BUSI401 CHAPTER 3

Question 1

A building’s superstructure includes all above-ground structural compo­nents. The superstructure can be considered an extension of the substructure above grade level.

This chapter will discuss common construction materials and structural systems used for superstructures of commercial and indus­trial buildings

Answer 1

A building’s superstructure includes all above-ground structural compo­nents. The superstructure can be considered an extension of the substructure above grade level.

This chapter will discuss common construction materials and structural systems used for superstructures of commercial and indus­trial buildings

Question 2

Engineers today are faced with the challenge of designing larger, more complex, functional, and durable superstructures than at any other time in history. Clients are demanding innovative architecture for buildings with superstructures that are lighter, more structurally efficient, and aesthet­ically pleasing. At the same time, there is pressure to manage the capital expense for new buildings, improve energy efficiency and environmental sustainability, and reduce long term operating costs.

To resolve these conflicting demands, the design team, which may include both architects and civil engineers, will translate the desired performance goals into a computerized model of design options. The sophistication of the modelling process will depend on the goals, the building’s mass and complexity, and the modelling tools available to the design team.

Answer 2

Engineers today are faced with the challenge of designing larger, more complex, functional, and durable superstructures than at any other time in history. Clients are demanding innovative architecture for buildings with superstructures that are lighter, more structurally efficient, and aesthet­ically pleasing. At the same time, there is pressure to manage the capital expense for new buildings, improve energy efficiency and environmental sustainability, and reduce long term operating costs.

To resolve these conflicting demands, the design team, which may include both architects and civil engineers, will translate the desired performance goals into a computerized model of design options. The sophistication of the modelling process will depend on the goals, the building’s mass and complexity, and the modelling tools available to the design team.

Question 3

An _______ approach is now the norm for contemporary building design. Each building component’s performance requirement is evaluated with the goal of _ _ _ _ _ _ _ _. This requires _ _ _ _ _ _ _ _, such as the substructure, super­structure, mechanical systems, and interior design.

Answer 3

An integrated approach is now the norm for contemporary building design. Each building component’s performance requirement is evaluated with the goal of the most efficient overall structure possible. This requires harmonizing the individual components, such as the substructure, super­structure, mechanical systems, and interior design.

Question 4

Canada has a number of national codes that are intended to regulate building construction and operation. These codes are collectively known as _ _ _ _ _ _ _ _ _ _ . These Codes form the basis for all of Canada’s provincial and municipal building codes.

Answer 4

Canada has a number of national codes that are intended to regulate building construction and operation.

These codes are collectively known as the National Model Construction Codes.’ These Codes form the basis for all of Canada’s provincial and municipal building codes.

Question 5

The _ _ _ _ _ _ _ _ _ _ regulates the design and construction of new buildings and significant renovations for existing buildings.

Answer 5

The National Building Code (NBC) regulates the design and construction of new buildings and significant renovations for existing buildings.

Question 6

The National Building Code (NBC) requirements are linked to goals for _ _ _ _ _ _ _ _ _ _ _ _ _. It sets minimum performance standards for construction materials and systems, such as the building envelope.

Answer 6

The National Building Code (NBC) requirements are linked to goals for public safety and health, accessibility, fire prevention and structural protection of buildings. It sets minimum performance standards for construction materials and systems, such as the building envelope.

Question 7

The NBC provides guidelines for the following building components and construction situations:

Answer 7

The NBC provides guidelines for the following building components and construction situations:

Fire protection, occupant safety, and accessibility

Structural design

Environmental separation

Heating, ventilation, and air-conditioning

Plumbing services

Safety measures at construction sites and demolition sites

Housing and small buildings

Question 8

The National Plumbing Code is organized in a similar fashion to the NBC. The Plumbing Code requirements are linked to goals for public safety and health, and protection of buildings and facilities from water and sewage damage.There are three components to the safety objectives: ______, _____, and ________.

Answer 8

There are three components to the safety objectives: safety in use, fire safety, and structural safety.

Question 9

The ________ provides minimum fire safety requirements for buildings, structures, and areas where hazardous materials are used. The Code also provides direction on fire protection and fire prevention for building operations.

