major earthquake. Given the proximity to pedestrian walkways, it may be prudent to consider a lighter weight architectural finish in these areas.
The free-standing backup power generator near grid line 5 is surrounded by concrete masonry walls with no roof covering. While it is disconcerting to be able to see light through some of the block joints, I understand that the walls are well reinforced and grouted. Cantilevered walls traditionally see very large horizontal forces during an earth quake. Several cross bracing steel pipes could be connected from the long block wall to the angled walls at the top edge, without interfering with generator operation. The less equipment stored in this area, the less likely the generator will be damaged by this stored equipment.
Observations - Roof Framing
The roof system consists of two parallel lines of wood trusses,
with a common support point along grid line C. The westerly work
shop and medical office roof is framed with flat rafters, pitched
towards the valley on grid line D. Additional joist framing over
the dispatch office area forms another flat section that is pitched
to the corner at line A and line 1b.
The trusses are mainly nailed together, with some bolts at the center area. Some of the truss diagonals do not meet at a concentric point, indicating that secondary bending stresses are present. The only sign of overstress was seen in the permanent deflection of the grid line 2 lower truss chord where a climbing rope is attached, near grid line D. There is actually a second diagonal truss member missing at this location. In order for the framing to work as a true truss, at least two members with vertical slopes must meet at a point.
There are no end plates or perpendicular ties from the truss ends to the concrete wall on grid line D. The trusses are simply toenailed to the 3-2x10 supporting beam system. There is no blocking adjacent to the truss ends. Blocking is especially important to have along grid line C, where both sets of roof trusses are supported. Solid wood blocking will prevent the trusses, and the parallel roof rafters, from twisting over during horizontal deflection of the roof assembly.
The original roof framing applied to the trusses is straight sheathing. Straight sheathing does not have a very high diaphragm strength value, and is not adequate for a building of this size. From gird line B to line C, the roof sheathing appears to have been replaced at one time with a blocked plywood diaphragm. The roof from line C to line D has a layer of rigid insulation above the original sheathing, then a layer of plywood above that. With proper nailing, plywood can develop all the strength this type of building needs. However, in all cases the plywood never gets to the outside shear resisting walls. Cricket framing just inside of the line D wall causes a change in roof slope, with the plywood stopping several feet from the wall.
Diagonal tie rods were observed in the bay between the line 5 wall and the first truss, connecting the outside wall to the lower chord of the truss. Since the out-of-plane strength of the wall is not very strong, these tie rods are of limited value. In general, actual earthquakes have shown tie rods to not be very effective, since they stretch and allow significant building damage to occur.
The most significant item missing from the roof system is a proper shear transfer system. It is critical that the roof diaphragm forces be transferred to the shear-resisting wall elements along the exterior walls. But only one anchor bolt was observed on line E. The few shot-in nails observed along line D are not equal in capacity to one anchor bolt.
At the flat roof sections that could be observed, the framing is simply toenailed into the supporting beam system. There are no shear transfer clips, no end blocking, and no anchor bolts. In addition, there are no perpendicular ties along either the concrete or the masonry walls. These ties have proven to be critical in keeping concrete walls and roofs attached during a major earthquake. While the building does have a post and beam independent support system for the roof framing, some of the posts are attached to the wall, and failure of one could damage/collapse the other.
The amount of shear resisting wall along the north side with the four truck bay openings is inadequate by inspection. Several steel frames, with appropriate footings, will be needed to straddle the entry doors and develop the needed seismic resistance for this wall.
A storage attic has been created between grid lines 3 and 4, using the office walls as load bearing elements. Significant quantities of materials, old drawings, file cabinets, plus a furnace and other elements are packed into this area. The 2x10 floor joists appear adequate to handle the vertical load, but the apparent lack of lateral support for this small diaphragm could cause some problems in the office area. A ladder system is in place to go over the truss elements and dodge the surface mounted electrical lines.
The storage attic stairs are too steep and have sloping treads. While stairs just to service equipment are exempt from the Uniform Building Code, the storage use suggests they should be closer to meeting the code maximum rise / minimum run provisions.
