CHAPTER 8: BRICK MASONRY

A. Answers to Questions in the Text

1. There are 3 syllables in "Masonry." It should never be pronounced "masonry."

2. The most common types of masonry units are brick, concrete masonry units, and stone.

3. The soft mud process molds bricks by forcing a relatively moist clay into rectangular molds. The dry-press process uses a high-pressure machine to force a relatively dry clay into molds. The stiff mud process, the most widely used of the three, extrudes a rectangular column of clay from a die and cuts it into individual bricks with a wire.

4. Mortar serves as an adhesive, holding masonry units together; as a cushion to create full bearing between units; as a sealant to keep weather from penetrating between units; and as a part of the finished surface of the wall.

5. The most common type of mortar is made up of fine sand, which gives bulk and strength to the mortar; Portland cement, a binder, which gives strength and weather resistance; hydrated lime, which imparts workability, and water, which activates the cement and lime and also imparts workability.

6. Mortar joints are tooled to give a neat appearance of the desired type, and to compress the face of the mortar and make it more weathertight.

7. A structural bond ties two or more wythes of masonry into a single wall.

B. Additional Questions

1. Sketch an elevation view of a portion of a brick wall laid in running (or common, English, Flemish) bond. (Answer: see Figures 8.18 and 8.19)

2. What are the dimensions of a Roman brick that is made to utilize 3/8" mortar joints? (Answer: 3-5/8" by 11-5/8" by 1-5/8")

3. What is a frogged brick? (Answer: It has a depression in one broad side of the brick)

4. What are some common uses for custom brick shapes? (Answer: Water tables, sills, jabs, copings, angles, radials, arches, stair nosings)

5. What grade of bricks should be chosen for use in the Great Lakes area? (Answer: SW)

6. Why does a mason lay leads rather than just laying bricks from one corner of the building to the other? (Answer: Leads establish plumb, straight, properly aligned corners and set the course of levels for the wall. If leads were not laid, it would be very difficult to produce a flat, plumb, accurate, level, well-crafted wall.)

C. True-False Questions

1. Quicklime is mixed with Portland cement, sand, and water to make mortar. (F)

2. Masonry units should always be wetted before laying. (F, under most conditions units should be laid without wetting.)

3. Bricks shrink considerably during drying and firing. (T)

4. There is no truly standard size. (T)

5. Mortar color is of little consequence to the finished appearance of a brick wall. (F)

6. Flemish bond is purely decorative. (F)

7. It is possible to build a brick dome without using formwork. (T)

8. It is almost impossible to build brick walls that contain steel reinforcing bars. (F)

D. Multiple Choice Questions

1. Portland cement mortar that is growing hard before use may be retempered with water and used:

a. within 30 minutes of its mixing

b. within one hour of its mixing

c. within 90 minutes of its mixing

d. within a half day of its mixing

e. within a day of its mixing

(Answer: c)

2. The most common mortar joint thickness is:

a. 1/8" (3.2mm)

b. 1/4" (6.4mm)

c. 3/8" (9.5mm)

d. 1/2" (12.7mm)

e. 5/8" (15.9mm)

(Answer: c)

3. Which of the following mortar joint profiles are appropriate to use outdoors in a severe climate:

a. weathered

b. concave

c. vee

d. flush

e. raked

f. stripped

g. struck

(Answer: b,c)

4. The most common type of large-scale reinforced brick masonry wall is characterized by:

a. Two wythes separated by a continuous cavity

b. Vertical reinforcing bars grouted into hollow bricks

c. Low-lift or high-lift grouting of a continuous cavity that contains the reinforcing bars

d. Welded wire fabric reinforcing

e. Special bricks that contain steel reinforcing

(Answer: a,c)

5. In English bond brickwork:

a. Every 6th course consists entirely of headers

b. Each course consists of alternating headers and stretchers

c. Every 4th course consists entirely of headers

d. Every 2nd course consists of alternating headers and stretchers

e. Every 2nd course consists entirely of headers

f. The head joints are always staggered from one course to the next

(Answer: e and f)

 

CHAPTER 9: STONE AND CONCRETE MASONRY

A. Answers to Questions in the Text

1. The main classifications of building stone are igneous, sedimentary, and metamorphic. Limestone is a soft, even-grained, sedimentary stone that cannot be polished. Marble is a metamorphic stone that was once limestone; it is somewhat harder, usually has pronounced vein patterns, and can be polished to a mirror sheen. Granite is a very hard igneous stone that can also be polished.

