HomeMy WebLinkAbout04/07/1992, C-13 - PALM STREET PARKING STRUCTURE - CRACK EVALUATION 4llipl`�I��III��p`I '� MffTING DATE
CIo san L..Js oBIspo April 7. 1992
COUNCIL AGENDA REPORT ITEM NUMB6i:
FROM:
David F. Romero, Public Works Dire
Wayne A. Peterson, City Engin
SUBJECT:
Palm Street Parking Structure - Crack Evaluation
RECOMMENDATION:
By motion:
1) Receive and file the structural engineering report from H.J.
Degenkolb Associates regarding cracks in the Palm Street Parking
Structure,
2) Appropriate $15, 000 from the parking fund undesignated fund
balance to the parking program operating budget to pay for
repairs recommended in the Degenkolb report.
DISCUSSION:
Shortly after the completion of the Palm Street Parking Structure, staff
noted cracks in the shear walls. The designer, Conrad & Associates, and
later Dave Carter, a local structural engineer were contacted and asked
to comment on the cracks and recommend any corrective work that should
be done to protect the building. Both indicated that the cracking was
normal and recommended epoxy injection. When the proposed contract to
carry out the repair was brought to the Council in March 1991, staff was
requested to have another structural engineer look at the building and
evaluate several different things.
This past November staff hired Loring A. Wyllie, Jr. , a structural
engineer and principal of the firm H.J. Degenkolb Associates, Engineers
of San Francisco, following the City adopted procedure for hiring
professional consultants. The amount of the contract for the work was
$4,200. Mr. Wyllie visited the site in early January and recently
submitted the attached report. The executive summary summarizes the body
of the report and answers Council's questions.
In brief, Mr. Wyllie says that the project is reasonably designed and
structurally sound. The cracking is a result of shortening of the
building which is typical of the design. Constraints on the designer,
caused by the site, caused him to place the shear restraining walls at
the ends of the long spans. By locating them there, they attempted to
hold the decks back as they began shrinking shortly after they were
placed. If the shear walls had been placed at the center of the walls
as they were in the Marsh Street structure, there may have not been any
cracking. Since the westerly wall has already been sealed on the outside
and most of the water penetration has been stopped, and since little
water penetration was noted on the north wall, he advises us to monitor
the cracks in these walls and to watch for evidence of corrosion. If
evidence is noted, he recommends we apply a seal on the exterior of these
walls. He feels that it is unlikely that such work will be necessary.
043- 1
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The sub-grade walls next to the Call America building are more of a
problem due to the ground water penetration and the adjacent electrical
panel. He advises that we should relocate the electrical panel and seal
the wall on the inside to prevent air and water from attacking the
reinforcing steel. An exterior water seal would be better but due to
the close proximity of the adjacent building it cannot be installed. It
takes the combination of air and water to cause corrosion, therefore by
sealing the inside of the wall the source of air would be limited. The
parking structure will have to be shut down for 1-2 days when the panel
is relocated, therefore that part of the work will be scheduled on a
weekend.
Mr. Wyllie noted two diagonal deck cracks in the northeast corner of the
building. These he feels should be attended to immediately. He
recommends that they be injected with epoxy to protect the post tension
tendons.
CONCURRENCES:
This work is considered maintenance and requires approval only for the I
electrical permit to relocate the panel. That permit will be applied for
prior to the work being carried out.
FISCAL IMPACT:
The engineer's preliminary cost estimate for corrective work includes:
$ 9,000 for relocating the electrical panel
3 . 000 for sealing the sub-grade wall
$15, 000 Total
I
Each portion of the proposed corrective work would involve a separate
specialty trade and would be contracted as a minor capital project in the
parking program budget. The $15, 000 required would come from the parking
fund undesignated fund balance, which is projected to be $860, 000 at the i
end of 1992-93 based on the recent Mid-year Budget Review (page C-23) .
I
Attachment:
Degenkolb Report Executive Summary
Full Degenkolb Report is
Available in the Council Office
mpelm
-13-A
Degenkolb
EVALUATION OF CRACKING
PALM STREET PARKING STRUCTURE
SAN LUIS OBISPO, CALIFORNIA
EXECUTIVE SUMMARY
The Palm Street Parking Structure in San Luis Obispo is a three-story post-tensioned
concrete parking facility constructed in 1987. It contains reinforced concrete shear
walls along the center of the south and east streetfront facades and along the entire
west wall and both ends of the north wall at property line conditions. The concrete
shear walls, particularly the two ends of the west and north property line walls, have
extensively cracked.
