Chapter 23 Concrete Construction-II (Site-Cast and Precast

23

CHAPTER

528

Concrete Construction–II (Site-Cast and Precast Concrete Framing Systems)

CHAPTER OUTLINE

23.1 TYPES OF ELEVATED CONCRETE FLOOR SYSTEMS

23.2 BEAM-SUPPORTED CONCRETE FLOORS

23.3 BEAMLESS CONCRETE FLOORS

23.4 POSTTENSIONED ELEVATED CONCRETE FLOORS

23.5 INTRODUCTION TO PRECAST CONCRETE

23.6 STRUCTURAL PRECAST CONCRETE MEMBERS

23.7 TOTAL PRECAST CONCRETE CONSTRUCTION

23.8 FIRE RESISTANCE OF CONCRETE MEMBERS

This chapter is a continuation of Chapter 22 . It begins with a discussion of commonly used types of elevated site-cast concrete slabs. (Slabs-on-ground were covered in Chapter 22 .)

This is followed by coverage of precast, prestressed concrete elements such as hollow-core slabs, solid planks, double-tee units, and inverted-tee beams. Construction systems related to these elements are also discussed.

23.1 TYPES OF ELEVATED CONCRETE FLOOR SYSTEMS

Site-cast reinforced-concrete framing systems consist of horizontal elements (elevated floor/roof slabs and beams) and vertical elements (columns and walls). Approximately 80% to 95% of the cost of materials and formwork of a concrete structural frame is in the horizon-tal framing elements of the frame. Consequently, the choice of the elevated floor system is the most important item in a concrete structure.

Elevated concrete floor systems can be classified as (a) beam-supported floors and (b) beamless floors. They are further divided into several types, Figure 23.1 .

23.2 BEAM-SUPPORTED CONCRETE FLOORS

A reinforced-concrete floor slab with beams on all four sides can either be a one-way slab or a two-way slab . They are also called one-way solid slabs or two-way solid slabs to distinguish them from one-way joist slabs or two-way joist slabs, which are not completely solid.

The transfer of loads from a solid slab to the four supporting beams is represented approximately by 45° lines originating from the slab corners, Figure 23.2 (a). If the slab panel is a square, each supporting beam receives the same amount of load, Figure 23.2 (b). If the slab panel is rectangular, one pair of beams carries a greater load than the other pair.

Preliminary Thickness of One-Way and Two-Way Slabs

Estimate the thickness of a one-way slab by dividing the slab span by 24. Thus, if the slab span is 12 ft, use a slab approximately 6 in. thick.

Two-way slab thickness is span/36. Use the longer of the two spans. Thus, if a slab panel measures 16 ft * 21 ft, the slab thickness is approxi-mately 21/36 = 7/12 ft, or a 7-in.-thick slab.

Also see Appendix B , and consult a structural engineer for exact sizes.

NOTE

529

ONE-WAY SOLID SLAB If the ratio of the long dimension to the short dimension of a four-side-supported slab panel is greater than or equal to 2.0, most of the load on the slab is transferred to the long pair of beams, that is, the load path is along the short dimension of the slab panel, Figure 23.2 (c). The load path along the long dimension of the slab is negligible.

Because the load is effectively transferred along one direction in Figure 23.2 (c), the slab behaves as a one-way slab. The reinforcement in a one-way slab is placed along the short direction, referred to as the primary reinforcement to distinguish it from the nominal rein-forcement placed along the perpendicular direction, called the secondary reinforcement . The purpose of secondary reinforcement is to resist stresses caused by concrete shrinkage and thermal expansion and contraction of the slab.

ELEVATED CONCRETE FLOORS

Beam-supported floors Beamless floors

Two-wayslab

One-wayjoist floor

Two-way joist floor(waffle slab)

Two-wayjoist floor

Flat plateOne-way

slabFlat slab

FIGURE 23.1 Types of elevated concrete floors.

(a) Transfer of the load on a slab to four supporting beams

(b) For a square slab, the load transferredto all four beams is equal

Beam

45º angle

Beam

45º angleBeam A

Beam A´Load on this slab area istransferred to Beam A´

Beam BBeam B´

45º angle

Load on thisslab area istransferredto Beam B

Load on this slabarea is transferredto Beam B´

Load on this slab area istransferred to Beam A

Long dimension

Sh

ort

dim

en

sio

n

Beam

45ºangle

Most of the load fromthe slab is carried inthis (short) direction

long dimension of slab

short dimension of slab2.0, the slab behaves as a one-wayslab because most of the loadfrom the slab is carried along theshort direction

≥ (c) If

FIGURE 23.2 Distinction between one-way and two-way concrete slabs supported by beams on all four sides.

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Part 2Materials and Systems of Construction

TWO-WAY SOLID SLAB If the ratio of the long to the short dimension of a four-side-supported slab panel is less than 2.0, the slab is considered to behave as a two-way slab . However, real two-way slab behavior occurs when the ratio of the two dimensions is as close to 1.0 as possible (between 1.0 and 1.25). In a two-way slab, both directions participate in carrying the load. Rein-forcement is, therefore, provided in both directions as primary reinforcement. Although not common, both one-way and two-way slabs may occur in the same floor, Figure 23.3 .

BEAM-AND-GIRDER FLOORS One-way and two-way solid slabs become increasingly thick and hence uneconomical as their span increases. Generally, the use of a slab thicker than 8 in. is discouraged because it cre-ates a large dead load on the floor. For a one-way slab, an 8-in. slab thickness is reached with a span of approximately 16 ft. For a square two-way slab, a span of approximately 24 ft requires an 8-in.-thick slab.

