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Steel Design Guide

Steel-Framed Open-Deck

Parking Structures

Steel Design Guide

Steel-Framed Open-Deck

Parking Structures

CHARLES H. CHURCHES

Structural Engineer Churches Consulting Engineers Washington, Pennsylvania

with additional material contributed by

EMILE W.J. TROUP

Structural Steel Fabricators of New England Canton, Massachusetts

CARL ANGELOFF

Manager/Market Development Bayer Corporation Pittsburgh, Pennsylvania

A M E R I C A N I N S T I T U T E O F S T E E L C O N S T RU C T I O N , I N C.

v

Preface Acknowledgements

This design guide is specifically focused on structural engi- neering issues in the design of open-deck parking struc- tures and does not deal in depth with parking usage or geometric topics. General parking topics and their imple- mentation in steel-framed parking structures are covered in a separate publication, Innovative Solutions in Steel: Open- Deck Parking Structures (formerly titled A Design Aid for Open-Deck Steel-Framed Parking Structures), also pub- lished by the American Institute of Steel Construction. This design guide approaches the development of steel- framed parking structures in the same sequence as a designer would approach the design development. For this reason, the discussion of the steel framing system is deferred until after the section dealing with deck selection. The issues discussed in this design guide are:

• Deck Systems
• Framing Systems
• Mixed Use Structures
• Fire Protection Requirements
• Barriers and Facades
• Stairs and Elevators
• Corrosion Protection
• Structural Maintenance

AISC would like to thank the following people for assis- tance in the production and review of this design guide. Their comments and suggestions have been invaluable.

Rashid Ahmed Edmund Baum Tom Calzone Charles Carter William Corbett John Bakota John Cross Thomas Faraone Christopher Hewitt

Kenneth Hiller Scott Kennedy Gerald Loberger, Jr. Billy Milligan William Pascoli Kimberly Robinson Len Tsupros Gail Vasonis Michael West