2. The quarry block is sliced into slabs using a gang saw. The faces of the slab are ground flat, then polished smooth in a grinding and polishing machine. Finally, the edges of the slab are squared by a circular saw.

3. Stone is laid differently than brick in that it is sometimes irregular and each piece must be chosen and trimmed to fit; stones are often too heavy to lift by hand and must be lifted with lewises and hoists; mortar joints in fine stonework are raked out and later pointed with non-staining mortar; and acids are not used to clean many types of stonework.

4. Concrete masonry units are the least expensive of any masonry units in North America, and they are very large, replacing twelve modular bricks, which results in much faster wall construction.

5. 29' - 3 5/8"

6. Vertical reinforcing bars are inserted into the hollow cores of the CMUs and the cores are grouted solid. Horizontal reinforcing is usually in the form of heavy wire joint reinforcing assemblies that are laid into the bed joints of mortar. Bond beam blocks may be used to allow ordinary reinforcing bars to be placed horizontally as well as vertically.

B. Additional Questions

1. Distinguish rubble tone from ashlar stone. (Answer: Rubble stones are irregular in shape. Ashlar is squared.)

2. Which can safely be sliced into the thinnest sheets, granite, limestone, marble, or sandstone? (Answer: granite)

3. Why is marble or limestone stonework never washed with acid, as brickwork is? (Answer: Marble and limestone react with acid and are consumed by it.)

4. Why are Type I, moisture-controlled concrete masonry units required in some types of construction? (Answer: They shrink less after installation than Type II units.)

5. How is the curing of concrete masonry units accelerated after they are molded? (Answer: They are steam-cured in an autoclave.)

C. True-False Questions

1. Many of the most highly prized marbles fall into the lowest grade category for that stone. (T)

2. Rubble stone cannot be coursed in a wall. (F)

3. There are no artisans left who have the skill and knowledge to carve ornamental stonework. (F)

4. A so-called "standard" building stone is 15 5/8" high, 23 5/8" long, and 3 5/8" deep. (F)

5. Limestone is easily cut just after it is quarried, but becomes harder with age. (T)

6. Ashlar stone is very irregular. (F)

7. Marble work should be cleaned with muriatic acid after the mortar has hardened sufficiently. (F)

8. There is no truly standard size of concrete block. (F)

9. Leads are not constructed for concrete masonry walls. (F)

D. Multiple Choice Questions

1.Type I (moisture-controlled) concrete masonry units should be specified:

a. Where appearance is important

b. In damp weather

c. For underground work

d. In severe climates

e. Where shrinkage of a wall must be controlled

(Answer: e)

2.One standard CMU occupies the same volume as:

a. Two modular bricks

b. Four modular bricks

c. Six modular bricks

d. Eight modular bricks

e. Ten modular bricks

f. Twelve modular bricks

(Answer: f)

3. Typical machines for working building stone include:

a. Reciprocating gang saws

b. Circular Saws

c. Lathes

D. Hand-held pneumatic chisels

e. Grinding and polishing wheels

f. Planers

(Answer: All of the above)

4. Which of the following types of masonry units are still in production in the U.S.?

a. Glass Blocks

b. Terra-cotta

c. Autoclaved cellular concrete

d. Hollow gypsum tiles

e. Unglazed hollow clay tiles

(Answer: a,b,c)

 

CHAPTER 10: MASONRY LOADBEARING WALL CONSTRUCTION

A. Answers to Questions in the Text

1. A cavity wall prevents penetration of water through a masonry wall by putting a continuous vertical airspace between the outer and inner wythes of masonry. When water reaches the cavity, it has no place to go but down. At the bottom of the cavity, the water is caught by a flashing and drained out through weep holes. The most critical aspects of cavity wall construction are keeping a clean cavity that is free of mortar droppings, providing weep holes at frequent intervals, and providing proper flashings.

2. External flashings should be installed at intersections of masonry walls with roofs or decks, to prevent penetration of water. Internal flashings should be installed wherever water might penetrate or accumulate inside a wall, such as over lintels and shelf angles, under copings and sills, at intermediate floors that bear on the wall, and at the base of the wall. The function of internal flashings is to catch and drain to the outdoors any water that penetrates the wall.

3. Weeping holes should be provided just above each internal flashing, in the outside wythe of the wall. Their function is to drain water that accumulates over the flashing. They may be constructed by leaving out the mortar in head joints, laying short pieces of rope in mortar joints and then pulling them out, or by inserting plastic or metal tubes in the mortar.