Because 16-ft and 24-ft dimensions are rela-tively small for column spacing, one-way and two-way slabs are generally used in a beam-and-girder floor, Figure 23.4 (a), or in a two-way beam-and-girder floor , Figure 23.4 (b).

BAND BEAM FLOOR A concrete floor that cannot be constructed with a flat form deck becomes uneconomical. There-fore, the floor systems shown in Figure 23.4 are relatively uncommon because of the complexity of the formwork resulting from deep beams around slab panels.

Two-wayslab

Two-wayslab

Two-wayslab

One-wayslab

One-wayslab

FIGURE 23.3 Although not common, one-way and two-way slabs can occur in the same floor.

(b) Plan (looking up) and section of a two-way beam-and-girder floor

(a) Plan (looking up) and section of a beam-and-girder floor. Forformwork economy, the depth of the beam and girder is generallykept the same. If the two bay dimensions are unequal, thegirder is generally placed along the smaller dimension.

Column

Column

Girder

Girder

Beam

Column

Ap

pro

x. 3

5 f

t ty

pic

al

Approx. 30 ft typical

Girder

Column

Ap

pro

x. 3

0–3

5 f

t ty

pic

al

Column

Column

Girder

Beam

Column Column

Column

Column

Approx. 30–35 ft typical

FIGURE 23.4 Plan and section through beam-and-girder floors.

Chapter 23Concrete Construction–II

(Site-Cast and Precast Concrete Framing Systems)

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Slab

Beam

Column

8- to 10-ft-wide beam

18- to 20-ft-wide slab

7- to 8-in.-thick slab

Column

Projection of beambelow slab made using2 x 8 or 2 x 10 lumber

Column

Slab

FIGURE 23.5 Plan (looking up) and section through a typical banded slab, which yields a more economical form-work than the beam-and-girder system in Figure 23.4 . The projection of the beam below the bottom of the slab is generally obtained by using dimension lumber, such as 2 * 8, 2 * 10, and so on. This illustration also shows typical spans and member thickness (also see Appendix B ).

A one-way slab floor with wide and shallow, continuous beams, referred to as band beams (in contrast with the conventional narrow beams ), gives more economical formwork, Figure 23.5 . Because the beams are wide, the slab span is reduced, reducing the slab thick-ness. Additionally, because the beams are shallow, the floor-to-floor height is smaller, reducing the height of columns, interior partitions, and exterior cladding. A smaller floor-to-floor height also reduces the overall height of the building, which reduces the magnitude of lateral loads on the building.

ONE-WAY JOIST FLOOR A concrete floor that results from extremely economical formwork consists of closely spaced, narrow ribs in one direction supported on beams in the other direction, Figure 23.6 . Because the ribs are narrow and closely spaced, the floor resembles a wood joist floor. It is, therefore, called a joist floor or a ribbed floor , but it is more commonly known as a one-way joist floor to distinguish it from the two-way joist floor described later.

A one-way joist floor is constructed with U-shaped pans as formwork placed over a flat-form deck. The gap between the pans represents the width of the joists, which can be adjusted by placing the pans closer together or farther apart, Figure 23.7 . The pans are generally made of steel or glass fiber–reinforced plastic (GFRP) and can be used repeatedly.

The vertical section through a pan tapers downward for easy stripping and has support-ing lips at both ends. Pan widths and heights have been standardized to give two categories of one-way joist floors:

• Standard-module one-way joist floor • Wide-module one-way joist floor

STANDARD-MODULE ONE-WAY JOIST FLOOR Standard-module pans are 20 in. and 30 in. wide, Figure 23.8 . These dimensions have been standardized so that, with 4-in.- and 6-in.-wide joists, the center-to-center spacings between

532

joists are 2 ft and 3 ft, respectively. A slab thickness of 312 in. is often used (to provide a 1-h

fire-rated floor), although structural considerations require a minimum thickness of only 2 in. with 20-in. pans and 21

2 in. with 30-in. pans. The slab is designed as a one-way slab resting on the joists, which, in turn, are designed as beams.

Pans of various depths are available to create joists of various depths to suit different joist spans. Because the load on each joist is small, reinforcement requirements are also small. No stirrups are generally used in joists, thus requiring the concrete to resist the entire shear. The increase in shear near-joist supports is resisted, if needed, by increasing the joist width, Figure 23.9 . Special pans with closed ends on one side are available to produce the widen-ing of joists.

A 4- to 5-in.-wide load-distribution rib is often provided in the center of joists.

WIDE-MODULE ONE-WAY JOIST FLOOR Wide-module pans are available in 53-in. and 66-in. widths, Figure 23.10 . They are gener-ally used with 5-ft and 6-ft center-to-center joist spacings, giving joist widths of 7 in. and 6 in., respectively. However, almost any joist width can be created to suit the load and span conditions. Joist widths of 8 and 9 in. are also common.

Joist

Distributionrib

Beam

Void for joistU-shaped pan

Reinforcementfor beam

FIGURE 23.6 A one-way joist floor. Observe that the bottom of the beams and joists is at the same level to allow the entire slab to be formed on a flat-form deck.

FIGURE 23.7 Formwork for a one-way joist floor showing U-shaped pans. The space between the pans represents the width of the joists. Notice that reinforcement for beams has already been laid. Reinforcement for joists and the slab is yet to be completed (also see Figure 23.11 ).