The cracks in these shear walls and at selected locations adjacent to stairways are the
direct result of concrete shrinkage and creep of the post-tensioned slabs which have
caused the structure to shorten by possibly as much as 1-1/2 inches in its long
direction. When the slabs shorten, the walls near the comers of the structure offer
restraint and the cracks in the walls are the result. With the location of wall dictated
by site constraints, the cracking that has occurred is normal and was to be expected.
We would estimate that 80 to 90% of the ultimate shortening that caused these sacks
has now occurred, so minimal additional cracking should be expected.
It is our opinion that the cracks in these shear walls have not in any way weakened
the structure to resist seismic forces. The structure appears to have been reasonably
designed and constructed and should be serviceable for many years.
1
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Degenkolb
It is our recommendation that a waterproof membrane be installed on the below
grade portions of the wall in the northeast corner of the structure which leaks
following all rainstorms. We also recommend that several floor and roof slab cracks
in that corner,as illustrated in Photographs No 5 and 6, be sealed to prevent water
penetration. We do not believe that cracks in the walls need to be epoxy-injected at
this time. We do recommend that when exterior above grade walls are repainted, that
all cracks in those walls be sealed with the sealant recently used on the west wall
which has prevented further water penetration through the above grade wall cracks.
We also recommend that the walls be periodically monitored,and if rust stains are
observed indicating that corrosion of reinforcing steel is underway, then additional
preventative measures such as epoxy injection should be considered.
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PARKIlI ORRUCTILM, r'
SAN!LMS'OBISPO; CA LTFORNlA
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Attention: Wayne A:.;Petersoncity i
":. Reference: PALM STREET-VARKINGISTRUCIURE:.:
SAN'qLUIS,OBISPO CALIFORNIA.:
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[DEGENICOLB JOB N0='9119114Jt r
Gentlemen:. .
W. e are pleased toosubmiE our:report,Evaluation of Crrnclan&:Palm.Street:Parksng 4 Fr
Stiue&14 SDi'is Obispo;Cdlifornia: „-
y- Please call if there are,anylquestions or if you desuse additional'Information. It has
been oiff: td'ba of,semce to the City of,San^LuisJOlrispo. r '
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JMI L JsDEGENKOI BASSOCIATES.ENGINEERS
H J Deg�kolb Assocutts,
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LSuite,90011
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Eiiigency916.552.1440
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Degenkolb
EVALUATION OF CRACKING
PALM STREET PARKING STRUCTURE
SAN LUIS OBISPO, CALIFORNIA
EXECUTIVE SLJNIIdARY
The Palm Street Parking Structure in San Luis Obispo is a three-story post-tensioned
concrete parking facility constructed in 1987. It contains reinforced concrete shear
walls along the center of the south and east streedwnt facades and along the entire
west wall and both ends of the north wall at property line conditions. The concrete
shear walls, particularly the two ends of the west and north property line walls, have
extensively cracked.
The cracks in these shear walls and at selected locations adjacent to stairways are the
direct result of concrete shrinkage and creep of the post-tensioned slabs which have
caused the structure to shorten by possibly as much as 1-1/2 inches in its long
direction. When the slabs shorten, the walls near the corners of the structure offer
restraint and the cracks in the walls are the result With the location of wall dictated
by site constraints, the cracking that has occurred is normal and was to be expected.
We would estimate that 80 to 90% of the ultimate shortening that caused these cracks
has now occurred,so minimal additional cracking should be expected.
It is our opinion that the cracks in these shear walls have not in any way weakened
the structure to resist seismic forces. The structure appears to have been reasonably
designed and constructed and should be serviceable for many years.
1
Degenkolb
It is our recommendation that a waterproof membrane be installed on the below
grade portions of the wall in the northeast corner of the structure which leaks
following all rainstorms. We also recommend that several floor and roof slab cracks
in that comer, as illustrated in Photographs No 5 and 6, be sealed to prevent water
penetration. We do not believe that cracks in the walls need to be epoxy-injected at
this time. We do recommend that when exterior above grade walls are repainted, that
all cracks in those walls be sealed with the sealant recently used on the west wall
which has prevented further water penetration through the above grade wall cracks.