vi

• Chapter 1—Introduction ............................................................................................................................................................ Table of Contents
• 1.1 Overview of Open-Deck Parking Structures ......................................................................................................................
• 1.2 Major Components of Interest to a Structural Engineer ....................................................................................................
• 1.3 Code Considerations .......................................................................................................................................................... - 1.3.1 Code Applicability .................................................................................................................................................. - 1.3.2 Relevant Code Sections for Open-Deck Parking Structures .................................................................................. - 1.3.3 Code Definitions .................................................................................................................................................... - 1.3.4 Fire Protection and Height...................................................................................................................................... - 1.3.5 ADA Guidelines......................................................................................................................................................
  • Chapter 2—Deck Systems for Parking Structures ..................................................................................................................
  • 2.1 Types of Deck Systems ...................................................................................................................................................... - 2.1.1 Cast-in-place reinforced concrete .......................................................................................................................... - 2.1.1.1 Clear Cover and Permeability ................................................................................................................ - 2.1.1.2 Curing ...................................................................................................................................................... - 2.1.1.3 Joints, Cracks and Drainage .................................................................................................................... - 2.1.1.4 Steel Deck................................................................................................................................................
    • 2.1.2 Cast-in-Place Post-Tensioned Slabs and Toppings ................................................................................................
    • 2.1.3 Precast Double Tees................................................................................................................................................
    • 2.1.4 Other Systems ...................................................................................................................................................... - 2.1.4.1 Filigree .................................................................................................................................................. - 2.1.4.2 Hollow-Core Plank ................................................................................................................................
• 2.2 Deck System Selection by Climactic Zone ......................................................................................................................
• 2.3 Concrete Durability ..........................................................................................................................................................
• 2.4 Plaza Deck Systems ..........................................................................................................................................................
• 2.5 Deck System Design Parameters ...................................................................................................................................... - 2.5.1 Cast-in-Place Conventionally Reinforced Concrete on Stay-in-Place Metal Forms .......................................... - 2.5.1.1 Deck Slope ............................................................................................................................................ - 2.5.2 Cast-in-Place Post-Tensioned Slabs and Toppings .............................................................................................. - 2.5.3 Precast Double Tees.............................................................................................................................................. - 2.5.4 Filigree Precast with Post-Tensioned Deck .......................................................................................................... - 2.5.5 Filigree Precast with Conventionally Reinforced Slab ........................................................................................ - 2.5.6 Precast Hollow Core Slabs with Field Topping .................................................................................................. - 2.5.7 Deck Renovation ..................................................................................................................................................
• Chapter 3—Framing Systems ..................................................................................................................................................
• 3.1 Introduction ......................................................................................................................................................................
• 3.2 Economy............................................................................................................................................................................ - 3.2.1 Relationship Between Deck Type and Bay Size Geometry ................................................................................
• 3.3 Plan Framing Design ........................................................................................................................................................ - 3.3.1 Cast-in-Place Conventionally Reinforced Slab Poured on Stay-in-Place Metal Decking .................................. - 3.3.2 Cast-in-Place Post-Tensioned Slab Framing Plan ................................................................................................ - 3.3.2.1 The Effect That Post-Tensioning Forces Have on Members and Their Connection ............................ - 3.3.2.2 Construction Loads................................................................................................................................ - 3.3.2.3 Camber .................................................................................................................................................. - 3.3.2.4 Connection Design ................................................................................................................................ - 3.3.2.5 Member Design in Direction of Primary Reinforcing ..........................................................................
  • 3.3.3 Precast Double Tee Deck...................................................................................................................................... vii
  • 3.3.4 Cast-in-Place Post Tensioned Slab on Filigree Forms ........................................................................................
  • 3.3.5 Cast-in-Place Conventionally Reinforced Slab on Precast Forms ......................................................................
• 3.4 Other Framing Considerations ..........................................................................................................................................
  • 3.4.1 Connection Type: Rigid or Semi-Rigid ................................................................................................................
  • 3.4.2 Composite Beams ................................................................................................................................................
  • 3.4.3 Shored Versus Un-Shored Composite Beams ......................................................................................................
    • 3.4.3.1 Cast-in-Place Post-Tensioned Deck ......................................................................................................
    • 3.4.3.2 Cast-in-Place Slab on Metal Deck ........................................................................................................
    • 3.4.3.3 Cast-in-Place Slab on a Filigree Deck ..................................................................................................
  • 3.4.4 Non-Composite Beams ........................................................................................................................................
  • 3.4.5 Castellated Beams ................................................................................................................................................
  • 3.4.6 Perimeter Beams ..................................................................................................................................................
  • 3.4.7 Steel Joists ............................................................................................................................................................
  • 3.4.8 Control/Expansion Joints ......................................................................................................................................
• 3.5 Vertical Framing Design ..................................................................................................................................................
  • 3.5.1 Lateral Load Considerations ................................................................................................................................
  • 3.5.2 Braced Frames ......................................................................................................................................................
    • 3.5.2.1 Length Changes Due to Thermal Effects ..............................................................................................
    • 3.5.2.2 Shortening of the Deck Due to Concrete Shrinkage and Creep ..........................................................
    • 3.5.2.3 Length Changes and How They Relate to Bracing ..............................................................................
  • 3.5.3 Shear Walls ..........................................................................................................................................................
• 3.6 Erection Considerations ....................................................................................................................................................
  • 3.6.1 Considerations for All Steel-Framed Parking Structures ....................................................................................
  • 3.6.2 Considerations for Deck-Specific Types ..............................................................................................................
• Chapter 3 Tables ..........................................................................................................................................................................
• Chapter 3 Figures ........................................................................................................................................................................
• Chapter 4—Mixed-Use Structures ..........................................................................................................................................
• Chapter 5—Fire Protection Requirements..............................................................................................................................
• Chapter 6—Barriers and Facades............................................................................................................................................
• 6.1 Impact Requirements ........................................................................................................................................................
• 6.2 Railing Code Requirements ..............................................................................................................................................
• 6.3 Facade Options..................................................................................................................................................................
• 6.4 Perimeter Protection..........................................................................................................................................................
  • 6.4.1 Precast Architectural Panels..................................................................................................................................
  • 6.4.2 Open Steel Member Design..................................................................................................................................
  • 6.4.3 Cable Barrier Design Calculations ......................................................................................................................
• Chapter 7—Stairs and Elevators..............................................................................................................................................
• 7.1 Stair Locations and Requirements ....................................................................................................................................
• 7.2 Elevators............................................................................................................................................................................
• Chapter 8—Corrosion Protection for Exposed Steel in Open-Deck Parking Structures ..................................................
• 8.1 General Overview............................................................................................................................................................
• 8.2 Environmental Factors ......................................................................................................................................................
• 8.3 High-Performance Coating Systems ................................................................................................................................ viii
  • 8.3.1 Overview.................................................................................................................. ............................................
  • 8.3.2 Selection................................................................................................................................................................
    • 8.3.2.1 Factors That Affect Cost and Performance ..........................................................................................
    • 8.3.2.2 Recommended Coating Systems ..........................................................................................................
    • 8.3.2.3 Moderate Performance Coating Systems ..............................................................................................
    • 8.3.2.4 Low-VOC Alternative ..........................................................................................................................
• 8.4 Galvanizing ......................................................................................................................................................................
• Chapter 9—Life-Cycle Costs of Steel-Framed Parking Structures ......................................................................................
• Chapter 10—Checklist for Structural Inspection of Parking Structures ............................................................................
• AppendixA1—Example: Post-Tensioned Deck Parking Garage........................................................................................
• AppendixA2—Example:Cast-in-Place Concrete on Metal Deck........................................................................................
• AppendixA3—Example: Precast—Twin Tee Deck..............................................................................................................
• AppendixB—Protective Coating System Specification......................................................................................................
• AppendixC—Bibliography of Technical Information on Painting....................................................................................
• AppendixD—Recommended Resources on Parking Structures........................................................................................