533

Because a wide-module floor has a larger spacing between joists, the slab thickness required is also larger. Therefore, a wide-module floor is generally used where a minimum fire rating of 1 h (required slab thickness = 31

2 in.) or 2 h (required slab thickness = 412 in.) is needed.

Figure 23.11 shows the overall view of a wide-module one-way joist floor with rebars in place. As shown in this figure, the joists in a wide-module floor are generally designed with shear reinforcement (stirrups). Therefore, the widening of joist ends at beams is unnecessary. Special end caps are available to close the open ends of pans near the beams, Figure 23.12 .

Taper 1:12

Lip

20 in.

14, 16, 20and 24 in.

14, 16and 20 in.

Stiffener

Taper 1:12

30 in.

Stiffener

FIGURE 23.8 U-shaped standard-module pans (consult the manufacturer for available sizes). Note that the pans have open ends on both sides, except the pans used adjacent to beam or the distribution rib, which have closed ends (see Figure 23.9 ).

Normal widthof pan

Open endof joist

Closed end of joist

Pan with one endclosed and narrower toproduce wider joist end

Widened endof joist

Distribution rib

FIGURE 23.9 A one-way joist floor with standard pans. Observe that the joist width has been increased near the beams to accommodate greater shear in the joists at that location. The beams in this slab are deeper than the joists. This is unusual because it increases the cost of formwork com-pared with the slab with the same depth of joists and beams, as shown in Figure 23.6 . A distribu-tion rib has been provided in the center of the slab.

Preliminary Depth of a One-Way Joist Floor

Estimate the depth of a one-way joist floor by dividing the joist span by 18. Thus, if the span of joists is 36 ft, use 24-in.-deep joists, which includes the thickness of the slab. Using a 41

2@in. -thick slab and 20-in.-deep pans for a wide-module floor gives a floor depth of 241

2 in. Where the spacing of col-

umns in either direction is une-qual, the supporting beams should preferably be oriented along the shorter direction and the joists in the longer direction.


NOTE

534


FIGURE 23.11 An overall view of a wide-module one-way joist floor formwork with rebars in place—ready to receive concrete. Observe the use of shear reinforcement (stir-rups) in the joists.

53 in.

14, 16, 20and 24 in.Taper 1:12

66 in.

14, 16, 20and 24 in.Taper 1:12

Stiffener

Stiffener

FIGURE 23.10 Wide-module pan dimensions. Consult manufacturers for available sizes.

Space forbeam

End cap

Space for joist

U-shaped pan

FIGURE 23.12 Wide-module pans and end caps. Because both ends of the pans are open, end caps are needed to close the gap to form a void for the beam.



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TWO-WAY JOIST FLOOR (WAFFLE SLAB) A two-way joist floor, also called a waffle slab , consists of joists in both directions, Figure 23.13 . For the same depth of joists, a waffle slab yields a stiffer floor than a one-way joist floor. It is, therefore, used where the column-to-column spacing lies between 35 and 50 ft. A waffle slab is best suited for square or almost square column-to-column bays. When left exposed to the floor below, the waffle slab provides a highly articulated ceiling.

(a)

(b)

Beam

Column

Beam

Column

Beam

Beam

Joist

Joist

FIGURE 23.13 Two-way joist floor (waffle slab) supported on beams on all sides. (a) Isometric from below. (b) Plan (looking up) and section through the slab. Note that a waffle slab can also be constructed without beams, as shown in Figure 23.15 .

Preliminary Depth of a Two-Way Joist Floor

Estimate the depth of a waffle slab by dividing the longer span by 22. Thus, if the two-way joist floor span is 44 ft, use a 2-ft (i.e., 24-in.)-deep floor that includes the thick-ness of the slab. Using a 41

2@in. -thick slab for a 2-h rated floor and 20-in.-deep domes gives a floor depth of 241

2 in. Also see Appendix B , and

consult a structural engineer for exact sizes.

NOTE

536

The formwork for the slab consists of glass fiber–reinforced plastic domes placed on a flat form deck with the lips butting each other, Figure 23.14 . Like the pans, the domes can withstand repeated use. Dome dimensions have been standardized to produce 3-ft, 4-ft, and 5-ft center-to-center distances between domes in a variety of depths. The domes have a wide supporting lip on all sides and are laid on a flat-form deck.

(a) Three-dimensional view of fiberglass domeused as form for a waffle slab

(b) Section through dome

Depth of domes: 14 in.,16 in., 18 in., and 20 in.

Lip

30 in. for 3 ft,41 in. for 4 ft, and52 in. for 5 ftcenter-to-center dome

3 ft, 4 ft, or 5 ftWidth of lip =3 in. for 3 ft,3-1/2 in. for 4 ft,and 4 in. for 5-ft dome

FIGURE 23.14 Standard GFRP dome sizes for a waffle slab. Consult manufacturers for available sizes.

Each question has only one correct answer. Select the choice that best answers the question.

1. A reinforced concrete, four-side-supported slab panel is a. a one-way slab. b. a two-way slab. c. a four-way slab. d. (a) or (b). e. (b) or (c).

2. A reinforced-concrete, four-side-supported slab panel measuring 15 ft * 22 ft is a. a one-way slab. b. a two-way slab. c. a four-way slab. d. (a) or (b). e. none of the above.

3. A reinforced-concrete, four-side-supported slab panel measuring 15 ft * 32 ft is a. a one-way slab. b. a two-way slab. c. a four-way slab. d. (a) or (b). e. none of the above.