Answer 9

The National Fire Code provides minimum fire safety requirements for buildings, structures, and areas where hazardous materials are used. The Code also provides direction on fire protection and fire prevention for building operations.

Question 10

The building enclosure has a great influence on the durability of the building. Well-designed and properly constructed wall and roof assemblies are critical to mitigating the damaging effects of moisture and humidity. Prolonged exposure to moisture will cause all materials to degrade more rapidly. Thus, when the building enclosure performs well, the structure is protected and the overall result is a more durable building

Answer 10

The building enclosure has a great influence on the durability of the building. Well-designed and properly constructed wall and roof assemblies are critical to mitigating the damaging effects of moisture and humidity. Prolonged exposure to moisture will cause all materials to degrade more rapidly. Thus, when the building enclosure performs well, the structure is protected and the overall result is a more durable building

Question 11

________ provides fire protection provisions for new and significantly renovated buildings, whereas the ________ focuses on fire prevention for existing buildings and structures.

Answer 11

NBC provides fire protection provisions for new and significantly renovated buildings, whereas the National Fire Code focuses on fire prevention for existing buildings and structures.

Question 12

The ________ is intended to complement NBC, by setting minimum requirements for energy efficiency for commercial and residen­tial buildings

Answer 12

The National Energy Code is intended to complement NBC, by setting minimum requirements for energy efficiency for commercial and residen­tial buildings

Question 13

Force can be defined as any natural or human action (a push or a pull) that causes an object to move or undergo a shape change. There are five factors that influence the effect of forces that act on building structures:

Answer 13

1. Direction of the force

2. Magnitude of the force

3. Location of the force

4. Duration of the force

5. Dynamic forces

Question 14

Magnitude of the force — normally measured as Newtons. One newton is the force required to support 100 grams of mass. One thousand Newtons (1 kN) is equivalent to approximately 220 pounds of force.

Answer 14

Magnitude of the force — normally measured as Newtons. One newton is the force required to support 100 grams of mass. One thousand Newtons (1 kN) is equivalent to approximately 220 pounds of force.

Question 15

_________ are a special class of forces that occur as a series of regular or irregular cycles.

Answer 15

Dynamic forces — a special class of forces that occur as a series of regular or irregular cycles. Earthquakes, reciprocating machinery, human activity (e.g., walking, dancing), and wind are all examples of dynamic forces.

Question 16

When designing a structure, there are a number of load classifications defined in the building code that must be considered:

Answer 16

Dead loads

Occupancy or live loads

Snow loads

Wind loads

Temperature

Earthquake loads

Question 17

The forces that we consider in designing structures arise from natural phenomenon. In order to predict such forces, we must rely on statistical methods. A key concept in this regard is the return period, which is used to identify a period of time to be considered for a particular event to occur. For example, wind and snow loads used in structural designs are usually based on the 1 in 50-year event (also called a 50-year return period — equivalent to an event that has a 2% chance of occurring in any one year). Earthquake loads are based on a 2,500-year return period, which is equivalent to an event that will have a 2% chance of occurring in a 50-year period.

Answer 17

The forces that we consider in designing structures arise from natural phenomenon. In order to predict such forces, we must rely on statistical methods. A key concept in this regard is the return period, which is used to identify a period of time to be considered for a particular event to occur. For example, wind and snow loads used in structural designs are usually based on the 1 in 50-year event (also called a 50-year return period — equivalent to an event that has a 2% chance of occurring in any one year). Earthquake loads are based on a 2,500-year return period, which is equivalent to an event that will have a 2% chance of occurring in a 50-year period.

Question 18

_________ is the force that pulls the material apart under stress. _________ causes elongation.

Answer 18

Tension

Tension is the force that pulls the material apart under stress. Tension causes elongation.

Question 19

Another way to visualize a _______ force is to place the palms of your hands together and then slide one past the other.

Answer 19

Another way to visualize a shear force is to place the palms of your hands together and then slide one past the other.

Question 20

A _____ force causes materials to distort.

Answer 20

A shear force causes materials to distort.