DESIGN GUIDE 18 / STEEL-FRAMED OPEN-DECK PARKING STRUCTURES / 1

1.1 Overview of Open-Deck Parking Structures

Steel-framed parking structures are increasing in popularity. The recent trend toward steel has prompted industry analyst Dale Denda of the Parking Market Research Company to comment that "exposed steel-frame construction is back as a recognized option for multi-story parking structures." (Parking Today, June 2001) Recent advances in coating technologies and design innovations need to be evaluated and considered for the parking structure. In addition, the structural engineer needs to be able to intelligently evaluate the merits of various framing systems in order to provide professional guidance to garage owners and other members of the project team. Today, owners and architects are choosing steel framing systems for their lower construction costs, reduced life- cycle costs, rapid construction, long term durability and a clean, open feel conducive to personal security. It falls to the structural engineer to optimize these benefits in the final design by taking advantage of high-performance coatings, innovative structural techniques, reduced structure weight (often at least 20 percent) and enhanced seismic perform- ance. Today's parking structure framing systems primarily fall into three categories:

• Cast-in-place concrete framing supporting a post-ten- sioned concrete deck
• Precast/Prestressed concrete framing supporting precast double tees
• Fabricated structural steel framing supporting a post-ten- sioned cast-in-place, conventionally reinforced concrete deck on stay-in-place metal form or precast deck Other deck systems have been utilized in various areas of the country including concrete filigree panels (a precast panel form system) and short-span reinforced concrete on removable forms. Structural steel framing has been used to support all of these types of concrete deck systems. This allows the structural designer to choose the optimal deck system for a given project and still enjoy the benefits of a steel framing system.

1.2 Major Components of Interest to a Structural Engineer

In order to effectively design an open-deck steel-framed parking structure the structural engineer will need to evalu- ate a number of issues. These include:

• Relevant provisions of the governing building code for the location of the parking structure
• The geometry of the parking stalls as a function of opti- mum bay sizing
• The possible configuration of ramp systems to allow for smooth traffic flow within the parking structure These three design components are introduced and dis- cussed as part of the general parameters affecting parking design in a separate publication, Innovative Solutions in Steel: Open-Deck Parking Structures (formerly titled A Design Aid for Open-Deck Steel-Framed Parking Struc- tures), also published by the American Institute of Steel Construction. They are summarized in this introductory section as they impact structural design. Nine components of the structural design process have been identified and a separate section has been allocated to each. These are:
• Deck Systems
• Framing Systems
• Mixed-Use Structures
• Fire Protection Requirements
• Barriers and Facades
• Stairs and Elevators
• Corrosion Protection
• Structural Maintenance Four appendices are included that provide design exam- ples, additional resources relating to high-performance coating systems, discussion of the benefits of steel-framed parking structures and additional resources for the designer of a parking structure.

1.3 Code Considerations 1.3.1 Code Applicability Over the past several decades designers have been faced with a variety of differing building codes based on the loca- tion of the constructed project. Variations existed between model building codes and local jurisdictions within areas of adoption of model building codes. The International Code

Chapter 1

Introduction

DESIGN GUIDE 18 / STEEL-FRAMED OPEN-DECK PARKING STRUCTURES / 3

When evaluating tier area and structure height, the impact of any future vertical expansion should be taken into account. When parking is being provided on the lower floors of a mixed-use structure, the lower parking floors must be fire separated from the upper floors and fire rated.

1.3.5 ADA Guidelines

The Americans with Disabilities Act establishes design guidelines for addressing the needs of persons with disabil- ities to access all newly constructed structures. Current ADA guidelines impacting parking include:

• The provision, size and location of a required number of physically disabled accessible spaces
• The provision, size and location of physically disabled van access - Ramp slopes - Signage - Trip hazards - Exit paths Table 1-3 indicates the required minimum number of accessible spaces in any parking facility. These spaces must be at least 8 ft wide with a 5-ft-wide accessible aisle adja- cent to the space. Two accessible spaces may share the same accessible aisle if the spaces utilize 90° parking. Angled parking spaces must each have their own accessible aisle. Ceiling clearances are not impacted by accessible spaces and should conform to a 7 ft, 2 in. minimum or any applicable local codes. Accessible spaces are required to be the closest spaces to all accessible building entrances.

NFPA 88A Type II (000) IBC Type IIB

Fire Resistive

Requirement

None None

Definition of Open

Side

1.4 sq ft of each linear foot

distributed along 40% of

perimeter

50% of interior wall area of

exterior wall

sq ft/tier # of tiers sq ft/tier # of tiers

2 sides open unlimited 1 height<=75 ft 50,000 8

3 sides open unlimited 1 Height<=75 ft 62,500 9

4 sides open unlimited

1

Height<=75ft 75,000 9

Exception^1 unlimited height<=75 ft

1 the distance from any point on the deck may not be greater than 200 feet from an open side

Table 1-2 NFPA Building Code and International Building Code Guidelines for Height and Tier Area Perimaters

p y , p g y p g q

at least one accessible elevator ,a pedestrian ramp to grade level or a grade level accessible

structure.