4. Ordinary construction uses floors, roof, and interior walls of balloon frame construction, while Mill construction uses heavy timber framing for internal construction. The heavy timbers and decking used in Mill construction are slow to catch fire. Ordinary construction is usually protected with plaster or gypsum board over the wood components, and by firestopping between floors. Wood members that bear on the masonry walls in either system are firecut to prevent them from toppling the wall should they burn through in a fire.

5. Concrete masonry walls require control joints to prevent random cracking of the masonry when it dries and shrinks. Brick masonry walls require expansion joints to relieve stresses caused by expansion of the bricks as they absorb water. Joints should be located at points of weakness in the wall: at window and door openings, at changes of direction, at changes of height. In long, unbroken expanses of wall, joints should be located at intervals recommended by relevant trade organizations.

6. Masonry walls may be insulated on the exterior face, inside the wall construction, or on the interior face. Insulation on the exterior face is achieved by using an exterior insulation and finish system (EIFS), which covers the masonry on the outside with plastic foam insulation and a thin layer of synthetic stucco. Insulation inside the wall may take the form of foam insulation boards in the cavity, or insulating materials (molded plastic foam or loose fill) in the cores of concrete masonry units. Insulation on the interior face of the wall is installed between wood or metal furring strips, and may be plastic foam or fibrous batts.

7. Balloon framing minimizes the vertical shrinkage of the wood framing, which minimizes the tilting of floor and roof surfaces that might otherwise result.

8. When masonry work is carried out in subfreezing weather, the work areas should be sheltered from wind by temporary enclosures. These enclosures may need to be heated in very severe weather. The masonry units, sand, and water should be heated as required. The goal of all these measures is to prevent the temperature of the masonry from falling below freezing until the mortar is fully cured.

B. Additional Questions

1. What are the chief advantage and disadvantage of adjustable masonry ties? (Answer: Adjustable ties make it easy for the mason to tie courses whose bed joints do not line up. But they may be insufficiently rigid to tie the wythes firmly together.)

2. Why is it wise to spend a little extra to use proven materials for flashings in masonry walls? (Answer: the additional cost is very small as a percentage of overall construction cost, and the potential cost of replacing defective or deteriorated flashings in a masonry wall is extremely high.)

3. Where should building separation joints be located? (Answer: At discontinuities in the massing of the building, and at intervals of 150' to 200' (40 to 60m).)

4. What is the chief reason to build a masonry cavity wall? (Answer: A cavity wall is much more resistant to the passage of water than a solid wall. Additionally, a cavity wall may be thermally insulated by adding slabs of plastic foam insulation within the cavity.)

5. Sketch a detail section of the base of a cavity wall with a brick facing and an 8" concrete block backup. Label all parts.

C. True-False Questions

1. Bricks tend to expand after installation, whereas concrete masonry units tend to shrink. (T)

2. It is important to make all reinforcing bars and joint reinforcing fully continuous through expansion joints and control joints in order to maintain the structural integrity of the wall. (F)

3. Efflorescence on brickwork usually indicates serious problems in the wall. (F)

4. Chemical antifreeze admixtures should be used in mortar when outdoor temperatures are below freezing. (F)

5. Brick masonry has no reliable tensile strength. (T)

D. Multiple Choice Questions

1. The following spanning systems may be supported on masonry bearing walls:

a. Wood balloon framing

b. Wood platform framing

c. Heavy timber framing

d. Structural steel framing

e. Sitecast concrete framing

f. Precast concrete framing

(Answer: a,c,d,e,f)

2. The most common form of construction in American downtowns in the 19th century was:

a. Structural steel frame

b. Cast iron frame

c. Mill construction

d. Sitecast concrete frame

e. "Ordinary" construction

(Answer: e)

3. Where an internal flashing meets the exterior surface of a masonry wall:

a. The flashing should be cut back to be concealed just inside the face of the mortar joint.

b. The flashing should project at least 3/4" (19mm) outside the face of the wall.

c. The flashing should project at least 1" (25 mm) outside the face of the wall.

d. The flashing should be turned down to form a drip.

e. If the internal flashing is made of a flexible material, it should be cemented to a sheet metal flashing just before it exits from the wall.