4. In a band beam reinforced-concrete floor, the beams are a. narrow and deep. b. wide and shallow. c. located at a spacing of 8 to 10 ft on center. d. unnecessary.

5. A band beam floor is generally more economical than a beam-and-girder floor because a. it requires a smaller quantity of concrete. b. it requires a smaller quantity of reinforcement. c. its formwork is more economical. d. none of the above.

6. A standard-module one-way joist floor is constructed using pans that are a. 20 in. wide. b. 30 in. wide.

c. 40 in. wide. d. (a) or (b). e. (b) or (c).

7. Based on structural considerations only, the thickness of the slab in a standard-module one-way joist floor need not exceed a. 2 in. to 21

2 in. b. 3 in. to 312 in.

c. 4 in. to 5 in. d. 5 in. to 6 in.

8. Pans used as formwork for one-way joist floors have a a. U-shaped profile. b. Z-shaped profile. c. dome profile. d. any one of the above. e. none of the above.

9. A wide-module one-way joist floor is constructed using pans that are a. 66 in. wide. b. 60 in. wide. c. 53 in. wide. d. (a) or (b). e. (a) or (c).

10. In a one-way joist floor, the supporting beams run in one direction and the joists run in the other direction. If the spans in the two directions are unequal, the beams should preferably span along the a. shorter direction. b. longer direction.

11. The formwork used for a waffle slab has a a. U-shaped profile. b. Z-shaped profile. c. dome profile. d. any one of the above. e. none of the above.

12. Using standard formwork components for a waffle slab, the center-to-center distance between waffles (voids) is a. 3 ft, 4 ft, or 5 ft. b. 2 ft, 4 ft, or 6 ft. c. 4 ft, 5 ft, or 6 ft. d. 5 ft, 6 ft, and 7 ft. e. none of the above.

PRACTICE QUIZ

23.3 BEAMLESS CONCRETE FLOORS

A waffle slab is more commonly constructed as a beamless slab, Figure 23.15 . In a beamless waffle slab, a few domes on all sides of a column are omitted so that the thickness of the slab at the columns is the same as the depth of the joists. The thickening of the slab at the col-umns provides shear resistance (against the slab punching through the columns).



537

Column

Column

Joist

(a)

(b)

FLAT PLATE A flat plate consists of a solid slab supported directly on columns, Figure 23.16 . A flat plate is similar to a two-way banded slab, except that the beam bands in both directions are concealed within the thickness of the slab. Therefore, the spans that can be achieved economically with a flat-plate floor are smaller than those obtained from one-way or two-way joist floors.

Flat-plate slabs are suitable for occupancies with relatively light live loads, such as hotels, apartments, and hospitals, where small column-to-column spacing does not pose a major design constraint. Additionally, a drop ceiling is not required in these occupancies and HVAC ducts can be run within the corridors, where a lower ceiling height is acceptable.

FIGURE 23.15 A beamless waffle slab, i.e., a waffle slab supported on column heads (capitals). Note the similarity with a flat slab of Figures 23.17 . (a) Isometric from below. (b) Plan (looking up) and section through the slab. Note that a waffle slab can also be constructed with beams, as shown in Figure 23.13 . A beamless waffle slab may be provided with spandrel beams, if needed.

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A flat-plate slab results in a low floor-to-floor height, and its formwork is economical. Because the beams are concealed within the slab thickness, columns need not be arranged on a regular grid—a major architectural advantage. However, a flat plate is a two-way system; hence, the column spacing in both directions should be approximately the same. A slab thickness of approximately 6 in. is generally needed for 15-ft * 15-ft column bays and approximately 8 in. for 20-ft * 20-ft bays with residential loads.

FLAT SLAB A flat slab is similar to a flat plate, but it has column heads, referred to as drop panels , Fig-ure 23.17 (a). The primary purpose of drop panels is to provide greater shear resistance at the columns, where the shear maximizes.

(a) A typical flat slab

(b) Minimum drop panel dimensions

(c) Drop depth of panel based on dimension lumber

Drop panelSlabthickness

Drop depth tobe at least0.25(slabthickness)

(Column spacing)/6minimum

Column

Dimension lumberPlywood form

Slab

Droppanel

FIGURE 23.17 Typical details of a flat slab and minimum code requirements for drop panel dimensions.

FIGURE 23.16 A flat-plate slab in an office building under construction. Observe the use of perimeter beams, which may be required with heavy exterior wall cladding. Flat-plate slabs without perimeter beams are common. Columns may be rectangular, square, or round.

Preliminary Depths of Flat-Plate and Flat-Slab Floors

Estimate the depth of a flat-plate floor by dividing the longer clear distance between columns by 30. Thus, if the distance between columns is 20 ft, use an 8-in.-thick slab.

For a flat-slab floor, esti-mate the thickness of the slab by dividing the longer clear distance between columns by 35. Thus, if the distance between columns is 24 ft, use an 8-in.-thick slab.


NOTE



539

Structurally, the drop panel must extend a minimum of one-sixth of the slab span in each direction, and its drop below the slab must at least be 25% of the slab thickness, Figure 23.17 (b). For formwork economy, the drop depth is also based on lumber dimensions, Figure 23.17 (c). With round columns, however, manufacturers supply col-umn forms that have built-in drop panels and column capitals, Figure 23.18 .