Question 21

________ is a force that causes rotation or twisting. A _______ force must be applied perpendicular to the axis of rotation.

Answer 21

Torsion is a force that causes rotation or twisting. A torsional force must be applied perpendicular to the axis of rotation.

Question 22

_______ is not a single force but rather a combination of forces including tension, compression, and shear that act simultaneously within a beam.

Answer 22

Bending is not a single force but rather a combination of forces including tension, compression, and shear that act simultaneously within a beam.

Question 23

The three most important performance goals to consider when choosing a building’s superstructure are as follows:

Answer 23

The three most important performance goals to consider when choosing a building’s superstructure are as follows:

Economy of construction

Serviceability: occupant comfort and use of building

Durability: expected lifespan of the building and maintenance costs

Question 24

____________ must also be considered. It is expected that occupancy, wind, and snow loads will act together on a structure. However, it is not expected that these loads will act at their full extent simultane­ously. Structural designers recognize that a wind storm will not occur at the same time as a major snow fall. This is accounted for using special load combination cases.

Answer 24

Load combinations must also be considered. It is expected that occupancy, wind, and snow loads will act together on a structure. However, it is not expected that these loads will act at their full extent simultane­ously. Structural designers recognize that a wind storm will not occur at the same time as a major snow fall. This is accounted for using special load combination cases.

Question 25

Most materials used in the construction of building structures behave according to _ _ _ _ _ _ _ _ . This theory is based on the simple observation that all loads cause materials to deform.

Answer 25

Most materials used in the construction of building structures behave according to “Elastic Material Theory”. This theory is based on the simple observation that all loads cause materials to deform.

Question 26

Temperature — temperature effects have to be considered. Materials will expand and contract and if not accounted for can result in large forces particularly in long structures. Often buildings will contain _ _ _ _ _ _ _ to account for such movements in structures that arise due to changes in temperature.

Answer 26

Temperature — temperature effects have to be considered. Materials will expand and contract and if not accounted for can result in large forces particularly in long structures. Often buildings will contain expansion joints to account for such movements in structures that arise due to changes in temperature.

Question 27

The forces that we consider in designing structures arise from natural phenomenon. In order to predict such forces, we must rely on statistical methods. A key concept in this regard is the return period, which is used to identify a period of time to be considered for a particular event to occur. For example, wind and snow loads used in structural designs are usually based on the 1 in 50-year event (also called a 50-year return period — equivalent to an event that has a 2% chance of occurring in any one year). Earthquake loads are based on a 2,500-year return period, which is equivalent to an event that will have a 2% chance of occurring in a 50-year period.

Answer 27

The forces that we consider in designing structures arise from natural phenomenon. In order to predict such forces, we must rely on statistical methods. A key concept in this regard is the return period, which is used to identify a period of time to be considered for a particular event to occur. For example, wind and snow loads used in structural designs are usually based on the 1 in 50-year event (also called a 50-year return period — equivalent to an event that has a 2% chance of occurring in any one year). Earthquake loads are based on a 2,500-year return period, which is equivalent to an event that will have a 2% chance of occurring in a 50-year period.

Question 28

_____ is a force that causes rotation or twisting. A ________ force must be applied perpendicular to the axis of rotation.

Answer 28

Torsion is a force that causes rotation or twisting. A torsional force must be applied perpendicular to the axis of rotation.

Question 29

Buildings must be designed to resist the calculated loads that are imposed upon them. The building as a whole is an interconnected network of structural elements that create a ________ such that loads are carried into the _________ and ultimately into the ground.

Answer 29

Buildings must be designed to resist the calculated loads that are imposed upon them. The building as a whole is an interconnected network of structural elements that create a load path such that loads are carried into the substructure and ultimately into the ground.

Question 30

_____________ such as those caused by wind or snow can occur during construction. Buildings that are partially constructed may require temporary bracing because portions of the building may be susceptible to collapse. For example, in a pre-cast tilt-up building wall panels must be supported with temporary shoring posts before the roof is installed.

