This column is based on experience gained in the design, construction, and investigations of over 400 parking structures over the past 40 years. Most of the principles that I present are recognized and accepted in the design and construction of precast, prestressed concrete parking structures by experienced design engineers and precast concrete manufacturers.

1. Design and Construction Best Practices
Yielding good parking
structure performance
Thomas J. D’Arcy
his column is based on experience gained in the design, construction,
and investigations of over 400 parking structures over the past 40
years. Most of the principles that I present are recognized and accepted
in the design and construction of precast, prestressed concrete parking struc-
tures by experienced design engineers and precast concrete manufacturers.
However, I am not aware of whether these principles have ever been gath-
ered in one place and in one article.
During the course of these experiences, I have investigated numerous
cast-in-place, post-tensioned, and precast concrete parking structures, which
has reinforced my philosophy.
Successful parking-structure design has several key concepts, but four of
the most important that relate to good performance are:
• having good functional design;
• creating efficient structural layout, design, and detailing;
• letting the structure breathe;
• getting the water off.
Good functional design
Parking patrons should be able to travel through a structure to find a
parking stall and exit in a logical, easy-to-negotiate manner with the aid of
clear way-finding signage. Signs should have a strong contrast between the
letters and background and should be appropriately located so as to provide
smooth traffic flow.
The typical parking-structure design employs a 60 ft to 62 ft (18 m to
19 m) span with parking stalls oriented 90 degrees to the two-way traffic
lanes. Angled parking stalls and one-way traffic lanes are not necessarily pre-
ferred by the parking patron and are used primarily where the site will not
This completed stair tower is part of a
allow 60-ft-long bays. Parking stalls vary in size, but an 8 ft 6 in.–wide parking structure on the National Institutes
(2.6 m) stall is today’s typical width. of Health’s campus in Bethesda, Md.
Typically, ramps are in the interior of the structure, with a 6% to 7% Courtesy of National Institutes of Health.
slope. For extremely large structures, exterior speed ramps may be required
to speed entrance to and exit from the structure. Spiral speed ramps can be
made of cast-in-place or precast concrete, but concrete spirals are an expen-
sive solution and are often difficult to construct. Because of the difficulty in
placing concrete and reinforcing on a curving slope, deterioration problems
are frequently observed in spiral, cast-in-place concrete ramps.
The views and opinions expressed herein
I also do not recommend the use of bumper blocks. They create a trip-
are those of the author and do not nec-
ping hazard, are prone to deterioration, and make cleaning the floor
essarily reflect those of the Precast/
difficult.
Prestressed Concrete Institute or its
Spandrels and guardrails should be designed to support a 10 kip
employees.
(44.5 kN) horizontal load located 24 in. (0.6 m) from the floor level.
PCI Journal | M a r c h – A p r i l 2008 1

2. Proper location of lighting fixtures hung on pendants slightly above
(2 in. [50 mm]) the bottom of the double-tee stem has been proved to pro-
vide lighting levels that are the same as other structural systems.1
Personal safety is an important consideration in the design of a park-
ing structure. The key consideration is making would-be criminals feel
uncomfortable in the structure. This is achieved by avoiding large structural
elements and making the structure more open, as in the case of the 909
Walnut Street Parking Structure discussed in “Precaster Plays Central Role
in Design of Rooftop Garden” on pages XX to XX of this journal. Stair
shafts should either be completely framed and open or located on the exte-
rior of the structure without solid walls. All elevators should be glass-backed
and, again, exposed to the building’s exterior where possible. When used,
ramp walls should be open, not solid. The first application of open-ramp
walls, also known as litewalls, was in the Dallas Galleria parking structure in
Dallas, Tex., in 1980. They have proved to be an efficient, economical solu-
tion.
Many other design concepts are important to functional design, but
those noted previously are most frequently overlooked by less-experienced
designers.
Efficient structural layout, design, and detailing
One of the basic tenets of good precast concrete parking-structure design
is to module the bays to accommodate the typical double-tee widths avail-
able in the geographic area of the structure. If 12-ft-wide (3.6 m) double tees
are the local standard, then 36 ft or 48 ft (11 m or 15 m) bays should
be employed; for 10-ft-wide (3 m) tees, space bays 30 ft or 40 ft (10 m to
12 m) apart; and for 15-ft-wide (5 m) tees, use 30 ft or 45 ft (10 m or 14 m)
Open ramp walls are used in the City
column spacing.