(Answer: b,d,e)

4. End dams are used:

a. To prevent water from leaking in the ends of walls

b. To prevent water from leaking in at the ends of masonry units

c. To keep mortar from dropping into the cavity

d. To keep weep holes clear

e. To keep water from running off the ends of sill and lintel flashings

(Answer: e)

5. Movement joints should be located in masonry walls"

a. at discontinuities where cracks would tend to form

b. every forth course

c. every sixth course

d. every seventh course

e. at a change of brick bond

(Answer: a)

6. Internal flashings should be located in masonry walls:

a. below copings and sills

b. above lintels and shelf angles

c. below lintels and shelf angles

d. at the base of the wall

e. every sixth course

(Answer: a,b,d)

 

CHAPTER 11: STEEL FRAME CONSTRUCTION

Answers to Questions in the Text

1. Steel is any of a range of iron alloys that contain less than 2% carbon, while iron may contain a larger proportion of carbon.

2. Steel structural shapes are produced by passing steel between rollers while it is still hot enough to be formed easily. The different weights and thicknesses of a shape are produced by varying the spacing between the rollers.

3. The fabricator receives steel shapes from the mill and prepares them for specific building frame by cutting them to length, punching or drilling connector holes as required, adding connecting angles and plates, and cambering and priming the components as required. The erector receives these prepared building parts from the fabricator and assembles them on the building site.

4. A W21 X 68 is a wide-flange steel section nominally 21" in depth and weighing 68 pounds per linear foot.

5. In a moment connection, the flanges or a beam or girder are attached to the adjoining member with welds (or, rarely, with bolts) that are as strong as the flanges themselves. One member cannot rotate without rotating the other member. In a shear connection, the welds or bolts are designed to transmit only the vertical load from the beam or girder to the adjoining member, and small rotations are possible between the two members. Moment connections are used for joints that must be made rigid to contribute to the overall rigidity of the building, or to create structural continuity between members. Shear connections are less expensive than moment connections and are used for all other connections.

6. The top flange of a beam is often coped so its top surface can lie in the same plane as the top of a girder to which it is connected. The bottom flange is coped in a knife connection to allow the beam to be inserted between the connecting angles by sliding it in from above.

7. Composite construction allows a steel beam or deck and the concrete fill above to act together as a single structural unit, which allows the use of a lighter steel member than would otherwise be the case.

8. Steel is advantageous in a fire because it is incombustible and does not contribute fuel to the fire as a wood beam does. Steel does, however, lose much of its strength at elevated temperatures, so must be protected by incombustible materials that will insulate it from the high temperatures of a fire.

9. Trusses, dome, arches, cable net structure, space truss

Answers to Exercises

3. A department store is classified as Use Group M, Mercantile. Assuming it has no automatic fire suppression system, we see from the table in Figure 1.1 that the lowest type of construction that might permit a four-story building with 17,500 square feet per floor is Type 2A, but we must correct the allowable area per floor (22,800) with a 10% adjustment for a four-story building as specified by Figure 1.2. This gives 20,520 square feet per floor, which is greater than the 17,500 we wish to build, so Type 2A construction will suffice. Moving to Figure 1.3, we can now determine on line 9 that lower-floor columns must have 2-hour protection, on line 11 that floor beams must be 1 1/2 hours, roof beams (line 12) must be 1 hour, and walls around shafts and stairways (lines 5 and 6) must be of 2-hour construction.

Additional Questions

1. Why are the edges of steel members beveled in some cases before welding? (Answer: A beveled edge allows access for the welding electrode so multiple passes can be made to fill the joint with weld metal to the same thickness as the member being connected).

2. What is the difference between an AISC Type 1 connection and an AISC Type 2 connection? (Answer: The Type 1 is a moment connection, and the Type 2 is a shear connection.)

3. Where is a seated connection likely to be used, and why? (Answer: A seated connection is used to connect a beam or girder to a column web, because it allows better access for pneumatic wrenches in this restricted location than does a framed connection.)

4. In what circumstance is a butt plate used in a column-to- column connection? (Answer: Where the column changes from one nominal size of section to another.)

5. Why are columns spliced at waist level above the floor? (Answer: This avoids conflict with beam-column connections, and is convenient for the ironworkers to reach.)