Answer 30

Environmental loads such as those caused by wind or snow can occur during construction. Buildings that are partially constructed may require temporary bracing because portions of the building may be susceptible to collapse. For example, in a pre-cast tilt-up building wall panels must be supported with temporary shoring posts before the roof is installed.

Question 31

Taller buildings tend to have _ _ _ _ _ _ _ frequency — the _____ the ________, the more susceptible the building is to resonance by the wind. Under certain conditions, the building can sway back and forth in the wind like a tall reed, causing discomfort for the occupants particularly in the upper floors.

Answer 31

Taller buildings tend to have lower natural frequency — the lower the frequency, the more susceptible the building is to resonance by the wind. Under certain conditions, the building can sway back and forth in the wind like a tall reed, causing discomfort for the occupants particularly in the upper floors.

Question 32

Resonance wind can be counteracted by the use of __________. Some tall buildings have _______ located in the top floor. An example of such a damper is on the top of the Wall Centre in Vancouver. There is a “slosh tank” that is comprised of a large tank of water with a series of baffles placed in such a way that causes the water to slosh back and forth in opposition to the natural sway frequency of the building.

Answer 32

This effect can be counteracted by the use of dampers. Some tall buildings have tuned mass dampers located in the top floor. An example of such a damper is on the top of the Wall Centre in Vancouver. There is a “slosh tank” that is comprised of a large tank of water with a series of baffles placed in such a way that causes the water to slosh back and forth in oppo­sition to the natural sway frequency of the building.

Question 33

In basic terms, the forces of wind and earthquake tend to _____, ______, and ______ buildings. ______, ____, and the connections ____________ members provide resistance to racking.

Answer 33

In basic terms, the forces of wind and earthquake tend to rack, overturn, and slide buildings. Shear panels, braces, and the connections between horizontal and vertical members provide resistance to racking. Overturning and sliding are prevented by proper foundation design and connection of the building to this foundation with anchor bolts

Question 34

______ and _____ are prevented by proper foundation design and connection of the building to this foundation with anchor bolts.

Answer 34

Overturning and sliding are prevented by proper foundation design and connection of the building to this foundation with anchor bolts.

Question 35

Deflection is the amount of movement a building material or building system experiences as a result of wind or other loads. For example, if a building moves two inches off centre as a result of wind force, then it is said to have deflected two inches.

Answer 35

Deflection is the amount of movement a building material or building system experiences as a result of wind or other loads. For example, if a building moves two inches off centre as a result of wind force, then it is said to have deflected two inches.

Question 36

_______ is the amount of movement a building material or building system experiences as a result of wind or other loads.

Answer 36

Deflection is the amount of movement a building material or building system experiences as a result of wind or other loads.

Question 37

Buildings rely on _____, _____, or _____ to resist lateral forces, which include those forces caused by wind or earthquakes.

Answer 37

Buildings rely on bracing, frames, or shear walls to resist lateral forces, which include those forces caused by wind or earthquakes.

Question 38

_____ and ____ are generally used in steel buildings. Steel tends to be quite flexible and usually experiences greater lateral deflection under wind or seismic loading.

Answer 38

Frames and bracing are generally used in steel buildings. Steel tends to be quite flexible and usually experiences greater lateral deflection under wind or seismic loading.

Question 39

Masonry and concrete structures generally rely on ________ to resist lateral forces.

These materials are very brittle when compared to steel and thus deflect much less.

Specially placed reinforcing steel within the concrete or masonry is required to reduce diagonal cracking these types of shear walls.

Answer 39

Masonry and concrete structures generally rely on shear walls to resist lateral forces. These materials are very brittle when compared to steel and thus deflect much less.

Specially placed reinforcing steel within the concrete or masonry is required to reduce diagonal cracking these types of shear walls.

Question 40

ECONOMIC CONSIDERATIONS

The real opportunity to minimize costs lies in optimizing the selection of the building configuration and structural system. There are also market-driven factors that affect the selection of a structural system, such as local availability of materials and supplies and the familiarity of local builders with various structural systems. Building costs increase if the project requires resources (materials, machinery, workers, and design expertise) from outside the region of the project.