Center Parking Facility in Oklahoma City,
Okla. Single-stem and dog-leg T-beams (one leg shorter than the other leg)
Courtesy of Joel Menciano, copyright should be avoided. Reasons to avoid these components are that single-stem
Architectural Design Group Inc. T-beams are unstable and thus dangerous to store, ship, and erect, and dog-
leg T-beams are difficult to strip from formwork, are prone to crack, and
can create field problems. A good parking-structure layout will not include
either of these potentially problematic products.
All precast, prestressed concrete components should be designed with
performance in mind. Excessive superimposed loads only lead to large cam-
bers and, therefore, potentially larger camber-match problems. Repeated
studies have shown that the practical design live load (considering all stalls
and aisles are full of vehicles) is 28 lb/ft2 to 30 lb/ft2 (1.3 kN/m2 to
1.4 kN/m2). Designing for loads greater than that level doesn’t improve the
performance of the structure.
The final deflected state of all precast, prestressed concrete components
should be checked under the realistic live load. All precast concrete com-
ponents should also have an upward camber under dead load to make sure
that they do not creep downward to create drainage issues. It should also be
noted that a design based solely on stress limits can produce a member that
deflects when subjected to dead load.
In my practice, I design members using a 30 lb/ft2 (1.4 kN/m2) live load
and check movement deflection under live load and the final deflected posi-
tion when loaded. If a specified load is larger, provide additional reinforcing
to satisfy the ultimate load requirements. For instance, if three strands per
leg are required for the 30 lb/ft2 load and four strands per leg are required
for the larger ultimate-load capacity, stress the four strands to a level equiva-
lent to the three-strand level.
2 Mar c h – Ap r il 2 0 0 8 | PCI Journal

3. I also prefer a closer spacing (±3 ft [±1 m]) of flange weld plates in the
traffic aisle and a wider spacing in the remainder of the span. These provide
better dynamic load transfer.
Pretopped double tees (where a thicker flange replaces the field-placed
topping) are currently the predominant floor-member solution. In some
regions, field-placed topping is still the typical solution. Both solutions can
produce good performance. In each case, special considerations should be
made.
When placing topping in the field, it is important that the contractor
either follow the double-tee cambered position under the dead-load-
deflection position or provide extra topping near the tee ends where
required to maintain drainage slopes. The field-placed topping should have
a minimum strength of 4000 psi (28 MPa) and a maximum water–cement
ratio (w/c) of 0.40. Where required, ±6% retained air should be included.
If the topping concrete is placed using a pump truck, the air content should
This detail shows the concrete pour
be checked at the discharge end of the pump because pumping can reduce strip from PCI’s Parking Structures:
air content. The topping should be tooled, not sawed, and sealed over all the Recommended Practice for Design
joints between precast concrete members. and Construction. Note: PL = plate.
The pretopped double-tee solution offers the highest-quality concrete 1" = 1 in. = 25.4 mm.
at the riding surface (typical 5000 psi to 8000 psi [34 MPa to 55 MPa]
and lower w/c) and, therefore, a more
durable surface. Pretopped double tees
expedite on-site construction because
little or no field-placed concrete is
required. Connections between precast
concrete double tees have no concrete PL cast into spandrel panel
cover protection, however, so special (centered between tee stems)
attention should be made to protect
these connections against corrosion.
Sealant
Where aggressive environments
exist, such as marine environments Loose PL
or where deicing salts are applied, the
flange connections need to be protect-
ed. This protection can vary depending
on the severity of the environment.
Solutions range from the applica- 2"
tion of a shop paint to zinc coating,
to galvanizing, to the use of stainless
steel. When galvanizing is employed, PL cast into tee flange
care must be taken to avoid hydrogen (centered between tee stems)
embrittlement of cast-in embed hard-
ware.
Curing of pretopped concrete
double tees is important to minimize Dap/bearing PLs
shrinkage cracking. It should be noted
that some minor cracking is to be
expected. If the cracks are small, 0.007
in. (0.2 mm) or less, a hydrophobic Bearing pad
silane sealer can be used to seal the
cracks. Larger cracks may need to be
sealed with clear epoxy. These cracks Note:
are seldom structural in nature and - Shim tee as required
should not be a cause of rejection.