A flat slab is generally used where the live loads are rela-tively high, such as in parking garages or storage or indus-trial facilities.

23.4 POSTTENSIONED ELEVATED CONCRETE FLOORS

The reinforced-concrete elevated-slab systems described previously can also be posttensioned. The posttensioning of slabs reduces slab and beam dimensions, reducing the dead load of the floor. This is particularly helpful in seismic areas, where a lower dead load imparts a lower seismic load on the structure. Smaller slab and beam dimensions also result in a lower floor-to-floor height, reducing the cost of exterior cladding. Column and foundation sizes are also reduced.

Because building codes place a limit on the maximum amount of prestress that can be introduced, an elevated, posttensioned concrete slab has a significant amount of conven-tional reinforcement as well, Figure 23.19 . However, the amount of conventional rein-forcement in a posttensioned slab is lower than that required if the slab were not posttensioned, reducing reinforcement congestion in long-span and/or heavily loaded members.

Generally, the prestressing (posttensioning) tendons in a slab are distributed in a banded arrangement rather than being individually distributed throughout the span in a basket-weave arrangement . Generally, the tendons are grouped together along the support lines in one direction, and bands of tendons are placed at equal spacings in the transverse direction, Figure 23.20 .

Posttensioned (PT) concrete floors are being increasingly favored for all types of structures—parking garages, high-rise office buildings, hospitals, and condominiums. Figure 23.21 shows a one-way joist floor with PT supporting beams. Posttensioning reduces the depth of beams, which may be necessary in some situations to equalize the depth of the beams with those of the joists. The process of posttensioning in an elevated structure is similar to that of a PT slab-on-ground, covered in Chapter 22 .

Drop panelformwork

FIGURE 23.18 A flat slab with (round) drop panels and column capitals under construction.

Steel frame

Stirrup

Posttensioning tendons

Reinforcing bars

FIGURE 23.19 A cage for a con-crete beam consisting of conven-tional reinforcement and posttensioning tendons (red color) ready to be lowered into the beam form. Observe the drape (generally parabolic) provided in posttension-ing tendons to balance the stresses created by gravity loads on the beam. The steel frames shown are used to make the cage. They will be removed after the cage has been placed in the form. Reinforcement for the slab is yet to be completed. Figure 23.20 shows the same beam after its reinforcement has been low-ered and the reinforcement and post-tensioning tendons for the slab are in place.

Two Most Commonly Used Concrete Floor Systems

Although various site-cast concrete floor systems have been described in this text, the two most economical floor systems are the flat-plate floor and the one-way joist floor . For typical live loads of 50 psf, the one-way flat-plate floor is economical for spans of up to approximately 30 ft. For spans between 30 ft and 50 ft, a one-way joist floor is economical [23.1].

NOTE

540


Band consisting ofseveral posttensioningtendons

Band consisting of several posttensioningtendons

Band consisting ofseveral posttensioningtendons

FIGURE 23.20 Layout of reinforce-ment and posttensioning tendons (red color) in the slab of a building. The reinforcement and posttension-ing tendons for the beam are shown in Figure 23.19 . Note the banded arrangement of tendons in the slab.

23.5 INTRODUCTION TO PRECAST CONCRETE

As stated in the introduction to Chapter 22 , precast concrete members (except concrete tilt-up walls) are fabricated in a precast plant and transported to the construction site for assembly. Precasting, which is generally done in covered or sheltered spaces, is particularly helpful in climates that limit the use of site-cast concrete.

Additionally, because precasting is done at the ground level, the cost of formwork and shoring is considerably reduced. Formwork cost reduction is also achieved through the use of standard-size elements cast in permanent forms, which are reused several times more

FIGURE 23.21 A wide-module, one-way joist floor with postten-sioned (PT) supporting beams. Note the use of (conventional) reinforce-ment in the slabs and joists. The beams have both conventional rein-forcement and posttensioning ten-dons (red color). PT beams are commonly used in a one-way joist floor to reduce beam size so that joists and beams have the same depth.



541

than the formwork used for site-cast members. Precasting also allows greater quality control over the strength of concrete and surface finishes. Most surface finishes are more easily obtained in a precast plant than at the site—often several floors above ground.

Precast elements used as horizontal framing members are generally prestressed (i.e., pre-tensioned). To ensure rapid reuse of forms and prestressing equipment, Type III (high-early-strength) concrete is generally used with a relatively higher-strength concrete (5,000 to 6,000 psi). To accelerate the setting and hardening of concrete, members are steam cured. Consequently, most precast concrete members are made on a 24-h cycle.

Precast concrete also has many disadvantages. Its main disadvantage is the cost of trans-portation. Although precast members are generally lighter than corresponding site-cast members (because of prestressing), they are still fairly heavy. Transportation also limits the length and width of precast members.

Another disadvantage of precasting is the need for heavier hoisting equipment at the con-struction site and additional safety measures that must be observed during erection. Erection and assembly at the site also introduce the need for a more skilled workforce compared with site-cast concrete construction. Architecturally, the most limiting factor in the use of precast concrete is the difficulty in sculpting concrete at a large scale, which is more easily realized with site-cast concrete. Precast elements are generally straight, with standard profiles.

ARCHITECTURAL PRECAST AND STRUCTURAL PRECAST CONCRETE Precast concrete members are classified as (a) architectural precast concrete and (b) structural pre-cast concrete . Architectural precast refers to concrete elements that are used as nonstructural clad-ding elements. Their most common use is in precast concrete curtain walls (see Chapter 28 ).