Answer 40

ECONOMIC CONSIDERATIONS

The real opportunity to minimize costs lies in optimizing the selection of the building configuration and structural system. There are also market-driven factors that affect the selection of a structural system, such as local availability of materials and supplies and the familiarity of local builders with various structural systems. Building costs increase if the project requires resources (materials, machinery, workers, and design expertise) from outside the region of the project.

Question 41

__________ is a force that attempts to squeeze a material together. When a material is subjected to a ________ force it will shorten. Columns are typically subjected to __________ forces.

Answer 41

Compression
Compression is a force that attempts to squeeze a material together. When a material is subjected to a compressive force it will shorten. Columns are typically subjected to compression forces.

Question 42

________ is not a single force but rather a combination of forces including tension, compression, and shear that act simultaneously within a beam.

Answer 42

Bending is not a single force but rather a combination of forces including tension, compression, and shear that act simultaneously within a beam.

Question 43

________ is the amount of movement a building material or building system experiences as a result of wind or other loads.

For example, if a building moves two inches off centre as a result of wind force, then it is said to have ____ two inches.

Answer 43

Deflection is the amount of movement a building material or building system experiences as a result of wind or other loads. For example, if a building moves two inches off centre as a result of wind force, then it is said to have deflected two inches.

Question 44

Buildings rely on bracing, frames, or shear walls to resist lateral forces, which include those forces caused by wind or earthquakes.

_______ and _______ are generally used in steel buildings. Steel tends to be quite flexible and usually experiences greater lateral deflection under wind or seismic loading.

Masonry and concrete structures generally rely on ________ to resist lateral forces. These materials are very brittle when compared to steel and thus deflect much less.

Specially placed _ _ _ _ _ within the concrete or masonry is required to reduce diagonal cracking these types of shear walls.

Answer 44

Buildings rely on bracing, frames, or shear walls to resist lateral forces, which include those forces caused by wind or earthquakes. Frames and bracing are generally used in steel buildings. Steel tends to be quite flexible and usually experiences greater lateral deflection under wind or seismic loading.

Masonry and concrete structures generally rely on shear walls to resist lateral forces. These materials are very brittle when compared to steel and thus deflect much less. Specially placed reinforcing steel within the concrete or masonry is required to reduce diagonal cracking these types of shear walls.

Question 45

The goal of most commercial building owners and developers is to _ _ _ _ _ _ _. The size of the structural members used in construction will impact the rentable space in a building and the appeal of the space to tenants and other occupants.

Answer 45

The goal of most commercial building owners and developers is to maximize the rentable area of a new building while controlling costs. The size of the structural members used in construction will impact the rentable space in a building and the appeal of the space to tenants and other occupants.

Question 46

The durability of the structure is a function of _______ and is largely dependent on the __________, ________, and ___________.

Answer 46

The durability of the structure is a function of exposure and is largely dependent on the material properties, quality of the building, and its intended use.

Question 47

EXPLAIN: Building durability can also be considered an economic perfor­mance measure or design goal.

Answer 47

Building durability can also be considered an economic perfor­mance measure or design goal since it represents another trade-off between initial capital cost and eventual structural repairs and replacement.

Question 48

The durability issue arises in particular in smaller buildings, given the wider available choices for building superstructure. For example, an architect may design a three-storey office building using structural steel, reinforced concrete, or conventional wood-frame. Each building system will have a different cost base and expected service life. Reinforced concrete and steel frame buildings, if properly installed and protected from moisture, salt, etc., will usually have a longer physical service life than a similar wood-frame structure. It may be possible to attract tenants who are prepared to pay higher rents for space in a steel frame or concrete building since it may be a more attractive space or be able to support heavier floor loads to suit the tenant’s needs.

Answer 48

The durability issue arises in particular in smaller buildings, given the wider available choices for building superstructure. For example, an architect may design a three-storey office building using structural steel, reinforced concrete, or conventional wood-frame. Each building system will have a different cost base and expected service life. Reinforced concrete and steel frame buildings, if properly installed and protected from moisture, salt, etc., will usually have a longer physical service life than a similar wood-frame structure. It may be possible to attract tenants who are prepared to pay higher rents for space in a steel frame or concrete building since it may be a more attractive space or be able to support heavier floor loads to suit the tenant’s needs.