True-False Questions

1. Most steel beams are I-beam shapes. (F)

2. Open-web steel joists are made by cutting the web of a wide-flange section along a zig-zag path, then rejoining the two halves in a greater depth by welding. (F)

3. Riveting of steel building frames is obsolete. (T)

4. In slip-critical connections, the bolts grip the two steel components so tightly together that the load is transferred between them by friction. (T)

5. Plumbing-up refers to the process of raising sections of steel from the ground to their position in the building. (F)

6. Column baseplates for smaller columns are usually attached to the columns in the fabricator's shop. (T)

7. Plate girders are available from stock in a variety of sizes and spans. (F)

8. Rigid frames are economical for low-cost manufactured steel buildings. (T)

9. Tensile structures made of steel cables can be made rigid against wind uplift. (T)

F. Multiple Choice Questions

1. Steel structural`: shapes are produced by: a. casting b. forging c. die- forming d. rolling e. milling (Answer: d)

2. The width of the flange of a W12 X 14: a. is 12" b. is 14"

c. is the standard 6" dimension used for all W sections d. is up to the mill that produces it e. must be looked up in a handbook (Answer: e)

3. Mild structural steel is designated by the ASTM number

of:

a. A36

b. A242

c. A441

d. A572

e. A588

(Answer: a)

4. The following materials are good for fireproofing

steel

columns:

a. Concrete

b. Brickwork

c. Metal lath and plaster

d. Gypsum board

e. Spray-on fireproofing

f. Mineral fiber slabs

g. Intumescent coatings

(Answer: all of the above)

 

CHAPTER 12: LIGHT GAUGE STEEL FRAME CONSTRUCTION

A. Answers to Questions in the Text

1. Light gauge steel framing members are manufactured by feeding galvanized sheet steel from continuous coils through machines that fold it at room temperature into long members with the characteristic channel or cee shapes.

2. In general, the details of light gauge steel framing are similar to those of wood platform frame construction. However, there are some important differences: Metal members are joined with self-drilling, self-tapping screws or welds rather than nails. Web stiffeners are required to prevent steel joists from crushing where they tr4ansmit vertical loads from studs above to studs or foundation wall below. Steel joists may be nested to form box members when they are doubled. Diagonal bracing is achieved with special steel straps.

3. Foam plastic sheathing or insulating edge spacers on framing members should be used in severe climates to minimize heat flow through metal framing members. It is especially important to do this where framing members such as ceiling joists bridge from the interior space of the building to the outdoors.

4. Where construction must be noncombustible, gypsum boards are used for sheathing, and concrete decks are poured over corrugated steel decking for floors, in both cases replacing wood panel sheathing.

5. A prescriptive code makes it possible for a builder or contractor to determine the required sizes of light gauge steel framing members for a building without having to have the structure designed by an engineer or architect.

6. Wood Light Frame Construction and Light Gauge Steel Frame Construction share most of their disadvantages and advantages. The wood system is superior in its thermal properties, is more familiar to most residential builders, and is still cheaper at the time this is written. Steel framing members are lighter than equivalent wood members, as well as being straighter, unaffected by humidity, and not susceptible to decay or termites. Prices of steel framing components are less volatile than those for wood components. Steel framing is noncombustible, enabling its use in higher construction types than wood framing. Wood framing is not subject to corrosion, and is perhaps easier to join than steel. Unbraced steel members are more apt to buckle than unbraced wood members.

B. Additional Questions

1. What is the function of a web stiffener? (Answer: It prevents crushing of a joist end that must transmit a vertical load from a wall above to a wall below.)

2. How is a light gauge steel frame attached to a foundation? (Answer: L-shaped foundation clips are screwed to the perimeter joists and bolted to the foundation.)

3. What is gypsum sheathing, and why is it used? (Answer: It is a gypsum panel product with a water-resistant facing and core. It is used in place of wood panel sheathing when the sheathing must be noncombustible to comply with code requirements.)

4. Why is a mastic adhesive used between wood subflooring panels and metal joists? (Answer: It cushions the interface between the wood and metal, and it helps keep the floor from squeaking.)

5.What special precautions are taken to keep steel studs from buckling? (Answer: Tall steel studs are braced at 4' (1.2m) maximum intervals with lateral steel straps or channels, sheathing panels and interior gypsum panels, securely screwed to the studs, provide the major mechanism against buckling.