Number of Parking Spaces Minimum Number of Accessible Spaces

1 to 25 1

26 to 50 2

51 to 75 3

76 to 100 4

101 to 150 5

151 to 200 6

201 to 300 7

301 to 400 8

401 to 500 9

501 to 1,000 2% of total

1,001 and over 20 plus 1 for each 100 over 1,

Table 1-3 Minimum Number of Accessible Spaces

4 / DESIGN GUIDE 18 / STEEL-FRAMED OPEN-DECK PARKING STRUCTURES

clearly marked with signage with raised or Braille letters and standard symbols. Local ordinances generally exceed the ADA requirements for size of lettering on directional signs for vehicular traffic. All trip hazards, such as car bumpers and raised curbs must be eliminated from pedestrian pathways, with maxi- mum curb slopes being 8 percent. All multi-story parking structures require either at least one accessible elevator, a pedestrian ramp to grade level or a grade-level accessible structure. The reader is encouraged to become familiar with the full text of the ADA guidelines.

One out of every eight accessible spaces must be physi- cally disabled van accessible. Access to van-accessible spaces must meet the 8 ft, 2 in. requirement for ceiling clearance. The van-accessible space is still required to be only 8 ft wide but must be adjacent to an 8-ft-wide accessi- ble aisle. Van-accessible spaces may be grouped on one level of the parking structure, typically the ground level. Any ramp upon which parking or pedestrian traffic is allowed is recommended not to exceed a 5 percent slope with a 6 percent maximum slope allowed. All accessible routes must be clearly marked and, if the slope exceeds 5 percent, be slip resistant. All pedestrian paths must be

6 / DESIGN GUIDE 18 / STEEL-FRAMED OPEN-DECK PARKING STRUCTURES

detailed. Conversely, the conventionally reinforced con- crete deck is prone to cracking on the tension side. The degree of cracking of a reinforced concrete slab is affected by many variables such as the amount of reinforcing steel used, the reinforcing location, the concrete quality, the con- crete curing process, and joint-spacing. Section 2.1 contains a discussion of each type of deck system, Section 2.2 presents climactic considerations affecting each deck system and the tables in section 2. summarize deck characteristics.

2.1.1 Cast-in-place reinforced concrete

Cast-in-place reinforced concrete slabs have performed admirably in floor systems in enclosed conventional build- ings. In open-deck parking structures, however, concrete decks suffer from freeze-thaw cycles in cold climates, application of de-icing road salts, poor design, construction or inspection practices, and unsuitable aggregates. Certain basic precautions are required for a parking deck to survive for the long term. These include the use of:

• High-grade concrete and aggregate
• Proper curing procedures (7 days wet cure for optimum results)
• Concrete with a minimum compressive strength of 4,500 psi
• Adequate drainage of the deck surface
• Alow water/cement ratio concrete mix (0.40 or less)
• Adequate clear cover (1.5 in.) for the top reinforcing steel
• Low permeability for the cured concrete
• Proper placement of reinforcement

The minimum thickness for a cast-in-place, convention- ally reinforced slab in an open-deck parking structure is dependent on bay spacing. Reinforcing steel in a cast-in-place concrete deck must be protected. There are several options for protecting the reinforcing steel.

• Epoxy coating
• Galvanizing
• Use of stainless steel reinforcing bars
• Use of corrosion-inhibiting admixtures
• Use of Cathodic protection (may be cost prohibitive)

Recent research sponsored by FHWAindicates that a 75 to 100 year life can be expected for a concrete bridge deck by using stainless steel reinforcing, with or without cracks in the deck. It is difficult, however, to justify the increase in expense by using stainless steel for a parking structure.

2.1.1.1 Clear Cover and Permeability Two prominent causes of distress in cast-in-place concrete decks are excessive permeability and inadequate clear cover over reinforcing steel. Concrete is much like a “hard sponge” that will absorb moisture throughout its life. Fortunately, there are several ways to control penetration of chlorides into the deck. The permeability of the concrete itself can be reduced by:

• Awater-reducing admixture (also known as a superplastizer)
• Alow water-cement ratio (0.30 to 0.45)
• Amicrosilica fume additive
• Acalcium nitrate corrosion inhibitor
• Flyash or other pozzalan
• Proper curing procedure Recent studies have indicated that a low water-cement ratio may be the dominant factor in achieving a concrete with low permeability. Asilica fume particle is only one one-hundredth the size of a cement particle. It is easy to see how this additive can fill the voids in a concrete mix—voids that would otherwise conduct moisture. Silica fume, like cement, also hydrates as it cures, so the strength of the con- crete increases as well. The specifier of such high-performance concrete addi- tives to the concrete should be aware that their use may require changes in the way the concrete is placed, finished or cured. For example, shrinkage of superplastized concrete has been observed to be higher in some instances than that of conventional concrete, so the placement of control joints assumes added importance. Other families of products are intended to prevent chlo- rides from penetrating into the deck by application after the slab is cast and cured. Examples include: elastomeric waterproofing membranes, penetrating sealers, surface sealers, and coatings or overlays. Sealers, which must be periodically re-applied, seem to be more effective when they can penetrate into the concrete. Good penetration ( 1 / 8 in. to 1 / 4 in.) along with an adequate coverage rate affords better resistance to permeability and counters the loss of sealer at the surface due to normal wear from traffic on the deck.