Question 49

The building ________ has a great influence on the durability of the building.

Answer 49

The building enclosure has a great influence on the durability of the building.

Question 50

Wood offers the following advantages as a material for building superstructures:

Answer 50

Wood offers the following advantages as a material for building superstructures:

Very high strength in resisting compression in relation to its weight.

Wood has a greater capacity to carry short-term loads than sustained loads, which enhances its performance in response to cyclical forces such as seismic events and strong wind gusts.

Wood systems are usually constructed to have the ability to distrib­ute and mutually support loads, increasing their efficiency. In other words, wood structures have redundant load paths such that if one wall fails there will be another wall that can support the floor or roof albeit with an overall reduced capacity.

Smaller wood members are stronger per unit area than larger wood members — this is a key advantage of wood frame construction since the wood elements are easy to handle and frame.

Question 51

EXPLAIN: The cellular structure of wood provides a good strength-to-weight ratio, but is also a weakness.

Answer 51

The cellular structure of wood provides a good strength-to-weight ratio, but is also a weakness. Since wood can absorb significant amounts of moisture through its cell structure, it is not dimensionally stable; rather the size and shape change of wood elements will change with moisture content.

Question 52

The tendency of wood to change its dimensional stability and strength with absorption of moisture is called __________.

Answer 52

The tendency of wood to change its dimensional stability and strength with absorption of moisture is called hygroscopicity.

Question 53

Define tensile strength?

Answer 53

the resistance of a material to breaking under tension.

Question 54

These are long wood strips glued together under pressure to create structural beams and studs.

Answer 54

Parallel strand lumber

Question 55

These are thin sheets of wood glued together under pressure to create long span beams. These products appear like very thick plywood.

Answer 55

Laminated veneer lumber (LVL)

Question 56

There are two broad types of conventional wood superstructures:

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Answer 56

There are two broad types of conventional wood superstructures:

Platform framing, also known as light frame or stick built construction

Post and beam framing

Question 57

Explain platform framing

Answer 57

Platform framing uses light, closely spaced wooden members to build the structure upwards as a series of platforms. The ground floor is typically framed on a concrete foundation. Walls constructed from closely spaced 2 x 4’s or 2x 6’s are then built on top of the floor. The next floor, or platform, is then built on top of the walls. Each successive floor is then built up in similar fashion.

Question 58

The building as a whole is an interconnected network of struc­tural elements that create a _ _ _ _ _ such that loads are carried into the substructure and ultimately into the ground.

Answer 58

The building as a whole is an interconnected network of struc­tural elements that create a load path such that loads are carried into the substructure and ultimately into the ground.

Question 59

Platform framing is most commonly used for modern light frame super-structures. This method provides a safe working surface for carpenters to frame the building as they progress and shorter wall studs are readily available from any lumber yard.

Also, the platform forms a firestop in the wall cavity, a feature absent from balloon frame construction unless separate firestop blocks are added. Exterior load bearing walls are often built flat on the platform, and then tilted up and fastened in place.

Answer 59

Platform framing is most commonly used for modern light frame super-structures. This method provides a safe working surface for carpenters to frame the building as they progress and shorter wall studs are readily available from any lumber yard. Also, the platform forms a firestop in the wall cavity, a feature absent from balloon frame construction unless separate firestop blocks are added. Exterior load bearing walls are often built flat on the platform, and then tilted up and fastened in place.

Question 60

The second type of wood superstructure, _ _ _ _ _ _, uses heavier members with larger spacing between them. Loads are transferred to the foundation by columns.

Answer 60

The second type of wood superstructure, post and beam construction, uses heavier members with larger spacing between them. Loads are transferred to the foundation by columns.

Question 61

With the advent of new engineered wood components such as I-Joists and CLTs, as well as factory prefabrication of floor, wall, and floor structures, it is increasingly common for mid-rise (up to six stories) structures to be built with wood-frame construction. The National Building Codes now allows mixed-use buildings where the ground level and basement are reinforced concrete and the upper levels are wood frame.