Structural precast concrete, which is covered in this chapter, includes all elements of a building’s structural frame (floor/roof slabs, columns, and walls). Although an entire room or assembly of rooms can be precast, most structural precast concrete is used in standard elements that are assembled on site to form spaces.

STRUCTURAL PRECAST—TOTAL PRECAST AND MIXED PRECAST CONSTRUCTION A building can be constructed of all precast concrete members in which all structural com-ponents—columns, load-bearing walls, and floor and roof slabs—are of precast concrete. This system is referred to as total precast concrete construction . In mixed precast construc-tion, some elements of the building consist of precast concrete members, while the others are of cast-in-place concrete, steel, or masonry.

Mixed precast construction is far more common than total precast. It combines the ben-efits of both precast and conventional construction. In mixed precast, precast concrete is used only in floor and roof slabs. Using precast concrete floor/roof slabs yields significant savings because a large percentage of the cost of materials and formwork in a concrete struc-ture is embedded in floor and roof slabs (see Section 23.1).


13. A flat slab floor consists of a. a slab of constant thickness. b. a slab with beams on all four sides of the slab panel. c. a slab with drop panels at each column. d. none of the above.

14. A flat plate floor consists of a. a slab of constant thickness. b. a slab with beams on all four sides of the slab panel. c. a slab with drop panels at each column. d. none of the above.

15. Commonly used beamless concrete floors are a. one-way and two-way joist floors. b. flat-plate and flat-slab floors. c. beam-and-girder floors. d. all of the above.

16. In a posttensioned elevated concrete floor, prestressing tendons are combined with conventional steel reinforcement. a. True b. False

17. The primary reason for using posttensioned concrete floors is to a. reduce or prevent the cracking of concrete. b. improve safety against failure. c. reduce floor depth. d. reduce or prevent corrosion of reinforcement. e. reduce formwork cost.

18. The strength of concrete used in precast concrete members is generally a. 1 to 2 ksi. b. 2 to 3 ksi. c. 3 to 4 ksi. d. 4 to 5 ksi. e. 5 to 6 ksi.

19. Portland cement used in precast concrete members is generally a. Type V. b. Type IV. c. Type III. d. Type II. e. Type I.

20. Architectural precast concrete refers to concrete members used in building interiors. a. True b. False

PRACTICE QUIZ

542


23.6 STRUCTURAL PRECAST CONCRETE MEMBERS

Structural precast concrete members may be classified as

• Horizontal-spanning elements: These consist of (a) hollow-core slabs, (b) solid planks, (c) double-tee units, and (d) inverted-tee beams.

• Vertical-spanning members: These consist of (a) columns and (b) walls.

Horizontal-spanning elements are almost always prestressed, and vertical spanning ele-ments are generally not prestressed. A brief description of horizontal-spanning members and their use in mixed precast construction follows.

HOLLOW-CORE SLABS AND SOLID PLANKS Hollow-core slabs are precast, prestressed concrete slabs that contain voids in their central region. The voids reduce the dead load of the slab by 40% to 50% compared with a site-cast concrete slab for the same span.

Hollow-core slabs are produced in thicknesses of 4 in., 6 in., 8 in., 10 in., 12 in., and 14 in., with the 8-in.-thick slab being most commonly used. The width of the slabs is generally 4 ft. Narrow-width slabs are produced in the plant by cutting standard 4-ft slabs along their length to fit a space that is not a multiple of 4 ft. In addition to 4-ft-wide slabs, some plants make slabs that are 3 ft 4 in. and 8 ft wide.

Figure 23.22 shows a typical section through an 8-in.-thick, 4-ft-wide hollow-core slab. The number of voids and their cross-sectional profiles are manufacturer specific. Manufac-turers provide the load capacity of their slabs, along with suggested details for connecting the slabs with supporting members.

Hollow-core slabs are generally used in mixed precast buildings, where the supporting structure may be of steel, masonry or site-cast concrete. Figure 23.23 shows the use of hollow-core slabs in a steel-frame building. Their use with a site-cast concrete frame is

Approximate Spanning Capability of Hollow-Core Slabs

To determine the approximate thickness of a hollow-core slab floor, divide the span by 40. Thus, if the distance between supporting members is 25 ft, the thickness of a hollow-core slab is (25 * 12)/40 = 7.5 in. Hence, use an 8-in. slab. A 12-in. slab has an approximate spanning capability of 40 ft.

Due to lighter loads on roofs, the slab thickness for roofs may be determined by dividing the span by 50.


NOTE

Keyway between slabs filled withportland cement-sand grout

48 in.

40-in.- and 96-in.-wide slabs are also available

Prestressing strand. Number of strands and their locationmay vary from manufacturer to manufacturer

8 in. most commonly used thickness 4-in., 6-in., 10-in., 12- in. and 14-in.-thick slabs are alsoproduced

FIGURE 23.22 A typical section through a 4-ft-wide, 8-in.-thick hollow-core slab. The dimensions of voids, their number, and their profile are manufacturer specific.

Hollow-coreslab

FIGURE 23.23 Mixed precast concrete construction consisting of a steel-frame structure with hollow-core slabs.

543

shown in Chapter 12 ( Figure 12.34 ); Chapter 26 (Sections 26.5 and 26.6) shows their use in load-bearing masonry structures.