Double tee to L spandrel connection
PCI Journal | M a r c h – A p r i l 2008 3

4. When stripping double tees from their forms, it is important that lift loops, if employed,
are at the same elevation so that the product strips flat.
All sleeves through the stem for electrical conduit should be in the same location.
Electrical conduits are typically located on the bottom of the flange for easy inspection and
possible repair and are not included in a topping pour.
Two basic pretopped double-tee systems exist. The first employs pour strips at the ends of
the double tees or over beams and where camber-match problems may occur, such as where
a long double tee is adjacent to a short double tee. Where required, chord reinforcing can be
located in the end double-tee pour strips, which are thickened to improve drainage, or in the
flange at the double-tee end in dry systems. Pour strips are used to accommodate typical cast-
ing and erection tolerances, and strips must be tooled and sealed over all joints, just as for the
topped system. Concrete for the pour strips should be of high quality and carefully cured to
minimize cracking.
The second pretopped double-tee system is a completely dry system with no field-placed
concrete. While this system expedites all-weather construction, it makes tight demands on
casting and erection tolerances. Products must be cast and erected with tighter tolerances
than those listed in PCI standards.2 This system should only be attempted by precasters with
the required capability and understanding of the system demands.
All pretopped systems demand tight camber control in design and production.
Consideration must be made in design where double tees of varying length are placed
adjacent to one another. The camber of the shorter double tees must be designed to match
that of the longer double tees as much as possible. This may result in stresses that vary from
code levels, which is less important than a poor camber match as long as ultimate capacity is
achieved. Providing the required number of strands and adjusting the applied tension on the
strands can typically achieve reasonable camber match.
To minimize flange-shrinkage cracking, particularly in dry, windy conditions, a curing
compound may have to be applied immediately after the broomed texture is applied to the
double-tee surface.
Excessive warping of pretopped concrete double tees to create a drainage profile can also
cause flange cracking. Various tests have established that the overall warping between stems
should not exceed ¾ in. to 1 in. (19 mm to 25 mm), for a total of 2 in. (50 mm) across the
width of the double tee. This is appropriate for 60-ft-long (18 m) double tees with typical
stiffness. Shorter, stiffer double tees may require smaller allowances.
Plaza loading is always a design concern, and the correct and proper load must be identi-
fied. If landscaping is included, one should ask how big the planned shrubs and trees are.
This will dictate the amount of soil and the size of the planting containers. If it’s a street-
level parking deck for a shopping mall with direct access to the street, will delivery trucks be
allowed in the structure and what are their expected sizes? If trucks are not allowed, I have
always put up vehicle-control barriers made of a well-anchored, 10-in.-diameter (250 mm)
steel-tube frame crossing the entrance at the height of a tractor trailer’s windshield to con-
trol traffic.
Protecting exposed embeds in precast concrete components in aggressive environments
is important. Nearly all observed deterioration in precast concrete parking structures has
occurred from leakage at joints between members. Because cast-in plates are typically located
at the sides and ends of precast concrete members, water leakage, particularly of salt-laden
water, at these joints requires these plates and embeds to be protected. The typical means of
protection can vary by the degree of corrosiveness of the environment. In dry, stable envi-
ronments, such as the American Southwest, such cast-in embeds may just be painted in the
shop. As the environment becomes more aggressive, cast-in embeds may be zinc-coated, gal-
vanized, or made of stainless steel in the highest corrosion regions.
In practice, I seldom, if ever, specify galvanizing because of bad experiences with hydrogen
embrittlement caused by improper galvanizing processes. It’s disconcerting to have
no. 6 (19M) or larger galvanized bars snap off a plate like candy canes when the assembly is
inadvertently dropped. PCI’s parking manual and ASTM A767 address hydrogen embrittle-
ment, but to be conservative, I do not specify it.3,4
4 Mar c h – Ap r il 2 0 0 8 | PCI Journal

5. Earthquakes and wind pressure provide
lateral loads and forces to parking struc-
tures that must be resisted. Shear walls have 12 1
typically been the designer’s first choice. 1'−0"
However, in recent years, moment frames
1" 1'−0" 3" 9"
are increasing in popularity. Shear walls
should not be solid but should contain suffi- CIP pour strip
ciently sized openings to maintain personal (by others)
safety. In some instances, cast-in-place con-
crete shear walls have also been employed.