Answer 61

With the advent of new engineered wood components such as I-Joists and CLTs, as well as factory prefabrication of floor, wall, and floor structures, it is increasingly common for mid-rise (up to six stories) structures to be built with wood-frame construction. The National Building Codes now allows mixed-use buildings where the ground level and basement are reinforced concrete and the upper levels are wood frame.

Question 62

Wood frame superstructures offer a number of advantages for commercial construction:

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Answer 62

Wood frame superstructures offer a number of advantages for commercial construction:

Building materials are light and easy to handle

Building materials are widely available in standard or custom dimensions.

Wood lumber is easy to attach to other wood through nailing, use of steel hangers, or bolting the elements together.

A wood frame structure is considerably lighter than concrete structures — an advantage where underlying soils are weak and the overall building weight is a concern.

Wood frame structures can generally be erected faster than reinforced concrete frame structures and are more easily modified “on the fly” compared to concrete or steel.

Wood elements can be combined with light steel to create very strong, rigid, and light structural elements for roof and roof assemblies.

Question 63

The main limitations of wood framing for commercial buildings are as follows:

Answer 63

The main limitations of wood framing for commercial buildings are as follows:

Lower fire resistance relative to steel and concrete.

Member spans are limited. Consider steel, where very long beams can be created by welding or bolting together shorter members. Similarly, reinforced concrete can be constructed with very long continuous spans.

Potential shrinkage or swelling associated with changes in moisture content in the wood frame elements.

Wood is also relatively weak compared to steel and concrete and this becomes a factor in tall structures constructed from wood. On lower floors the forces can be so high that the crushing strength of wood is exceeded and steel plates or tube sections are required as bearing pad elements to distribute loads into the foundation.

Question 64

Steel structures offer a number of advantages for modern commercial building design:

Answer 64

Steel structures offer a number of advantages for modern commercial building design:

Steel is isotropic — meaning it has the same physical properties in all dimensions.

Steel elements have predictable consistency and a high strength-to¬weight ratio. North American steel mills have high levels of quality control thus these materials have high structural reliability. Steel building components are designed with tolerances of millimetres while pre-cast concrete and other building elements are designed with tolerances of centimetres.’

Use of structural steel allows considerable flexibility in design since the steel fabricator can produce almost any configuration using welded and bolted connections. The result is that a steel frame building can be a relatively light and open structure relative to other building materials.

The highly repetitive modular form of structural steel elements provides for speed of assembly and rapid erection at the building site on a much larger scale than other building systems using concrete or wood. A notable example of the speed of steel frame construction is the Empire State Building in New York, a 102-storey building constructed in only 402 days — at one point the builders averaged one floor per day of construction. In contrast, a concrete building would average one floor per week due to time required for the concrete to cure such that there was sufficient strength to support the next floor above.

When a steel frame building envelope is designed with poured concrete floors and elevator towers, the composite construction provides the durability of concrete with the convenience of steel, as well as providing enhanced fire resistance over the use of steel alone.

Long spans are possible with steel framing, allowing large, open office and retail floor plates with maximum flexibility for occupants.

Steel can also be used to create dramatic architectural features. Modern steel fabrication techniques allow for large scale bending of steel that can be used to create large scale structural forms with interesting and unique shapes.

Question 65

However, there are also some significant issues with use of steel in superstructure construction:

Answer 65

Steel elements in a building superstructure must be pre-ordered well in advance of the planned construction; changes to architectural drawings will often set projects back since steel must be re-ordered with additional cost.

Specialized labour is required to erect steel frame buildings; the availability of skilled trades may be limited in some locations.

Steel is not fire resistant; extreme heat will cause steel to lose its ability to resist compressive and tension forces.

The cost of steel can increase significantly between the date of order and delivery;

Exposed steel is subject to oxidation or rust.

Paints or galvanizing can be applied to protect steel from rust. If long term oxidation is expected and other protection is prohibitively expensive, the designer may increase the thickness of exposed members to account for loss of material during the structure’s design life.