DESIGN GUIDE 18 / STEEL-FRAMED OPEN-DECK PARKING STRUCTURES / 7

In recent years, there has been significant testing and evaluation of substances that seal concrete decks. Materi- als examined include latex products, epoxies, urethanes, linseed oils, silanes and siloxanes. The success of any sealant depends upon factors such as:

• Chemical formulation
• Concrete quality
• Surface preparation
• Conditions at the time of application
• Rate of coverage

Sealers are considered highly sensitive to these variables, which may help explain inconsistencies among test results and ratings that have been published by both producers and independent agencies. Perhaps the best advice for an owner or specifier is to evaluate a product both by independent agency data and local field experience, when available. Agood waterproofing membrane system, unlike a sealer, will bridge small cracks (perhaps up to 1 / 16 in. wide). A membrane system, which is usually applied in three or four layers (binder, membrane, wearing surface), may be as much as 4 or 5 times the initial cost of a penetrating sealer. Alife-cycle cost analysis is thus in order when selecting a deck surface treatment, and it must include consideration of other corrosion control measures being contemplated for the deck. The depth of clear cover over reinforcing steel largely determines their rate of corrosion. Even the top 1 / 2 in. to 1 in. of high-grade concrete can eventually become contami- nated by de-icing chlorides. Thus, it has been suggested that the top 1 in. of concrete be considered “sacrificial”. By increasing actual concrete cover to 2 in., dramatic reduc- tions in chloride penetration to the level of top reinforcing - and in rate of corrosion - have been observed in simulated long-term tests. Increasing concrete cover over negative moment rein- forcing steel better protects the bars, but will increase the width of any tension cracks that form on the surface. Care should be taken not to significantly exceed 2 in. of cover as cracking will occur in areas of negative reinforcement as the thickness approaches 3 in. Acover of 2 in. of actual cover allows for fabrication and construction tolerances to minimize crack width. The American Concrete Institute (ACI) recommends that top bar spacing in negative moment areas be reduced to as little as 4 in. All reinforcing steel must be strongly supported. Another technique for protecting reinforcing steel is epoxy coating or galvanizing. Research has shown that an epoxy coating with an optimum thickness from 5 to 10 mils can reduce the rate of steel corrosion up to 41 times. Epoxy

coatings are flexible, low in shrinkage and creep, and are virtually impermeable to chloride ions. One concern is damage to the coating during shipment and handling; dam- aged areas that expose the bar must be repaired. Galvanized bars have received mixed reviews over the years, but stud- ies have also found them to be somewhat effective in resist- ing chloride corrosion. It is important to note that, when galvanizing is selected as the means of protection for the reinforcing steel, all reinforcing steel in that deck must be galvanized, and the galvanized bars must not be in contact with any ungalvanized steel. Galvanized bars are more resistant to damage from abuse; they tend to repair them- selves. Both epoxy coated and galvanized reinforcing steel are used in bridge decks. Bridge owners looking for a 75- to 100-year life-span for critical bridges are likely to opt for stainless steel. As a chemical additive to concrete, calcium nitrite has been found to be effective in interrupting the electrolytic process that causes corrosion of reinforcing steel in con- taminated concrete. Even though chloride concentration at the level of the bars is far above the threshold level, corro- sion activity itself is inhibited and greatly diminished.

2.1.1.2 Curing The necessity of proper curing of the concrete deck cannot be understated. Improper curing techniques and/or the lack of an adequate curing period will often diminish deck per- formance. Steam heat-curing of concrete with a low water-cement ratio provides a 28-day compressive strength equal to that of moist curing, and equal or better resistance to water and chloride absorption and intrusion. Steam curing is often utilized for plant-cast deck systems such as precast double tees. Site-cast decks should be water cured for a minimum of 7 days. Curing compounds are not recommended, par- ticularly in warm weather as they do not prevent the escape of moisture and also prevent sealer penetration. The use of any deicer on the deck should be avoided for at least 6 months after concrete placement to minimize concrete scal- ing.

2.1.1.3 Joints, Cracks and Drainage Leakage of water chlorides through cracks or joints accel- erates corrosion of reinforcement and deterioration of a concrete deck. Leaks also provide the major access for cor- rosive chlorides to the supporting steel or concrete frame. The primary difference between how these leaks impact a concrete and steel frame is in the amount of time that elapses before the damage becomes obvious. Leakage into a concrete frame will be hidden from view, but will require expensive restoration in the long term. Leakage onto a steel frame will result in short term visible surface corrosion that

