Building Construction (2nd Edition)

Building Construction (2nd Edition)
Chapter 7-Structural Systems
Test Review

  • Reinforced concrete of protected steel framing are found in Type I fire-resistive buildings.
  • Use of unprotected steel results in a Type II building classification (non-combustible).
  • Wood an masonry together are found in ordinary construction Type II and mill construction Type IV.
  • 3 methods are used to reinforce concrete: ordinary, PRE-tensioned, and POST-tensioned.
  • With ordinary reinforcing, wet concrete is poured around steel bars within a framework.
  • Design engineers determine the number of reinforcing bars, diameter of bars, and depth of concrete around bars.
  • Standard size reinforcing bars vary from 0.375" to 2.257".
  • Vertical steel reinforcing bars are known as stirrups, which resist diagonal tension.
  • Concrete beams are frequently cast in the shape of a tee (more efficient/lightweight).
  • The primary function of reinforcing steel is to resist tensile forces but they can support some compressive forces.
  • With prestressing, a compressive force is applied to the concrete BEFORE a load is applied by tightening or preloading the reinforced steel.
  • Preloading of reinforcing steel creates compressive stresses in concrete to counteract tensile stresses from applied loads.
  • Initial prestressed forces applied to reinforcing bars are slightly higher than what is needed to support the concrete and applied loads.

  • 2 methods of prestressing concrete are PRE-tensioning and POST-tensioning.
  • In PRE-tensioned concrete, steel strands are stretched between anchors creating a tensile force in the steel BEFORE applying concrete.
  • PRE-tensioned concrete usually has a slight upward deflection.
  • With POST-tensioning, reinforcing bars are NOT tensioned until AFTER the concrete is hardened.
  • A general rule is that reinforcing steel should only be cut to rescue trapped victims.
  • Reinforcing bars in POST-tensioned concrete is NOT bonded to the concrete and are likely to spring out of the concrete if cut.
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  • Cast-in-place concrete (AKA site-cast concrete) is poured into forms at the building site.
  • Most cast-in-place concrete is proportioned at a central bulk plant and mixed in mixing trucks enroute to the work site.
  • Power-driven mixers may be used on work sites for small cast-in-place jobs.
  • When cast-in-place concrete arrives at the work site, a slump test is performed to check the quality of the concrete.
  • A slump test checks moisture content by measuring the amount that a small cone shaped concrete settles after it is removed from the standard test mold.
  • Concrete may be subjected to compression testing to determine quality (requires hardening).
  • Reinforcing steel overlaps construction joints which occur between successive pourings of cast-in-place concrete.
  • Common cast-in-place structural systems include flat slab, slab & beam, and waffle construction.
  • Flat slab concrete frames are simple systems consisting of a slab supported by concrete columns.
  • Flat slab concrete varies from 6 to 12 inches thick.
  • Shear stresses exist where columns intersect with flat concrete slabs.
  • In flat slab concrete frame buildings with heavy live loads, supporting columns are reinforced with additional concrete in the form of drop panels or mushroom capitals.
  • Flat plate concrete systems can be found when buildings will support light loads as in residential or small commercial buildings.
  • Slab & beam frame consists of concrete slab supported by beams or beams & girders.
  • Waffle construction concrete uses square forms to produce a thicker slab but eliminates the weight of unnecessary concrete on the bottom half of the slab.
  • Reinforcing steel is placed in the bottom of waffle construction formwork to provide reinforcement in 2 directions (AKA 2-way slabs).
  • Precasting plants produce precast parts (most common) and whole modular units PRIOR to delivery to work site.
  • From a construction standpoint, precast concrete structures have more in common with steel frame buildings than cast-in-place buildings.
  • Precast concrete was not common until after World War II.
  • Precast concrete is produced as slabs, beams, columns, and wall panels.
  • Precast slab types include solid, hollow-core, single-tee, and double-tee.
  • Precast solid slabs are used for short spans up to 30 feet.
  • Precast tee-slabs are used for spans up to 120 feet.
  • Precast slabs can be supported by girders & columns, wall panels, or both.
  • Precast exterior wall panels are commonly used with a steel framework.
  • Precast elements are usually lighter than similar cast-in-place elements.
  • Precast elements can be connected by bolting, welding, and POST-tensioning.
  • A corbel is a ledge that projects out from a column and supports a precast beam.
  • Precast beams can be secured to columns with steel angles, cast into a column, or with POST-tensioned steel cables.
  • Precast units may have a cement topping up to 1 1/2 inch thick.
  • Buildings supported by concrete frame are usually enclosed by NON-bearing curtain wall.
  • NON-bearing exterior curtain walls can be made of aluminum, glass, steel panels, and masonry.
  • Curtain walls make it difficult to identify the structural system by observation alone.
  • Stucco and exterior insulation finish systems (EIFS) may appear visually to be concrete.
  • Concrete structural systems can have fire-resistance ratings from 1 to 4 hours.
  • Fire resistance of concrete is affected by concrete density, thickness, quality, and load supported.
  • Structural lightweight concrete has lower density and lower thermal conductivity which results in better insulation against fire and heat than standard concrete.
  • Cast-in-place concrete is inherently more fire resistant than precast due to continuity of the concrete.
  • Concrete which is supported by NON-fire-resistive members or that has openings not enclosed by rated fire doors or shutters, is considered NON-fire-resistive.
  • Concrete cannot withstand an explosion.
  • Because steel is a very dense material, it is not efficient to use a solid slabs or panels.
  • Sheets of steel may be used as floor decking or in exterior curtain walls.
  • Connection of the beam to a column transfers the load between members and determines structural rigidity.
  • Beam & girder steel frames are classified as rigid, simple, or semi-rigid.
  • Rigid steel frames resist bending forces.
  • Simple steel frames primarily support a vertical force.
  • Semi-rigid steel frames provide some diagonal support by using diagonal bracing or shear panels.
  • Shear panels are reinforced wall located between columns and beams that provide lateral support and should be continuous from foundation to the highest story needed.
  • Steel trusses are more economical than beams for carrying loads across greater spans.
  • Steel trusses are frequently used in 3 dimensional space frames.