Connections between hollow-core slabs and supporting members are made using site-cast concrete fill and reinforcing steel (see Sections 26.5 and 26.6). In addition to the concrete fill used for connections, a site-cast concrete topping is generally used over the slabs. The topping provides structural integration of slab units and increases the floor’s fire resistance and sound insulation. It also functions as a leveling bed, particularly with units with uneven camber. In many buildings, however, topping is omitted for the sake of economy. Topping, when used, is generally 2 in. to 21

2 in. thick and reinforced with welded wire reinforcement (WWR). Hollow-core slabs are produced through a long-line extrusion process. The machine, housed

in a precasting enclosure, lays concrete on a casting bed over prestressing tendons, which have already been pretensioned to the required level of stress. It forms continuous voids as it travels along the length of the bed. The concrete used is high-strength, zero-slump concrete.

After casting, the slab is steam cured and cut to customer-specified lengths. The cut lengths of slabs are removed from the bed the morning after casting, and the process just described is repeated on a 24-h basis.

Solid precast concrete planks are similar to hollow-core planks and are generally used where superimposed loads are relatively light.

DOUBLE-TEE UNITS A typical section through a double-tee unit is shown in Figure 23.24 . The width of units is generally 8 ft, with a center-to-center distance of 4 ft between beams. Double-tee units with widths of 9 ft, 10 ft, and 12 ft are also available from some manufacturers. Overall depths of double-tee units vary from 16 in. to 36 in. to give different spanning capabilities.

Double-tee units are used where the spans are large and cannot be provided economi-cally with site-cast concrete construction or hollow-core slabs. They are commonly used for hotel and bank lobbies. As with hollow-core slabs, a topping of concrete and WWR may be used on double-tees for structural integration and leveling.

Another common use of double-tees is in multistory parking garages, where a minimum distance of 60 ft between columns is generally necessary. In garages, double-tees are gener-ally supported on site-cast or precast inverted T-beams. Double-tee units are cast on a long line of steel formwork. Several units are made in one casting operation by dividing the bed with separators.

INVERTED-TEE BEAMS Precast inverted-tee beams are generally used as supporting members for hollow-core slabs or double-tee units. Like hollow-core slabs and double-tee units, they are prestressed. They are commonly used in total precast construction (see Figure 23.28 ).

23.7 TOTAL PRECAST CONCRETE CONSTRUCTION

Figure 23.25 shows a total precast building under construction, whose structural frame comprises double-tee units, inverted-tee beams, columns, and walls. The construction process of a total precast building has much in common with structural steel construction; that is, members are brought into position using a crane, as seen in Figure 23.26 , and are connected together either through welds or bolts. Steel embeds are included in precast members to facilitate member bolting and/or welding.

Figure 23.27(a) shows the top of a lower-floor column with threaded bolts projecting out, and Figure 23.27(b) shows the bottom of an upper-floor column with an embedded

Approximate Spanning Capability of Double-Tee Units

To determine the approximate depth of a double-tee unit floor, divide the span by 28. Thus, if the span of units is 60 ft, the approximate depth of the floor is (60 * 12)/28 = 26 in.

Standard available depths of double-tee units are gener-ally 14 in., 18 in., 24 in., and 30 in. The manufacturer should be consulted for more precise information.


NOTE

Prestressing strands,which are profiled uptoward the ends

W = 8 ft (most common); 9-ft, 10-ft and 12-ft-wide

units available

0.5 W

FIGURE 23.24 Typical section through a double-tee floor.

Single-Tee Units

Single-tee units were used for-merly (generally for long spans), but they are not com-monly used today because of erection instability problems.

NOTE

544

Threadedbolt

Lifting hook, which willbe cut after the columnis in position

Steel base platewith four holes

Block-out(pocket)

(a) (b)

FIGURE 23.27 (a) Top of the lower-floor column with threaded bolts. (b) Bottom of the upper-floor column (two stories high). The block-outs (pockets) in the columns allow a bolted connection between the lower- and upper-floor columns.

Set-back

FIGURE 23.26 A double-tee floor unit being flown into position in the total precast building in Figure 23.25 . The setbacks in double-tee stems reduces floor height.


FIGURE 23.25 A total precast concrete parking garage under construction.



545

base plate containing holes to engage the bolts. The block-outs in the column above the holes are filled with concrete after the connection has been made. Figure 23.28 shows a close-up of the connection between the lower- and upper-floor columns.

Walls in a total precast building carry gravity loads and also function as shear walls. Where needed, the walls are provided with projecting brackets to receive inverted-tee beams, Figure 23.29(a) or double-tee units, Figure 23.29(b).

23.8 FIRE RESISTANCE OF CONCRETE MEMBERS

The fire resistance of a concrete member is a function of three variables:

• Thickness of the member • Type of coarse aggregate • Cover over reinforcement and prestressing tendons

Block-out

FIGURE 23.28 Close-up of the bolted connection between the lower- and upper-floor columns. The block-outs (pockets) will be filled with concrete and finished smooth to match the column surface.

(a) (b)

Projecting bracket tosupport inverted-tee beam

Inverted-teebeam Double-tee

floor unit

Projection in wall tosupport double-tee floor

FIGURE 23.29 Projections in precast concrete walls to support (a) inverted-tee beams and (b) double-tee floor slabs.

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A thicker member has greater fire resistance. For the same member thickness, a light-weight aggregate gives greater fire resistance than a normal-weight aggregate. Among the normal-weight aggregates, carbonate aggregate (i.e., limestone) gives greater fire resistance than noncarbonate (i.e., siliceous) aggregates.