Moment frames can be made of the
/4" chamfer
C SLEEEVE
exterior framing by combining columns and
1
/2"
Typical
spandrels into one-story-high units. Interior
L
moment frames have also been successfully
3
employed. Connections to the base and
between frames vary depending on the size
1
/2"
and type of lateral load. Welded, doweled,
111/2"
and bolted connections are typical for wind
1'−0"
loads and lower-level seismic loads. For
/2"
6"
1"
moderate and higher seismic loads, current
1
B
codes call for emulation of cast-in-place
/2"
6"
construction and special hybrid frames are
1
1'−0"
111/2"
also allowed. To create emulated connec- D
tions, proprietary splice-sleeve or threaded
connections or post-tensioning are typically
required.
T
In practice, I connect all precast concrete E601
members to adjacent precast concrete mem-
bers to ensure transfer of loads and prevent
loss of bearing. Several of the precast con-
CIP pour strip
crete structures that I have designed have (by others)
been subjected to severe seismic events with 9
E6.01
little or no damage to the structure because
I have employed this practice. 1'−1" 21/2"
Another important issue is maintain-
ing stability during erection. Appropriate
68
bracing and timely completion of necessary
connections during the erection process are
essential. In multistory structures, an erection bracing plan should be The author and fellow engineer
completed prior to starting erection. Jaime Irragori developed the through-
column bolted connection between
the spandrel and column to allow for
Letting the structure breathe structural breathing. This connection
has resolved the typical spandrel end
For the most part, parking structures are exposed to the full range of tem-
cracking. Note: CIP = cast in place.
perature extremes of their environments. Therefore, the structural members 1" = 25.4 mm; 1' = 0.3048 m.
and their connections must be able to respond to expansion created by the Courtesy of Consulting Engineers
hottest days and contraction created by the coldest days. Group.
Precast concrete structures, with their many joints, provide an excellent
opportunity to respond to these movements. Connections between precast
concrete members must be designed to accommodate thermal movements.
Rigid connections of spandrel members to columns or walls—because of
their direct exposure to the sun, particularly on the south and west sides of
a structure—were initially identified as a connection region where struc-
tural breathing was essential. To resolve this issue, I and fellow engineer
Jaime Irragori developed a through-column bolted connection between the
PCI Journal | M a r c h – A p r i l 2008 5

6. spandrel and column. This connection has resolved the
typical spandrel end cracking and, since its initiation, has
been employed successfully in thousands of precast con-
crete parking structures.
Other connections between rigid elements also need
to accommodate temperature-related strains. Long con-
nection plates welded only at their ends have proved suc-
cessful. Ongoing PCI research on thermal movements
in precast concrete parking structures, “Volume Change
T1.2 T1.7 T2.0 T3.0 T4.0 T5.0 T6.0 T7.0 Movements and Forces in Precast Concrete Structures”
0.3
by Gary Klein and Richard Lindenberg, has also proved
that each connection location between double-tee flang-
0.2 es can provide relief and will open slightly (0.10 in. [2.5
mm] or less), thus minimizing thermal-stress buildup
0.1 in a floor system. This also explains why, in many cases,
Movement (in.)
much less movement is experienced in expansion joints
0
in precast concrete structures.
-0.1 I also have a philosophy on expansion joints: “When
in doubt, leave them out.”
-0.2 In precast concrete structures designed with connec-
tions that will accommodate expected strains, expansion-
-0.3 joint spacing can exceed 300 ft (90 m), with spacing of
T1.2 T1.7 T2.0 T3.0 T4.0 T5.0 T6.0 T7.0 about 340 ft to 350 ft (104 m to 107 m) performing
DT Location satisfactorily. Cast-in-place concrete structures, particu-
Cooling Sept '04-Jan '05 Warming Jan '05-Sept '05 larly post-tensioned ones, require much shorter distances
between expansion joints. When expansion (contrac-
tion) joints are employed, vertical shear transfer between
the adjacent double tees must be provided. If not pro-
Results shown here are for research on
thermal movement at double-tee flange
vided, the joint will surely fail.
joints in a parking structure. Note: 1 in.
= 25.4 mm. Courtesy of Gary Klein and Getting the water off
Richard Lindenberg.