It is clear that the station needs more storage area than presently exists. For example, I found Santa Claus hid ing above the staff quarters t-bar ceiling. Future remodeling plans should develop some seismically designed, effective and easily reachable cold storage area.
Observations - Interior
The back office area suspended t-bar ceiling and lights are hung
on tension wires only - off an assortment of wood elements running
from the interior partition to the outside wall on line 5 (photos
23 & 24). There are no elements present that can handle upward
forces. Therefore, this ceiling can come down during a significant
earthquake. While the ceiling panels are relatively light weight,
the light fix tures are more dangerous to occupants. This is
also true for the medical area suspended ceiling, and the crew
quarters ceiling.
The interior walls/partitions have a variety of connection details to the trusses or roof elements, and seem to connect in a haphazard to non-existent manner. The two 2x4 out-of-plane braces for the partition wall separating the shop from the medical room are too vertically positioned to be effective. The heavy cabinets in the work shop could distort this wall out of plane.
The line D partition separating the shop and the truck bay is framed to an existing 6x10 with joist hangers that just barely make a connection. Again, the shop cabinets, if not anchored to the floor slab, could damage or bring this wall down.
The wood paneling used on the interior partitions is not nailed to the wall top plate. In some places, the paneling just runs up past the top plate. With no nailing, it can contribute very little horizontal resistance to earthquake Loads.
The line of beams along grid line D have metal clips to the 8x8 wood posts, and a horizontal 3/16-inch diameter wire"staple" at the beam joints. With a radiused surface at the point the stresses are highest - where the metal wire penetrates the beam - this type of connection can withdraw from the wood as it is loaded, similar to the way the old J-bolts fail during an earthquake.
The 911 equipment cabinet is against the concrete wall along line D. With no solid roof tie, this wall could fail during a large earthquake, taking the equipment with it.
There is surface-mounted wiring and plumbing everywhere. Continuing electrical work has apparently been done over the years. With phone wires, electrical wires and emergency equipment wires everywhere, it will make repair work go slower than usual, and therefore add to the cost of repairs. At some locations, the plumbing and tube-enclosed wiring sys- tems appear to be more substantial than the structural connections (see photo 30).
The florescent light fixtures over the truck bays are suspended from the trusses with small lengths of chain. During a seismic event, these fixtures will sway into each other, breaking the bulbs.
Conclusions and Recommendations
The 1991 edition of the Uniform Building Code (UBC) is the current
standard for building design. While the Uniform Code for Building
Conservation and the State Historical Code provide alternate,
less stringent, evaluation procedures fo buildings a community
is trying to save for historical or cultural reasons, these codes
are generally not applied to fire stations. An Essential Facility
is defined by the UBC as "those structures which are necessary
for emergency operations subsequent to a natural disaster"
. The UBC lists specifically hospitals, medical facilities, fire
and police stations, emergency vehicle garages, and standby power-generating
equipment for essential facilities. The Uniform Building Code
requires that any seismic design or evaluation of Essential Facilities
use earthquake loads that are 25% larger than a typical building
is subjected to. The UBC also requires Essential Facility buildings
to be subjected to wind loads that are 15% larger than those applied
to a typical Building.
The usual seismic evaluation of a one-story building is performed in order to develop simplified lateral earthquake forces that can be applied to the roof and detailed to suc- cessfully reach the foundation. Through this process, the strength of the roof diaphragm, all of the connections to the walls (both parallel and perpendicular), and the wall capacity itself, are checked and compared to known values. A typical wood roof and concrete/non-plywood shear wall building is subjected to a horizontal force equivalent to 0.183 times the force of gravity. With the additional UBC factor for essential structures, the fire station would be required to resist 0.23 G's of force. This translates into a seismic design force at the roof of about 37,900 pounds. It is well known that the actual earthquake forces may be much highe.r But since they are also applied for a very short period of time, experience to date indicates that this simplified method of analysis is sufficient to identify the seismic weaknesses in an existing structure.