C. True-False Questions

1. Steel framing is noncombustible and does not have to be protected against fire. (F)

2. Steel framing members may be cut with a shear or an abrasive-blades saw. (T)

3. Steel framing is not capable of framing such intricate shapes as wood framing. (F)

4. Finish nails are used to attach wood baseboards and trim to steel framing. (F)

5. Steel framing members are available in more limited range of sizes than wood members. (F)

6. Cold-formed steel framing and light gauge steel framing generally refer to exactly the same system of construction. (T)

7. The term "light gauge" refers to the relative thinness of the material from which the framing members are made. (T)

D. Multiple Choice Questions

1. Cold-formed steel framing members may be used for:

a. Rafters

b. Joists

c. Studs

d. Headers

e. Sole plates and top plates

(Answer: all of the above)

 

2. Light gauge members may be joined with:

a. Nails

b. Screws

c. Welds

d. Special clinching tools

e. Special pneumatically-driven pins

(Answer: b,c,d,e)

 

3. The holes that are punched through steel framing members are for:

a. Electric wiring

b. Ventilation

c. Plumbing

d. Insulation

e. Bracing

f. Vapor control

(Answer: a,c,e)

 

4. Web stiffeners are required at:

a. Midspan of all joists

b. Midheight of all studs

c. Midheight of studs over 10' (3m) tall only

d. Exterior and interior joist bearings

e. Eave overhangs

(Answer: d)

 

5. Light gauge metal floor framing accepts the following subfloor materials:

a. Corrugated steel decking

b. Plywood

c. OSB

d. Gypsum sheathing

e. Particle board

(Answer: a,b,c)

 

 

Chapter 13: Concrete Construction

Answers to Questions in the Text

1. Cement is the gray powder added as a binder to aggregates and water to create concrete.

2.A satisfactory concrete mix is made of fresh cement; clean, well-graded aggregates; and clean water. Admixtures are sometimes added to the mix to regulate its properties or the properties of the finished concrete. The concrete must be thoroughly blended in the correct proportions, with special attention given to avoiding excessive water content. To avoid segregation of its constituents, it must be handled without moving or dropping it excessive distances, and compacted in the forms without excessive agitation.

3. Freshly-poured concrete should be kept moist for as long as possible, at least 7 days and preferably longer. In cold weather it must be kept from freezing during this period of hydration to avoid cessation of the curing reaction. In hot weather, it may be necessary to cool the ingredients in the concrete, or to add ice in place of some of the mixing water, in order to prevent premature curing during placement, and concrete slabs may need to be protected from wind during the pouring and finishing.

4. If concrete has too low a slump, it will be difficult to handle and place in the forms, and will be likely to have voids. If the slump is too high, the concrete is likely to be weak and have poor surface qualities. water-reducing admixtures or air-entraining admixtures can be used to increase the slump without increasing the water content.

5. Steel reinforcing rods are placed in those regions of reinforced concrete components where tensile forces will be experienced, to resist these forces and control cracking of the concrete. The concrete is allowed to do most or all of the work of resisting compressive stresses.

6. Stirrups act to resist the diagonal tension forces that occur near the ends of concrete beams.

7. Column ties prevent the column bars (vertical reinforcing bars) from crackling outward under compressive loads.

8. Shrinkage-temperature steel controls cracking of one-way concrete slabs caused by concrete shrinkage and thermal stresses in the direction normal to the direction of the principal slab reinforcing.

9. Two-way slabs need no shrinkage-temperature reinforcing because they already have reinforcing in both directions.

10. Reinforcing in a beam is not under any stress until the beam is put under load, at which time it assumes its stress and elongates, causing tension and cracking in the surrounding concrete. The concrete on the same side of the neutral axis of the beam as the reinforcing is doing very little structural work. In a prestressed beam, the steel is stressed artificially before the beam is put under load, placing all the concrete in the beam in compression. Under load, the steel is stressed further, and some of the concrete is placed in further compression, but none of the concrete goes into tension, so no cracks occur. The structural efficiency of a prestressed beam is greater than that of a reinforced beam because all the concrete is working. A reinforced beam is easier and more economical to make under many circumstances, however.

11. Posttensioning is used instead of pretensioning primarily in cases where the concrete must be prestressed in place on the jobsite. Here there are no abutments against which to pretension the steel, so the cured concrete must serve to resist the prestressing force created by the posttensioning jacks.

Additional Questions

1. Why is important to have a proper grading of aggregate sizes in concrete? (Answer: In properly graded aggregates, the finer particles serve to fill the voids between the larger particles so the concrete will be dense and without voids that would damage its strength. If fine particles only were used, their very large surface area would required too much cement. If coarse particles only were used, there would be large, unfilled voids between them).

2 You are a peace Corps member, assigned to the emerging Republic of Dismalia, which desperately needs to create a domestic industry to manufacture portland cement. what kinds of natural resources will you be looking for in order to make concrete progress toward establishing this industry? (Answer: Lime-bearing materials such as limestone, marble, marl, or seashells; iron/silica/alumina-bearing materials such as clay or shale; and a source of fuel with which to heat the cement kilns).