Local building codes prescribe the minimum thickness of members to achieve a given fire resistance, Table 23.1 . In addition to meeting minimum thickness requirements, the member must satisfy minimum cover requirements. The minimum cover required for cor-rosion protection ( Table 19.1 ) is adequate to satisfy the fire-resistance requirements in most situations. However, the local building code must be referenced for precise information.

TABLE 23.1 FIRE RESISTANCE OF CONCRETE MEMBERS

Fire resistance Member Aggregate type 1 h 2 h 3 h

Minimum thickness (in.) of floor slabs or walls

Lightweight

Carbonate

Siliceous

2.5

3.2

3.5

3.6

4.6

5.0

4.4

5.7

6.2

Minimum thickness (in.) of columns Lightweight

Carbonate

Siliceous

8

8

8

9

10

10

10.5

11

12


21. In mixed precast construction, precast concrete members are generally used for a. shear walls. b. load-bearing walls. c. columns. d. floors. e. all of the above.

22. In total precast construction, precast concrete members are generally used for a. shear walls. b. load-bearing walls. c. columns. d. floors. e. all of the above.

23. A 12-in.-thick hollow-core floor slab will generally span up to approximately a. 40 ft. b. 30 ft. c. 25 ft. d. 20 ft. e. none of the above.

24. When a concrete topping is used over hollow-core slabs, its thickness is approximately a. 1 in. b. 11

2 in. c. 2 in. d. 3 in. e. 4 in.

25. The purpose of concrete topping on hollow-core slabs is to a. structurally integrate the slabs. b. level the top of slabs. c. increase the sound insulation of the floor. d. increase the fire rating of the floor. e. all of the above.

26. The most commonly used width of a double-tee floor unit is a. 8 ft. b. 6 ft. c. 4 ft. d. none of the above.

27. Double-tee floors are commonly used a. in residential occupancies. b. where spans lie between 30 ft and 40 ft. c. where spans lie between 40 ft and 50 ft. d. where spans are in excess of 50 ft.

28. The fire-resistance rating of concrete members is a function of a. member thickness. b. type of coarse aggregate in concrete. c. concrete cover for corrosion protection. d. all of the above.

PRACTICE QUIZ

As discussed in Chapter 9 , continuous building separation joints are required in many buildings. A preferred method is to use two columns and two beams at the joint. Often for the sake of economy, however, a single column with two beams is used.

The following text explains the construction of such a joint in a site-cast, reinforced-concrete building. (See Chapter 19 for similar detail in a steel-frame building.)

EXPAND YOUR KNOWLEDGE Building a Separation Joint in a Site-Cast Concrete Building

547

P

Separation joint

Beam Beam

ColumnColumnColumn

FIGURE 1 Section through a concrete floor at a building separation joint using one column at the joint (also see Chapter 9 ).

Slip-jointassembly; seeFigures 4 and 5

Separationjoint

ColumnColumn bracket

FIGURE 3 Three-dimensional view of Detail P.

Separation joint, approximately2 in. wide (see Chapter 9)

inSlip-joi tblyassemb

FIGURE 2 Detail P.

Slip joint made h with two steel bearing plates laid overeach other with no nchorage an between them. Each steelplate has a epoxied factory-e Teflon sheet; see Figure 5.

The entire assembly is installed in position at the timeof concrete ent. placeme Both plates are taped togetherbefore being p ed lace in position to prevent their

oncreting.movement during co

FIGURE 4 Detail of the slip joint at P. (Continued)

EXPAND YOUR KNOWLEDGE (Continued)

548

Coefficient of Friction Assume that two rectangular blocks are laid, one over the other. A weight, P, is placed on the upper block, which is then made to slide over the lower block with the help of a horizontal force, F. If F is the horizontal force that is just able to slide the upper block, then the coefficient of friction , f, between the surfaces of the blocks is given by

f =FP

If the force required to slide is small, f is small. The value of f between two Teflon sheets is nearly 0.05 (as reported by a sep-aration joint assembly manufacturer, Conserve Inc., Georgetown, South Carolina).

EXPAND YOUR KNOWLEDGE (Continued)

Building a Separation Joint in a Site-Cast Concrete Building (Continued )

Each steel bearing plate is factory epoxied to a Teflon sheet.Teflon sheet is used because of its very low coefficient offriction which allows movement between the column bracket and the concrete beam above.

Although the plates have been shown separated from eachother, they are taped together before being placed inposition. The upper plate is slightly larger than the lowerplate to provide protection to the Teflon sheet and to allow thespace between the plates to be filled with an elastomeric sealant(also see Figure 19.22).

Steel bearing plate

n sheet factoryTefloned to steel plateepoxie

Steel studs for embedmentin column bracket

Steel bearing plate—Teflonsheet factory epoxied tobottom of plate

Steel studs forembedment in beamembedm

FIGURE 5 Bearing plates in a slip-joint assembly.

P

F

1. Explain the difference between one-way slab and two-way slab actions.

2. Using sketches and notes, explain beam-and-girder floor systems.

3. Using sketches and notes, illustrate the two most commonly used reinforced-concrete floor systems.

4. Using sketches, explain the commonly used beamless concrete floor systems.

5. Sketch the following precast concrete structural members: a. Hollow-core units b. Solid planks c. Double-tee units d. Inverted-tee beams

6. Explain the difference between total precast and a mixed precast construction.

REVIEW QUESTIONS

Documents

Chapter 23 Concrete Construction-II (Site-Cast and Precast