We also recommend that the walls be periodically monitored,and if rust stains are
observed indicating that corrosion of reinforcing steel is underway, then additional
preventative measures such as epoxy injection should be considered.
2
Degenkolb
INTRODUCTION
The Palm Street Parking Structure is a three-story, ramped parking structure
constructed in 1987. The structure is about 287 feet by 113 feet in plan and is
constructed of post-tensioned concrete The structure consists of 5 inch thick post-
tensioned slabs spanning about 18 feet to 14 inch wide by 33 inch deep post-tensioned
beams. The beams span about 55 feet to columns located on the north and south
faces of the building and along a central column line. There are three stairways
located near the northeast, southeast and southwest corners of the structure. The
foundations are 30 inch diameter drilled, cast-m-place concrete piles which extend
three feet into unweathered rock. Photograph No. 1 shows the structure from its
southeast corner at the intersection of Palm Street and Morro Street. Figure 1 is a
plan of the building.
The building is braced for lateral forces by a series of reinforced concrete shear walls.
The wall on line 1 at the west end is a solid concrete wall subdivided into three parts
by 1 inch wide vertical expansion joints. There is also a solid wall subdivided into
two parts on line A from lines 1 to 7. Likewise, there is a solid wall on line 15 from
lines A to A.2 and on line A2 from line 15 to 17. All of these walls are property line
walls adjacent to other parcels and are solid without openings. There are also shear
walls on line C form lines 8 to 10 and on line 17 from A.8 to B.2.
The building was designed by Conrad Associates of Van Nuys, California, and the
drawings were first issued in October 1986. Cracking occurred in many of the
reinforced concrete shear walls and a November 9, 1990 report was submitted to the
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Degenkolb
City by David M Carter of Applied Engineering of San Luis Obispo. Applied
Engineering concluded that the cracks were caused by "a combination of stresses
caused by elastic shortening of the floor slabs due to prestressing, and creep in the
floor slabs due to the sustained compression from prestressing." They recommended
that the cracks be repaired by epoxy injection.
The cracks in the walls have not been repaired by epoxy injection. However, the solid
west wall was repainted on the exterior about one-and-a-half years ago and the cracks
were sealed with VIP'Ter-Polymer Sealant" #5000 Series before painting. Photograph
No. 2 is a view of this west wall
4
Degenkolb
OBSERVATIONS
The site was visited by Loring A. Wyllie,Jr. on January 8, 1992. Mr. Wyllie is a
Senior Principal of Degenkolb Associates and well qualified to investigate the cause of
cracks in a concrete structure. Mr. Wyllie has served for twenty years on Committee
318 of the American Concrete Institute which writes and maintains the Building Code
requirements for reinforced concrete construction in the United States. He is also past
president of the Structural Engineers Association of California and is a member of the
National Academy of Engineering. He has investigated the causes of cracking on
numerous similar concrete structures.
January 8 was an ideal day to inspect the parking structure. It was a clear, sunny day
but it had rained heavily the day before, so any water infiltration through cracks was
clearly evident
The exterior walls, particularly on lines 1, A and A.2, have a considerable number of
cracks. The longer walls are subdivided into segments by 1 inch wide vertical
expansion joints. The wall on line 1 is subdivided into three segments, there is a joint
at the junction of lines 1 and A at the corner, and the wall on line A has one
additional vertical joint The cracks in all of these walls have a diagonal direction
down from the comer of the building towards the middle. At the corners, a few of
the cracks are almost horizontal for a short length. The cracks are also greatest in the
lowest floor. Photograph No. 3 shows the wall on line A in the lowest floor adjacent
to the comer. The cracks themselves are quite fine, in the range of 10 to 30 mils, or
0.01 to 0.03 inches in width. This is less than 1 millimeter which is equivalent to
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Degenkolh
about 40 mils. The markings seen on the wall in Photograph No. 3 are stains from
water and moisture infiltration through the cracks in the past. The only cracks that
were leaking during our inspection were cracks on line A2 where the lowest story is
below grade and this wall retains earth of the adjacent property at that location.
Photograph No. 4 shows some of these leaking cracks. In the lowest story, electrical
equipment is adjacent to this leaking wall.