  • 2 common basic steel trusses are the open-web joint and joist girder.
  • Open-web steel joists are made with depths up to 6 feet and spans up to 144 feet (2 foot or less depth and 40 foot span is most common).
  • Top and bottom chords of open-web steel joists can be made from 2 angles, 2 bars, or a tee-shaped member with diagonal members being flat bars continuous round bar (AKA bar joint) welded to top and bottom chords.
  • Bar joists often support floors and roof decks.
  • Joist girders can take the place of steel beams for the primary structural frame.
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  • Steel rigid frames with inclined roof members are widely used for 1 story industrial buildings and farm buildings.
  • Steel rigid frames usually span 40 to 200 feet.
  • The top of a rigid steel frame is the "crown" and the points where inclined members intersect vertical members are the "knees".
  • Vertical members of steel rigid frames may or may not be rigidly connected to foundations depending on anticipated wind loads.
  • 1 story rigid frame structures must be braced diagonally to prevent lateral deflection.
  • Steel arches support roofs where large unobstructed floors are needed.
  • Steel arches can span in excess of 300 feet.
  • Steel arches may be girder arches (solid arch), or trussed arch (using truss shapes).
  • Steel wire strengths can be as high as 300,000 psi.
  • Suspension roof systems can use steel rods or cables to provide large unobstructed areas without obstruction of side vertical clearances (unlike arches).
  • Steel suspension systems may be used with cantilever roofs.
  • The cross section of steel columns can be very small compared to their length due to the high compressive strength of steel.
  • An unprotected slender steel column can buckle easily from the heat of a fire.
  • The most common steel column cross sections are hollow cylinder, rectangular tube, and wide flange.
  • Factors which affect buckling of steel columns include length, cross section, and top/bottom support.
  • With steel columns, the slenderness ratio is a comparison of the unbraced length to the shape and area of its cross section.
  • Steel columns used for structural support should not have a slenderness ratio less than 120.
  • Rigid connections at the ends of steel columns resist rotation and are more resistant to buckling.
  • Cross sectional shape of a steel column affects buckling potential.
  • Square steel columns have less tendency to buckle than tubular shapes.
  • Mass of steel and structural connection is determined by structural design used.
  • Rigid connections in beam & girder frames have greater mass at the point of connection.
  • Steel beam & girder and steel trusses frequently use steel web gussett and plates for rigid connections.
  • Steel gussett plates strengthen connections and increase steel mass at the connection.
  • Large beam & girder frames with repeating sections tend to be mutually supporting (redundancy).
  • The main interest in masonry for firefighters is its use in wall construction.
  • The most commonly encountered load-bearing masonry walls are brick, concrete block, and combinations of both.
  • Gypsum block and lightweight concrete block are limited to NON-load-bearing partition walls.
  • Masonry walls can be found in fire-resistive and NON-fire-resistive buildings.
  • The most basic masonry structure consist of exterior load bearing walls which support the floor and roof.
  • Interior floors and roofs made of wood joists and rafters, along with exterior masonry walls, is termed "ordinary construction".
  • Interior masonry walls provide lateral and vertical load support.
  • Wood and steel trusses commonly support roofs of masonry wall buildings.
  • Cast iron was frequently used for interior columns in the 19th Century.
  • Thickness of masonry walls varies from a minimum 6 inches, up to several feet.
  • Lower partitions of multi-story walls must be thicker to carry the increased dead load from upper portions.
  • NON-reinforced masonry walls are usually limited to 6 stories in height.
  • Steel or concrete structural frame is more economical than masonry bearing walls when the building is more than 2 to 3 stories in height.
  • Reinforced masonry bearing walls can be as high as 20 stories and only 10 inches thick.