Galvanized metal deck in some parking garages is per- forming well, no doubt a reflection on the attention given to crack control, joint seals and fastening of metal deck seams. At least a G-90 perforated galvanized deck is recommended (i.e., 0.90 ounces of galvanizing per ft 2 ) for parking deck applications, as is welding or mechanical fastening of the side lap seams. Button-punching of side lap joints appears to increase the likelihood of leakage through the seam and corrosion of the underside of the deck. For extra protection a high-performance, compatible paint system should be applied to the exposed underside of the deck after installa- tion in areas where road or marine salts are present. There are only three conditions for which composite metal floor deck should be used in open-deck parking garages:

• As a stay-in-place form only, not relied upon as tension reinforcement for the slab
• As tension reinforcement in temperate climates, but with tension reinforcing steel in the slab as well as a backup
• As the sole tension reinforcement for a slab in a deck system that has been designed, by necessity, to be leakproof An example of the last condition is the bottom level of a car park having finished occupied space below. Leakage through this level is unacceptable. Atypical solution is to sandwich waterproofing and insulating membranes between the structural slab and a good quality paving slab above. Ahigh priority should be placed on providing the best possible surface drainage for the paving slab, and use of a membrane system should be considered. Fortunately, an insulated structural slab in this application is not likely to be exposed to freeze-thaw cycles or extreme temperature changes.

2.1.2 Cast-in-Place Post-Tensioned Slabs and Toppings

Post-tensioning a site-cast concrete slab in a steel-framed parking garage minimizes intermediate joints and crack for- mation and helps to limit the width of cracks that do form. However, post-tensioning will increase elastic and creep shortening of the concrete slab. Bracing or shear wall locations should be near the center of mass of the slab to reduce the possibility of restraint cracks. Extra care should be taken to isolate the slab from any rigid elements near the outer portions of the slab. Post-tensioning can be done in one or both directions. Ideally, under real service loads, no tension should exist in the top of the slab in the direction(s) of post-tensioning. Some designers prefer not to post-tension in the direction of composite beams, as it is difficult to estimate the portion of

the post-tensioning force being absorbed by the composite beams themselves. Unpublished tests performed by Mulach Parking Systems showed a maximum stress increase of three percent. At the least, one would expect a non-uniform distribution of post-tensioning force across the slab. Indeed, unusual patterns of hairline cracking have been observed in a few post-tensioned composite decks. However, slabs that have not utilized longitudinal post-ten- sioning have been noted to exhibit significantly more crack- ing in the affected direction and post-tensioning in both directions is encouraged. The post-tensioned slab is somewhat more expensive than the conventionally reinforced, cast-in-place slab. In some regions there is reluctance to use post-tensioning due to a lack of availability of an experienced labor force and local concrete contractors with post-tensioning expertise. Design recommendations issued by the American Con- crete Institute and the Post-Tensioning Institute should be observed.

2.1.3 Precast Double Tees For the long-span parking module, 10, 12 or 15 ft wide by 24 to 32 in. deep precast, prestressed double tees supported by steel framing are typical. This system, with both its frame and concrete deck shop fabricated, has a very fast erection time when both products are delivered in a timely and coordinated fashion to the job site. Other advantages of double tees include:

• Better control and assurance of concrete quality due to prefabrication at a plant;
• Elimination of negative moments in the deck elements, as they are mostly simple span;
• Inherently low permeability and better resistance to pen- etration of chlorides if steam-cured, because steam cur- ing of the double tees decreases size of capillary pores.
• Low cracking as a result of the prestressed condition of the element One of the concerns about all precast parking structures is stability during erection. Asolution to that problem is to use double steel columns and beams at interior supports. Each double tee frames into its own beam at both ends, and this avoids the large torsional loads that occur when placing the first bay of panels onto a common beam and concerns about adequate flange width to accommodate tees from two sides. The two steel columns are normally spaced 3 ft apart and tied together to form a mini-frame, which provides lat- eral load resistance in the long-span direction. The space between the tee ends and supporting beams can be used as a drainage pipe chase. The tee ends are bridged by a well-

DESIGN GUIDE 18 / STEEL-FRAMED OPEN-DECK PARKING STRUCTURES / 9

detailed strip of high quality site-cast concrete, which is later sealed. If prestressed double tees frame onto one common beam, joints should be sealed with sealant systems that accommo- date movement and end rotations. Joint surfaces and instal- lation of sealers are especially important. Whatever the detail over the beams, a joint seal should be specified that is compatible with the behavior of the long-span double-tee deck system. With double-tee decks, particular attention must be given to the longitudinal joint at abutting flanges. Every foot of joint is a foot of potential joint breakdown, leakage and sub- sequent deterioration of embedded metals. It is recom- mended that a high quality traffic-bearing polyurethane or silicone sealant be applied to longitudinal joints. Care should be taken with silicone sealants as their softness pres- ents a possible trip hazard in pedestrian traffic areas. As a backup, all metal passing through the joint can be stainless steel, painted or galvanized for corrosion protection. In years past, site-cast structural toppings were placed on the precast deck to help prevent joint leakage and to provide a more true, jointless surface. Toppings are subject to crack- ing, delamination, initial shrinkage and debonding. They are placed on concrete panels that are themselves relatively stable. For these reasons, unless diaphragm action is required, precast, prestressed double tee decks in parking structures are often left untopped and protected with pene- trating sealers. In applications when the seismic response modification factor R is taken greater than 3, the need for a continuous diaphragm requires a reinforced topping slab. With untopped double tees, differential camber between adjacent panels must be more carefully controlled, and be limited to a 1 / 4 in. maximum in the driving lane area. Exces- sive differential camber compounds the wear and tear of joint seals; it can be controlled by minimizing the design prestress force and by field adjustment using jacking and shimming plus pour strips.