3. List briefly the steps usually taken to control the quality of concrete arriving at a construction site. (Answer: Require a certificate from the batch plant showing the composition of each load of concrete; run a slump test to check the amount of water in the concrete; pour test cylinders and cure and test them to check the structural strength of each batch).

4. What is the diameter of a #8 reinforcing bar? (Answer: 1")

5. How much tensile force can be safely carried by a #9 reinforcing bar with an allowable tensile stress of 22,000 pounds per square inch? (Answer: 22,000 pounds. This requires the you to remember the useful fact that a #9 bar has a cross-sectional are of exactly one square inch.)

6. The structural plans call for a #7 reinforcing bar, but none is available. There is room in the formwork to substitute two smaller bars for the #7. what number bars should you use? (Answer: Two #5. This question is a good one for a class of bright students with a good background in mathematics, who will realize that the areas of reinforcing bars are proportional to the squares of their diameters. 7 squared is 49, so to find two bars that are equivalent or better; the square of the number of each bar must be at least 49/2, or 24.5. The square root of 24.5 is just a bit less than 5, so two #5 bars will be almost perfect. One could also construct a question that would require that a student figure out that a #3 bar and a #4 bar together have the same area of steel as a single #5, as Pythagorus would have perceived very readily).

True-False Questions

1. Concrete was first used in the 19th century. (F)

2. Concrete shrinks during curing and drying. (T)

3. Because concrete cures by hydration, it is advisable to have as much water as possible in the mix. (F)

4. Concrete should be allowed to dry as quickly as possible after pouring. (F)

5. Reinforcing bars are usually cut and bent on the jobsite. (F)

6. In most situations, reinforcing bars are spliced by simply overlapping them by a specified amount. (T)

Multiple Choice Question

1. Air is used in concrete to:

a. Create a better surface appearance

b. Make the concrete easier to handle during placement

c. Increase the resistance of the concrete to freeze-thaw damage

d. Strengthen the concrete

e. Help evaporate excess water from the curing concrete (Answer: b,c)

 

2. In a group of concrete samples, the strongest will be the one:

a. Cured underwater for a year

b. Moist-cured for a week, then allowed to dry

c. Moist-cured for 3 days, then allowed to dry

d. Cured in the air for a year (Answer: a)

 

3. Two-way slabs are preferred in cases where:

a. Round columns are used

b. Shrinkage/temperature steel is required

c. Beams can run in only one direction

d. Column spacing are square or nearly square

e. The building will be more than 6 stories high (Answer: d)

 

CHAPTER 14: SITECAST CONCRETE FRAMING SYSTEMS

A. Answers to Questions in the Text

1. Steps in finishing a concrete slab: (1) Striking off or straightedging; (2) Floating; (3) Troweling. The troweling must be delayed until some hours after pouring because if the surface of the concrete is worked excessively while it is still very fresh, water and fines rise to the surface and spoil the finish.

2. Steps in forming and pouring a concrete wall: (1) Erecting and bracing the formwork for one side of the formwork (2) Erecting the reinforcing steel, (3) Inserting form ties and erecting the second side of the formwork, (4) Adding walers and bracing the second side of the formwork, (5) Pouring and vibrating the concrete. Steps 1 and 2 may be reversed.

3. One-way concrete framing systems span in one direction between parallel walls or beams. Two-way systems span in two mutually perpendicular directions, usually without beams, Steel and wood framing systems are one-way systems. Two-way systems are generally more efficient than one-way.

4. One-way systems: One-way solid slab, flexible but limited in span. One-way concrete joist system: Allows longer spans. Wide-module or skip joist system: Allows longer spans and uses less concrete than one-way joist systems. Slab bands or joist bands may be used with these systems; they serve to reduce the spans of the slabs and to conserve headroom in the building.

Two-way systems: Two-way solid slab: rarely used, but useful where loads are very heavy. Two-way flat slab: good for heavy loadings. Two-way flat plate: good for lighter floor loadings, very flexible in a column locations, conserves headroom in a building. Two-wya concrete joist or waffle system: Good for longer spans.

5. Posttensioning reduces the necessary depth of a structure and helps reduce cracking

B. Additional Questions

1. Specify a slab finish for each of the following situations:

a. A structural slab over which terrazzo will be installed.

b. A sidewalk.

c. An inclined outdoor automobile ramp in a snowy climate.

d. A factory floor.