  • Masonry units laid side-by-side in a horizontal layer is called a "course".
  • Horizontal courses of brick laid on top of each other is called a "wythe".
  • The simplest brick wall consists of 1 wythe.
  • A brick wythe is commonly used with a concrete block wythe (AKA Concrete Block Brick Faced-CCBF).
  • When bricks are placed horizontally, end-to-end, they create a "stretcher course".
  • When bricks are placed vertically, on end, they create a "soldier course".
  • Parallel wythes of bricks can be bonded together using a "header course" every 6th course, or by using corrosion-resistant metal ties to bond wythes together.
  • Horizontal bonding of brick and block is usually done with metal ties.
  • Exterior brick walls usually have a vertical cavity between exterior and interior wythes to prevent water seepage through mortar joints.
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  • Masonry walls are reinforced to permit taller buildings or to provide lateral stability.
  • Grout is a mixture of cement, aggregate, and water.
  • Grout can be used to fill cavities between 2 adjacent wythes of a brick wall or within the openings of concrete block for reinforcement.
  • Buttresses and pilasters can be used to reinforce masonry walls.

  • Support of masonry over an opening requires the use of a lintel, arch, or corbelling.
  • Lintels are beams (usually steel angles) over an opening in a masonry wall.
  • When the height of a masonry wall above an opening is shorter than a triangular section above the opening, a lintel must provide support for the entire weight of masonry above the opening.
  • Lintels (most common), and arches are common methods of supporting loads over masonry openings.
  • Corbelling is used only for architectural styling.
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  • A parapet is an extension of a masonry wall that projects above the roof.
  • Parapets are found on exterior masonry walls and fire walls when buildings have combustible roofs, but may also be used for architectural styling.
  • Parapets project 1 to 3 feet or more above the roof and usually have NO lateral support.
  • Live loads of a building are transferred to bearing walls and columns by joists, beams, or trusses.
  • Masonry buildings with wood interior framing are classified as "ordinary" or "heavy timber".
  • In many residential and small commercial buildings, wood joists or beams rest on an indentation in the masonry wall, called a beam pocket.
  • A "fire cut" is a cut made with a slight angle at the end of a wood joist or beam to allow the member to fall freely away from the wall in the case of structural collapse.
  • Wood roof trusses are frequently supported by pilasters.
  • Fire resistance of masonry depends on type of masonry and thickness.
  • Walls made of fire-rated concrete masonry units or bricks can have 2 to 4 hours of fire resistance.
  • Well-constructed masonry walls are usually last to fail in a wood-joisted building.
  • Masonry wall failure is usually due to deterioration of the wall and/or structural members.
  • Cracks in masonry walls are often due to foundation shift.
  • One method of reinforcing masonry walls is by using tension rods extending through the walls and then attached to "thrust plates" on the outside.
  • Masonry walls usually collapse as a result of interior framing collapse.
  • Collapsing interior framing produces horizontal forces against a masonry wall, resulting in tensile forces which the mortar cannot resist.
  • It should be assumed that a collapsing wall will fall out from a building a distance at least equal to the height of the wall.
  • Building codes require less clearance between buildings with masonry or fire-resistive exterior walls.
  • Wood is almost always used in a frame structural system.
  • Use of solid logs for log cabins is perhaps the only use of wood for solid wall construction.
  • The 2 types of wood framing systems most frequently encountered are timber framing and light wood framing.
  • Wood timber framing is NOT the same as a building with exterior masonry walls and heavy timber framing.
  • Other types of wood construction include pole, log, and prefabricated panel construction.
  • Due to basic strength limitations of wood, it is usually not economical to use wood frames in buildings over 3 stories in height.
  • Until water-powered saws were developed 2 centuries ago, producing individual wood boards was slow and laborious.
  • In heavy timber designs, basic structural support is provided by beams and columns made of heavy timber.
  • In heavy timber designs, columns are NOT less than 8 x 8 inches, and beams (except roof beams) are NOT less than 6 x 10 inches.