2.1.4 Other Systems

2.1.4.1 Filigree

The Filligree deck system consists of a precast, prestressed 2.5-in. concrete panel, usually cast off-site then shipped, erected and used as the formwork for a 3 1 /4-in. topping com- positely cast with the form. The system has been used in building construction for at least 35 years, originally sup- plied under the trade name “Filigree.” That system is still produced, and in some regions local precasters are supply- ing competitive systems. The precast form is usually supplied in 8-ft widths and lengths up to 40 ft, which can span two bays. The form is precast with steel elements protruding from it that develop

the composite action with the site-cast topping. Filigree has most of the required reinforcing steel and supports set into the panel, but the concrete contractor must add some nom- inal reinforcing steel in the negative moment region, over the beams in the topping slab. Using spans of 18-ft precast formwork, little or no shoring is required. The steel beams are also composite with the topping, which is cast around standard shear connectors. For the two-bay panel holes are cast at the plant for the shear studs, which are field welded to the beam flanges. Joints should be tooled in the cast-in- place topping immediately above the joints between the fil- igree panels. Parking garage owners should require some on-site pres- ence of the supplier of this deck system during construction. The “system” is not just the precast form but the two com- ponents. The site-cast topping, like all structural concrete toppings, is subject to differential shrinkage and movement, and the panels must fit tight and proper field concreting procedures must be followed. Minimal shoring, depending on the supporting framing scheme, is usually required. Con- tractors not familiar with this deck system should become thoroughly familiar with it, including seeking the assistance of the supplier and/or designer prior to start of construction.

2.1.4.2 Hollow-Core Plank Hollow-core precast plank has been popular as a floor sys- tem in residential buildings, either on steel framing, masonry bearing wall framing or concrete framing. How- ever, neither the concrete mix nor the plank configuration is particularly designed or controlled for the challenging exposure of the open-deck parking garage. The hollow cores in the plank may accumulate water, and the top and bottom elements are slender so there is minimal cover for prestressing steel. For these reasons, hollow-core plank is not recommended for open-deck parking structures.

2.2 Deck System Selection by Climactic Zone Deck system selection is a reflection of the particular cli- mactic and environmental conditions. Such durability con- siderations are summarized for U.S. exposures in Figure 2-1.

2.3 Concrete Durability The quality of concrete used in the deck system is very important. Care must be taken to ensure maximum con- crete durability. The following considerations should be taken into account when specifying the concrete material:

• The minimum 28 day concrete strength should be 4,500 psi
• The minimum cementious material content should be 61 / 2 bags per cubic yard

10 / DESIGN GUIDE 18 / STEEL-FRAMED OPEN-DECK PARKING STRUCTURES

• The minimum entrained air content should be 6 percent plus or minus
• The maximum water to cement ratio should be 0.
• The minimum of a 1.5 in clear cover at the top of the deck for all reinforcing steel
• Strict adherence to ACI chloride levels must be used for new concrete In addition to the above minimum concrete material parameters the following alternatives should be considered since even small increases in material costs during con- struction can reap large benefits in durability:
• An encapsulated post-tensioned system
• Calcium nitrate corrosion inhibitor
• Silica Fume
• Fly ash or other pozzalan
• Galvanized reinforcing steel
• Epoxy-coated reinforcing steel

As noted earlier, concrete parking decks require protec- tive coatings. Leaving a concrete parking deck untreated is similar to leaving an exposed steel column unpainted. Pro-

tective coatings come in two categories, sealers and mem- branes. The cost, application, and protection afforded is vastly different. It is important that the proper material be chosen for use that meets the needs and requirements of the structure and owner. Concrete sealers are a one step, light coating that is spray applied then brushed in to achieve maximum penetration on the concrete surface. They are designed to prevent water and water-borne salts from penetrating the concrete deck. The sealers themselves are not designed to be waterproof. Agood sealer should allow the concrete to breathe, or allow vapors to escape. Sealers are most effective in protecting un-cracked concrete surfaces. Concrete membranes are designed to be waterproof and are not a light one-step spray application like sealers but a heavy, multiple-step squeegee or troweled on application. Membranes are not designed to and cannot bridge cracks in the slab other than microcracks. There are also some one- step coatings available that are much heavier than a sealer but not as heavy as a three-step membrane. If a deck system has occupied areas below the deck regardless of whether or not the deck system has a propen- sity to crack, a membrane coating should always be used and a plaza deck system should be considered.

