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Yielding Good Parking Structure Performance

Yielding Good Parking Structure Performance



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 ...

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.



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    Yielding Good Parking Structure Performance Yielding Good Parking Structure Performance Document Transcript

    • Design and Construction Best PracticesYielding good parkingstructure 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 acceptedin 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 numerouscast-in-place, post-tensioned, and precast concrete parking structures, whichhas reinforced my philosophy. Successful parking-structure design has several key concepts, but four ofthe 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 aparking stall and exit in a logical, easy-to-negotiate manner with the aid ofclear way-finding signage. Signs should have a strong contrast between theletters and background and should be appropriately located so as to providesmooth traffic flow. The typical parking-structure design employs a 60 ft to 62 ft (18 m to19 m) span with parking stalls oriented 90 degrees to the two-way trafficlanes. 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 aallow 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 requiredto speed entrance to and exit from the structure. Spiral speed ramps can bemade 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 inplacing concrete and reinforcing on a curving slope, deterioration problemsare 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
    • 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
    • I also prefer a closer spacing (±3 ft [±1 m]) of flange weld plates in thetraffic aisle and a wider spacing in the remainder of the span. These providebetter dynamic load transfer. Pretopped double tees (where a thicker flange replaces the field-placedtopping) are currently the predominant floor-member solution. In someregions, field-placed topping is still the typical solution. Both solutions canproduce good performance. In each case, special considerations should bemade. When placing topping in the field, it is important that the contractoreither follow the double-tee cambered position under the dead-load-deflection position or provide extra topping near the tee ends whererequired to maintain drainage slopes. The field-placed topping should havea minimum strength of 4000 psi (28 MPa) and a maximum water–cementratio (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 pourbe 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 Designjoints 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 moredurable surface. Pretopped double teesexpedite on-site construction becauselittle or no field-placed concrete isrequired. Connections between precastconcrete double tees have no concrete PL cast into spandrel panelcover protection, however, so special (centered between tee stems)attention should be made to protectthese connections against corrosion. Sealant Where aggressive environmentsexist, such as marine environments Loose PLor where deicing salts are applied, theflange connections need to be protect-ed. This protection can vary dependingon 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 stainlesssteel. When galvanizing is employed, PL cast into tee flangecare must be taken to avoid hydrogen (centered between tee stems)embrittlement of cast-in embed hard-ware. Curing of pretopped concretedouble tees is important to minimize Dap/bearing PLsshrinkage cracking. It should be notedthat some minor cracking is to beexpected. If the cracks are small, 0.007in. (0.2 mm) or less, a hydrophobic Bearing padsilane sealer can be used to seal thecracks. Larger cracks may need to besealed with clear epoxy. These cracks Note:are seldom structural in nature and - Shim tee as requiredshould 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
    • 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.3,44 Mar c h – Ap r il 2 0 0 8 | PCI Journal
    • Earthquakes and wind pressure providelateral loads and forces to parking struc-tures that must be resisted. Shear walls have 12 1typically been the designer’s first choice. 1−0"However, in recent years, moment frames 1" 1−0" 3" 9"are increasing in popularity. Shear wallsshould not be solid but should contain suffi- CIP pour stripciently 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 SLEEEVEexterior framing by combining columns and 1 /2" Typicalspandrels into one-story-high units. Interior Lmoment frames have also been successfully 3employed. Connections to the base andbetween 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 Bcodes 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- Dtions, proprietary splice-sleeve or threadedconnections or post-tensioning are typicallyrequired. T In practice, I connect all precast concrete E601members to adjacent precast concrete mem-bers to ensure transfer of loads and preventloss of bearing. Several of the precast con- CIP pour stripcrete structures that I have designed have (by others)been subjected to severe seismic events with 9 E6.01little or no damage to the structure becauseI have employed this practice. 1−1" 21/2" Another important issue is maintain-ing stability during erection. Appropriate 68bracing and timely completion of necessaryconnections during the erection process areessential. In multistory structures, an erection bracing plan should be The author and fellow engineercompleted prior to starting erection. Jaime Irragori developed the through- column bolted connection between the spandrel and column to allow forLetting 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 Engineershottest days and contraction created by the coldest days. Group. Precast concrete structures, with their many joints, provide an excellentopportunity to respond to these movements. Connections between precastconcrete members must be designed to accommodate thermal movements.Rigid connections of spandrel members to columns or walls—because oftheir direct exposure to the sun, particularly on the south and west sides ofa structure—were initially identified as a connection region where struc-tural breathing was essential. To resolve this issue, I and fellow engineerJaime Irragori developed a through-column bolted connection between the PCI Journal | M a r c h – A p r i l 2008 5
    • 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
    • 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 meetwalls, beams, or spandrels and around columns. Urethane sealants are typically employed; however, they are sensitive toultraviolet light and will deteriorate on roof levels. I recommend the use ofsilicone sealants at exposed roof levels, while urethane can be employed atlower, covered levels. Where the first level contains occupied spaces, I typically seal all joints asnoted 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 sizesof 10 in. to 12 in. (250 mm to 300 mm) are essential to minimize cloggingof the drains. Full-width trench drains at the bottom of ramps should not beemployed because they represent a weakening of the floor diaphragm. Largeor multiple drains at the ends of the spans at the bottoms of ramps haveproved to be sufficient. While a 100% leak-free structure is difficult to achieve, the suggestionslisted, if properly implemented, will eliminate all but minor leaking and willminimize 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 parkingmaintenance 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
    • 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. J8 Mar c h – Ap r il 2 0 0 8 | PCI Journal