(Answers: a. Straightedged. b. Wood flat finish. c. Broom finish, d. Steel trowel finish).

2. What does a curing compound do? (Answer: It forms a coating on the surface of a freshly-finished concrete slab that retains the moisture in the slab to encourage full hydration of the concrete).

3. What is the major physical difference between a two-way flat slab and a two-way flat plate? (Answer: The two-way flat plate has no mushroom capital or drop panel).

C. True-False Questions

1. Welded wire fabric reinforcing should be cut completely at control joints in a concrete slab on grade. (T)

2. Floating can be done as soon as the straitedging operation is concluded. (F)

3. The center portions of forms ties remain permanently embedded in a concrete wall. (T)

4. Wood dowels are used to bond a concrete column to its footing. (F)

5. Concrete columns are often cat in paper tubes. (T)

6. The column grid supporting a two-way flat plate can be somewhat irregular in its spacing. (T)

D. Multiple Choice Questions

 

1. Column bars are offset at the top:

a. To bring the column back into verical alignment

b. To overlap the beam reinforcing

c. To overlap the reinforcing for the column in the next story

d. To compensate for curing shrinkage of the concrete

e. To allow for the thickness of the column ties

(Answer: c)

 

2. A one-way concrete joist system can be thought of as a very thick one-way slab with the concrete between the joist left out because:

a. The slab becomes stronger by leaving it out

b. Shear stresses in the slab are reduced in this way

c. It contains no reinforcing

d. This concrete isn't doing very much work structurally anyway

e. The ribbed structure simply looks better

(Answer: d)

 

3 Flying formwork is:

a. A large piece of reusable floor formwork supported on trusses and moved with a crane

b. What happens during pouring when a form is inadequately supported

c. Slabs cast at ground level and raised by jacks

d. Formwork supported on special steel outriggers that resemble wings.

e. Super-lightweight fiberglass pans

(Answer: a)

 

 

Chapter 15: Precast Concrete Framing Systems

Answers to Questions in the Text

1. Precast concrete framing might be chosen in cases where rapid erection, reduced dead weight, all-weather construction capability, quality control of concrete, or slenderness of structure are important considerations. Sitecast framing might be chosen where two-way action is desired, unusual shapes must be spanned, sculptural forms must be created, or highly customized surface textures poured.

2. Steam curing of precast elements develops the full strength of the concrete many times faster than curing at ambient temperature, which allows the precasting plant to reuse its casting beds on a 24-hour pouring cycle. This permits a casting bed to be many times as productive as if ambient-temperature curing were used, leading to lower product costs.

3. Hollow-core slabs may be produced by direct extrusion, by casting the concrete in two layers with dry aggregate introduced as a filler to create the voids (after which it is poured out for reuse), or by using inflatable tubes to form the voids.

Additional Questions

1. What are the standard precast, prestressed slab elements? (Answer: solid flat slab, hollow core slab, double tee, tee or single tee)

2. What is the reason to cast beams in L-shaped or inverted T shapes? (Answer: Slab elements bear upon the ledges provided by these shapes).

3. Why are stirrups usually left projecting from the tops of precast concrete beams? (Answer: The stirrups are cast into the concrete topping at the jobsite to create composite structural action)

True-False Questions

1. Single tees can span farther than double tees. (T)

2. Precast slab elements camber up from the casting bed as soon as the pretensioning force is released in the strands. (T)

3 Precast slabs must be topped with sitecast concrete. (F)

4 precast concrete beams and columns cannot have fire resistance ratings greater than 2 hours. (F)

Multiple-choice Questions

1. Ordinary, non-prestressed reinforcing is used in precast concrete elements for:

a. attaching weld plates

b. top reinforcing over cantilever points

c. shear reinforcement in stems of tees and double tees

d. reinforcement of flanges of tees and double tees

e. reinforcement of wall panels

f. extra reinforcing around openings in precast elements

(Answer: all of the above)

 

2. Continuous bending in a line of precast beams can be created by means of:

a. two-way structural action

b. posttensioning the beam tops at columns

c. composite structural action

d. shrinkage/temperature reinforcing

e. joining beams at points of zero moment (Answer: b, e)

 

3. Which of the following are used in connecting precast elements?

a. bolting

b. riveting

c. welding

d. grouting

e. posttensioning

f. direct bearing

(Answer: a,c,d,e,f)