2.4 Plaza Deck Systems Aplaza deck system is a multiple-layer system that pro- vides added redundancy and protection against wear for a

12 / DESIGN GUIDE 18 / STEEL-FRAMED OPEN-DECK PARKING STRUCTURES

System Estimated Foundation Load Cast-in-place concrete frame with post tensioned concrete beams and girders and one-way post tensioned slab

107 psf

Precast, pre-tensioned, pre-topped doubles on a precast concrete frame

96 psf

Precast, pre-tensioned, site-topped double tees on a precast concrete frame

113 psf

Non-prestressed cast-in-place composite concrete slab on precast, prestressed joists and beams and concrete columns

108 psf

Precast, pre-tensioned beams and girders with one-way post tensioned slab on site-precast columns

105 psf

Precast, pre-tensioned beams and girders with composite CIP/plank slabs and site-precast columns

111 psf

Structural steel frame with cast-in-place, one-way, composite, post tensioned slab

75 - 82 psf

Structural steel frame with cast-in-place conventionally reinforced deck on stay-in-place metal deck

55 – 75 psf

Precast, pre-tensioned, pre-topped double tees on a structural steel frame

96 psf

Cast-in-place. Non-prestressed, short-span concrete 125 psf Precast, prestressed short-span concrete 130 psf Cast-in-place, post tensioned, flat plate short-span concrete 125 psf

Table 2-2 Foundation Loads by System

membrane system. Plaza deck systems are more expensive than typical membrane systems, but they may be selected to:

• Protect occupied space below
• Reduce membrane maintenance
• Meet architectural and aesthetic needs of the deck

Unlike typical membrane systems, which are directly exposed to traffic, plaza deck systems have a membrane protected by a wearing surface and a secondary drainage system. The components of a plaza deck, from top to bot- tom, include:

• Wearing surface
• Slip sheet
• Drainage layer
• Insulation (over occupied space)
• Protective board
• Waterproofing membrane
• Structural Slab

The plaza deck system should be designed to drain both the surface water and any water that filters through the deck system and collects on top of the membrane. The drains must contain weep holes below the surface level to accom- modate the drainage from the membrane surface. Both the wearing surface and the sub-surface drainage layer should have a slope of 1 / 4 in. per ft and an absolute minimum slope of 3 / 16 in. per ft. If this minimum slope requirement is not met, the system will be highly susceptible to deterioration and leakage.

2.5 Deck System Design Parameters

Codes prescribe a minimum uniform live load of 50 pounds per square ft and a concentrated load of 2000 pounds applied over an area of 20 in. 2 at any point on the deck. The code-prescribed minimum live loads listed above must be considered in the design. Additionally, a well-designed deck must account for the realistic loading of the structure. Realistically, the typical live load on the structure is approx- imately 30-35 pounds per square ft. This is found by con- sidering a compact car in the smallest parking space in a garage (7.5 ft by 15 ft). This compact car space occupies an area of 113 ft2 and the weight of a compact car that could fit into a space that small is approximately 3,200 pounds. Allowing for an additional 500 pounds for four occupants, the realistic loading by the vehicle is a weight up to 3,

pounds or 33 pounds per square ft. This does not account for usually unloaded areas such as driving lanes, etc. Although conservative, a realistic live load on the order of 30 pounds per square ft must be checked as a rolling load or as pattern loading on slabs. This analysis will yield dif- ferent reinforcing patterns than a simple code-specified loading, and the more conservative of the two designs should be used. When designing a post-tensioned slab, in addition to the code-specified load, the slab must be checked using a live load of 20-25 pounds per square ft or skip loading, but permitting zero tension in the top of the slab. The foundation system for the parking structure must be investigated prior to selecting a deck system. Local soil conditions should be determined through soil borings and geotechnical testing by a qualified geotechnical engineer. If the site has poor soil conditions and requires deep founda- tions, a lighter deck would be beneficial, since it would be less costly and more easily installed. Relative weights of various framing systems are listed in Table 2-2. If site geol- ogy is such that the supporting underlying strata is not uni- form and differential settlement will likely occur, a deck system that can accommodate differential settlement must be used. If the site has large grade differentials, a retaining wall design should be incorporated within the structural design or the ground surface should be sloped back. The deck system must have both the continuity and the struc- tural diaphragm capacity to function as such.

Drainage Parameters for Parking Decks Next to concrete quality, the most important factor in garage deck durability is drainage. If a parking deck does not drain it will deteriorate rapidly in the areas where water and de-icing chemicals are permitted to pond. This type of deterioration will be more significant in geographic areas where freeze/thaw cycles are a frequent occurrence and large amount of de-icing chemicals are used. In order to achieve proper drainage the topics of deck slope and drain locations and selection must be addressed.

2.5.1 Cast-in-Place Conventionally Reinforced Con- crete on Stay-in-Place Metal Forms (see also discussion and figures in section 3.3.1) Typical Parameters

• Light gauge vented metal decking available in depth of 2 in. and 3 in.
• Gauges from 20 to 16
• Widths of 36 in.
• Galvanized
• Span Range 8 ft to 12 ft
• Slab Thicknesses 5 in. to 6 in. (minimum of 3 in. over top of flutes)

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