Other forms of cracking are evident in the structure. There are diagonal cracks in the
floor slabs radiating from the stairway opening in the northeast corner, as seen in
Photograph No. 5 at the roof slab. A large crack at a slab corner in the first
supported level near grid A-15 is seen in Photograph No. 6. The cause of this crack is
not certain There are also various cracks at some of the other walls and at the
stairways, particularly near grid C-3 and C-17. The wall on line C between lines 8
and 10 also has several diagonal cracks and some crazing near the floor at C-10.
Photograph No. 7 shows cracking at column C-2 where a solid concrete parapet above
the entry below has sacked at the column. This parapet was designed as a non-
structural parapet supported on the beam below, but its eccentricity from the column
has resulted in this minor cracking.
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Degenkolb
DISCUSSION
The cause of almost all of the cracking in the structure is the result of shortening of
the post-tensioned slabs due to prestressing, shrinkage of concrete and creep of the
prestressed concrete. This shortening of the slab is inevitable and must occur. When
it does, the rigidity of the walls located near the ends of the building, such as line A,
line A.2 from lines 15 to 17 and line 1 near lines A and C, try to resist that shortening.
The result is like a tug of war and since the walls are slightly weaker in shear and
flexure than the slab is in tension, the walls crack to relieve the stresses built up by
the shortening of the slab.
In order to put the magnitude of this slab shortening in perspective, let us briefly
examine each element causing the shortening. The following discussion deals with
the approximate 275 foot length between the ends of the walls on lines A and A.2.
Elastic shortening of the slab occurs when the post-tensioning of the prestressed
tendons occurs during construction. For a 275 foot length, this amounts to a slab
shortening of about 3/16 inch However,since there was a closure strip-between lines
10 and 11, most of this shortening was probably relieved by slight bending of the
columns, although this may be part of the cause of the cracks in the wall on line C
between 8 to 10, as these walls were interconnected at the time of prestressing to the
rigidity at the west end of the building.
The second element of the shortening is shrinkage of the concrete. When concrete
drys and cures, water leaves the concrete and it shrinks, or shortens. The process
goes on for years and is dependent on many factors. The actual ingredients of the
concrete mixture and their quantities affect the ultimate shrinkage.
7
Degenkolb
The thickness of the slab somewhat controls the rate the concrete shrinks. The
relative humidity of San Luis Obispo affects how much water dries out of the concrete
as it reaches equilibrium with the moisture in the air surrounding the concrete. The
amount of bonded mild reinforcing steel in the slab, which is very light but more in
the Palm Street structure than in many post-tensioned parking structures, affects the
actual shortening as the bonded mild steel must go into compression when the slab
shortens. All of these issues are complex and we would estimate that the 275 foot
length of the Palm Street structure might shorten in the neighborhood of 1/2 inch to
1 inch after the initial shrinkage during construction before the pour strips were
closed.
The third element is creep of the concrete. This is the result of the prestressing
tendons placing the concrete slab in compression and this sustained compression
causes a plastic flow or shortening of the concrete slabs over the years. For a 275 foot
length prestressed to about 200 pounds per square inch as in the Palm Street
structure, this creep might approach 3/4 inch
Thus, it can been seen that concrete shrinkage and creep are the major causes of this
slab shortening and combined they may represent an upper bound shortening of
about 1-1/2 inches for the long axis of the structure and about 1/2 inch for the short
length in the north-south direction. If the structure shortens towards the center, this
means about 3/4 inch needs to be relieved at the east and west ends by bending of
columns and walls and cracking or rotation of longitudinal walls on lines A and A.2.
Photograph No. 8 shows cracking in the concrete block masonry retaining walls at the
east end where movements of 3/8 to 1/2 inch are clearly visible. Some of the
stretched cable used for guardrails is sagging, as seen in Photograph No. 9, again as a
result of this shortening.
8
Degenkolb
This shortening is normal and must be expected in post-tensioned structures. What is
normally done to prevent this cracking is to place the required shear walls near the
center of the structure, as the designers did on lines C and 17, the two street facades.