Removing water from the surface of a parking struc-
ture is key to good performance. Ponded water—
particularly if it is saturated with road salts that were
either applied to the structure or tracked in by vehicles—
can attack the concrete surface and lead to deterioration
of the structure. If any cracks or failed sealant exist on the
surface of the parking structure, the salt-laden water will
seep in and corrode the reinforcement or embeds, which
will eventually lead to concrete spalling, loss of area of the
steel reinforcement, and potential major repairs.
First, maintain positive slope-to-floor drains to make
sure that water promptly drains. A cross-span slope of
1½% to 2% is essential to overcome the effects of cam-
ber. In addition, slope-creating crickets at the interior
beam line are necessary to eliminate ponding. At the
perimeter of the structure, the end double tee also needs
to slope to the beam line to minimize warping of the
double tees. In some cases, the bottom of the exterior
double-tee stem may fall below the bottom of the adja-
cent spandrel. However, because the first stem is inboard
2 ft to 3 ft (0.6 m to 0.9 m), it is typically not visible
from the exterior and is not a detriment to the exterior’s
appearance.
6 Mar c h – Ap r il 2 0 0 8 | PCI Journal

7. As noted, all joints between precast concrete members should be sealed.
This is also true of the joints between all precast concrete members in field-
topped systems. This includes end conditions in which double tees meet
walls, beams, or spandrels and around columns.
Urethane sealants are typically employed; however, they are sensitive to
ultraviolet light and will deteriorate on roof levels. I recommend the use of
silicone sealants at exposed roof levels, while urethane can be employed at
lower, covered levels.
Where the first level contains occupied spaces, I typically seal all joints as
noted previously, then I apply a traffic-bearing membrane to the surface above.
The size of the floor drains is also important. Large drains with grate sizes
of 10 in. to 12 in. (250 mm to 300 mm) are essential to minimize clogging
of the drains. Full-width trench drains at the bottom of ramps should not be
employed because they represent a weakening of the floor diaphragm. Large
or multiple drains at the ends of the spans at the bottoms of ramps have
proved to be sufficient.
While a 100% leak-free structure is difficult to achieve, the suggestions
listed, if properly implemented, will eliminate all but minor leaking and will
minimize deterioration.
Summary
When the concepts presented in this article have been followed, durable,
well-performing parking structures have resulted. In addition, appropriate
The Lehigh University Alumni parking
maintenance is necessary to ensure the inherent durability of a precast con- structure in Bethlehem, Pa., is an exam-
crete parking structure. ple of a well-performing parking struc-
ture. Courtesy of Lehigh University.
PCI Journal | M a r c h – A p r i l 2008 7

8. References
1. Monahan, Donald R. 2007. Precast Concrete Parking Structure Lighting
Study. PCI Journal, V. 52, No. 6 (November–December): pp. 89–98.
2. PCI Tolerance Committee. 2000. Tolerance Manual for Precast and
Prestressed Concrete Construction. MNL 135-00. Chicago, IL: PCI.
3. PCI Parking Structures Committee. 1989. Parking Structures:
Recommended Practice for Design and Construction. MNL 129-89.
Chicago, IL: PCI
4. American Society of Testing and Materials (ASTM). 2005. Standard
Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete
Reinforcement. ASTM A767/A767M-05. West Conshohocken, PA:
ASTM.
About the author
Thomas J. D’Arcy, PE, SE, is principal with
the Consulting Engineers Group Inc. in San
Antonio, Tex. He is a PCI Fellow, a PCI Medal of
Honor recipient, and was named a Titan of the
industry in 2004. D’Arcy was chairman of PCI in
2005. He currently serves on PCI’s Technical
Activities, Educational Activities, Building Code,
Research & Development, Continuing Education, Concrete Materials
Technology, and TRMD Steering committees.
Synopsis
Successful parking-structure design has several key concepts, but four
of the most important that relate to good performance are a functional
design; creating efficient structural layout, design, and detailing; letting
the structure breathe; and getting the water off of the structure. Based
on his experience gained in the design, construction, and investigations
of more than 400 parking structures over the past 40 years, the author
addresses these four topics.
Keywords
Design, loads, parking structures.
Reader comments
Please address any reader comments to PCI Journal editor-in-chief Emily
Lorenz at elorenz@pci.org or Precast/Prestressed Concrete Institute, c/o
PCI Journal, 209 W. Jackson Blvd., Suite 500, Chicago, IL 60606. J
8 Mar c h – Ap r il 2 0 0 8 | PCI Journal