Based on the calculations that will be performed in Phase Two, the design of particular assemblies or connections to develop adequate seismic resistance will be per formed. However, our experience in this area does allow us to identify the following deficiencies without performing any calculations.
1. The retaining wall is starting to break apart where it is exposed to the elements. The overhang along grid line1a should be removed, and the remainder stuccoed over. With no evidence of settlement or severe degradation elsewhere, this wall will continue to function for some time. The lack of an exterior waterproof coating may lead to future degradation and partition wood decay within the building.
2. The exterior stucco coating over the concrete, masonry and corrugated steel panel walls may debond and fall off during a major seismic event.
3. The wood-framed parapets need a new bracing system, with a horizontal stucco control joint at the roof line.
4. The concrete walls are in relatively good condition, with a nominal amount of rebar and compressive strength. Unfortunately, the weight of these walls does result in12,200 pounds of additional seismic force being introduced to the roof and other elements.
5. The wood/corrugated siding/stucco walls along grid lines 4 and 5 may not satisfy the lateral force requirements for out-of-plane earthquake loads, and may need to be rebuilt.
6. The roof needs to have a consistent plywood diaphragm system developed that will adequate unite the various framing elements, with proper nailing to chords, collectors and shear resisting elements. The plywood must be completely blocked at all edges, and extended where needed to the exterior walls. This will require the entire perimeter of the roofing surface to be uncovered, plus removal of all decayed wood prior to the addition of plywood panels. This work should be coordinated with revisions to the parapet bracing system.
7. Install blocking at the ends of all rafters, trusses and at shear collector elements, with appropriate shear transfer nailing, clips, brackets and anchor bolts. Perpendicular ties are needed at the masonry and concrete walls. Without a means of transferring horizontal forces between the roof diaphragm and the shear resisting walls, severe earth quake damage can result. Several support beam lines need to be bolted together to act as drag ties, delivering roof loads to the shear walls.
8. Install some cross-wall bracing at the top of the masonry emergency generator enclosure.
9. The roof trusses need to be evaluated for their ability to carry live plus dead loads. Some diagonal elements are missing, some members do not meet at a common point, and the trusses are not currently functioning as true point, and the trusses are not currently functioning as true trusses. This is not directly a seismic concern, however.
10. It may be cost effective to develop an interior shear wall system about both major axis of the building. For instance, the quarters partition walls on lines C and 2 could be properly framed up to the roof. This would intercept about half of the 37,900 pound seismic force, and reduce the lateral design forces on the exterior walls.
11. Several steel moment-resisting frames are needed along the Saratoga Avenue wall, straddling the truck doors, and tied together with collector elements. Some additional out-of-plane bracing is needed at the truck door headers.
12. The exterior wood framed walls on grid line A, line4 and line 5 will need to be retrofitted with the appropriately nailed plywood shear panels, supported by framing continuous to the roof diaphragm (see Item 5 above), and anchor bolted to the retaining wall below.
13. The t-bar ceiling and light fixture systems need compression elements added to handle vertical upward forces. All of the partitions need their paneling nailed at the top plate, plus perpendicular bracing elements designed to keep them in place. The truck bay hanging florescent light fixtures need better anchorage to stay in place.
14. The present storage loft appears to have developed over a period of time. A properly designed and larger storage loft, with code-conforming stairs, could be developed as part of the seismic retrofit work.
15. The tile accent roofs have lose, broken and missing pieces. The tile should either be eliminated, or removed and properly reinstalled. If the tile is to remain, a small wrought iron catcher rail should be installed over public ways
16. The surface-mounted wiring will have to be protected and maintained during any seismic retrofit or remodel ing work. This will increase the cost of repairs.
If and when you desire, we can commence Phase Two, the detailed calculations and the actual design of structural improvements. Some destructive examination work to uncover all existing framing conditions will be required at that point in time.
I would be happy to meet with you, to discuss the con tent of this report, its impact, and future options.
Sincerely yours,
C. Edward Meserve, S.E. 1995