This allows the structure to shorten towards the rigid walls and the columns are
generally flexible enough so only a few very fine cracks result from the shortening
effects. However, the Palm Street structure was built adjacent to property lines on
line 1, line A from 1 to 7 and on line A2 from lines 15 to 17. The building code
requires a solid wall at these locations, resulting in the restraint to the shortening and
the cracking that has occurred. These walls could have been isolated from the
structure to prevent the cracking,but then additional shear walls would have been
required, increasing the cost of construction. The cracking in the slab near grid A.2-16
and the cracking in stairways at grid C-3 and C-17 is also a direct result of this
shortening process; all of these cracks are in the direction consistent with the slab
shortening.
It should also be noted that the shear walls are particularly rigid due to the drilled
piles to unweathered rock on which they sit. This foundation provides a strong fixity
to the ground at the base of the wall, and the near horizontal nature of the cracks
seen in Photograph No. 3 are consistent with this base fixity of the walls.
It is also our opinion that most of the shrinkage and creep has now occurred. A
5 inch thick slab will undergo 50% of its ultimate shrinkage within the first four
months. Although both shrinkage and creep take eventually forever to reach their
ultimate values, we estimate that at least 80 to 90% of the shortening due to shrinkage
and creep has taken place. However, the slabs will annually be subjected to
9
Degenkolb
shortening and elongation due to temperature changes. Although concrete is a good
insulator and it takes prolonged exposure to a new temperature to cause the centers
of the slabs to change temperature, a 20° F change in temperature causes the 275 foot
long Palm Street structure to shorten or elongate about 3/8 inch Due to relatively
moderate temperature of San Luis Obispo, we would not expect these temperature
movements to cause major cracking in the future. We believe that most of the
cracking due to slab shortening that will occur has occurred.
10
Degenkolb
RECOMMENDATIONS
We were impressed with the VIP sealant that was installed on the west wall when it
was repainted. The west wall takes the direct thrust of most storms which come from
the southwest and no moisture penetration was visible on the west wall on January 8,
one day after a major rainstorm. We would recommend that the other exterior walls
be sealed with the same sealant when they are eventually repainted to minimize
moisture penetration which may eventually cause corrosion to reinforcing bars in the
wall. The existing stains on the above grade walls are not rust colored, so we do not
believe any significant corrosion has been caused to date.
We do not recommend that the City epoxy inject the cracks in the walls of the Pahn
Street Parking Structure. The cracks are less than 1 millimeter in width and epoxy
injection would be very expensive. In our opinion, there is no need for epoxy
injection. The walls should be periodically monitored, and if rust stains observed,
then epoxy injection should be considered at that time
11
Degenkolb
We do recommend that the City waterproof the wall in the northeast corner below
grade. Specifically, the walls on line A2 from lines 15 to 17 and the wall on line 15
from lines A to A.2 The walls should be waterproofed on the inside face with a
waterproofing system such as xypex or an ironite membrane to stop water penetration
and seal the wall. This will minimize potential corrosion of reinforcing steel by
reducing air flow and maintaining water around the reinforcing. The membrane will
also keep water away from the electrical equipment located in that corner.
It is our opinion that the cracks in the concrete shear walls have not in any way
weakened the walls to resist earthquake forces. The cracks, although noticeable, do
not reduce the capacity of the walls to brace the building for seismic forces.
Reinforced concrete is expected to crack when it resists forces, and certainly a strong
earthquake will cause more and probably wider cracks m these walls.
Finally, we would also recommend that the few roof slab cracks near the northeast
stair, illustrated in Photograph No. 5, be epoxy-injected or otherwise sealed to prevent
water penetration which might reach the post-tensioning tendons. The crack shown
in Photograph No. 6 should also be sealed.
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PHOTOGRAPH NO. I - THE PALM STREET PARKINGFROM
1THE CORNER OF PLAM AND MORRO STREETS
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PHOTOGRAPH NO. OF kI
WHlCH HAS BEEN REPAINTED
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PHOTOGRAPH NO. 3 - CRACKS IN THE WALL AT LINE A
FROM THE CORNER OF THE STRUCTURE
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PHOTOGRAPH NO. 4 - CRACKS ON LINE A2 WHICH WERE LEAKING
WATER AT THE TIME OF SITE INSPECTION
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PHOTOGRAPH°NO 5"=CBACKS,IN ROOF�SLAB NEAR S TAIIi°NEAR
LINE A.2164N THENORTHEAST CORNMR'j- ,' `'I_
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