  • Integrity of wood frame structures is affected by methods used to join joists, beams, and columns.
  • Factors affecting the design of connections for timber construction include specific gravity of wood, wood shrinkage, position of fasteners, and size of wood and fasteners.
  • Components of wood member connections include bearing blocks, steel straps/brackets, and mortise & tenon joints (older construction).

  • Heavy timbers cut from a single log are usually NOT available in lengths greater than 20 feet.
  • Glulams or timber trusses are used for greater spans of timber framing.
  • Post & beam construction uses columns (AKA posts), and beams with dimensions less than heavy timber but greater than light frame construction.
  • Posts in post & beam construction are usually 4 x 4 or 6 x 6 inches and space 4 to 12 foot apart.
  • Post & beam framing is square or rectangular and must be braced for diagonal stability.
  • Post & beam construction is usually more labor intensive than light frame construction.
  • Interior wood surfaces of post & beam and heavy timber construction is left exposed and thus eliminates combustible voids and avenues for fire spread.
  • The most popular form of wood framing is light wood frame construction.
  • Light wood framing uses 2 inch nominal lumber such as 2 x 4 or 2 x 8 inch members.
  • Vertical members of light wood framing are supported by joists or trusses, and inclined roofs are supported by rafters or light trusses.

  • The 2 basic types of light wood framing are balloon frame and platform frame.
  • Balloon framing can have open channels, due to continuous exterior wall studs, which permit fire spread from foundation to attic.
  • In platform framing (AKA western framing), exterior wall studs are NOT continuous from floor to floor.
  • Platform framing has double 2 x 4 inch members (AKA plates) laid horizontally along the top of the studs on each floor which act as fire stops.
  • Light wood framing is usually covered with plaster or drywall.
  • Fire in a balloon frame building is more difficult to control than in a platform frame building.
  • Platform frame buildings are easier to erect than balloon frame.
  • Wood shrinkage is greater in a direction perpendicular to the wood grain.
  • Platform framing has more horizontal members than balloon frame, which results in greater wood shrinkage (causes cracks/misalignment).

  • Exterior wall materials include sheathing, siding, building paper, and insulation.
  • Sheathing provides structural stability, insulation, and an under layer for siding, and may be made of plywood, or particle board.
  • Siding can be made of wood boards, aluminum, or of wood, asphalt, or asbestos cement shingles.
  • Foam insulation promotes rapid fire spread over its surface and may require facing with a thermal barrier such as gypsum, by code.
  • Fire spread due to combustion of foam insulation within a wall space is dependent on the amount of air within the space.
  • Loose fill insulation types include granulated rock wool, granulated cork, mineral wool, glass wool, cellulose fiber, and shredded wood.
  • Loose fill insulation can be treated with water-soluble salts to reduce combustibility, but will still smolder if involved in a fire.

  • Brick veneers must be tied to a wall at intervals of 16 inches.
  • Weep holes in brick veneers are 2 feet on center.
  • An air space of approximately 1 inch exists between a brick veneer and its supporting wall.
  • There is little difference between a brick veneer building and ordinary wood frame building in terms of fire behavior.
  • A rule of thumb to determine whether a brick wall is load-bearing is a bearing wall will have a header course every 6th course.
  • Concealed spaces must be opened to check for fire extension.
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