Chapter 3 - Superstructure

Question 1

What are the five factors that influence the effect of forces that act on building structures?

Answer 1

1. Direction of force
2. Magnitude of force
3. Location of force
4. Duration of force
5. Dynamic forces

Question 2

What is force?

Answer 2

Any natural or human action (push or pull) that causes an object to move or undergo a shape change.

Question 3

What is a superstructure?

Answer 3

All above ground components of a building

Question 4

When designing a structure, what are the different load classifications in the building code that must be considered?

Answer 4

1. Dead loads
2. Occupancy or live loads
3. Snow loads
4. Wind loads
5. Temperature
6. Earthquake loads
7. Other loads

Question 5

What are dead loads?

Answer 5

Loads that arise from the self-weight of the structure. This will include permanent fixtures like interior walls, lights, ducts and furniture. They are referred to as “dead loads” because they are believed to be relatively static and would not be expected to change by any significant extent over the service life of the building.

Question 6

What are occupancy or live loads?

Answer 6

Loads that arise from the use of the structure. Primarily, these will be comprised of the occupants of the building. However, in other structures like parking garages live loads would include the expected weight of the vehicles; libraries will include the weight of the books.

Question 7

What are snow loads?

Answer 7

Loads that arise from snow are considered as a separate load class

Question 8

What are wind loads?

Answer 8

Loads that arise from wind are considered a seperate load class.

Question 9

How does temperature effect building loads?

Answer 9

Temperature effects have to be considered when designing a structure. 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 temperatures.

Question 10

What are earthquake loads?

Answer 10

Earthquake loads are unique in that they arise from an acceleration of the ground below the structure. This is similar to the force that is felt when you accelerate your care from a dead stop. Earthquake motions are complex, they occur in all different directions and are frequency dependent. All buildings have a unique natural frequency ; as such each building will pick up a different acceleration during a given earthquake. Therefore, when designing for earthquakes you must determine the nature frequency of the structure before you can determine the design forces.

Question 11

What consists of other loads?

Answer 11

Blast, machinery impact, fire or rain may all have to be considered depending on the circumstance and geographical location. Fires do not impose loads on a structure, rather a fire, after a certain period of time will weaken the structure. The structure must therefore be designed to resist a certain load after the fire has burned for a period of time. Typically this is in range of two hours, thus the term “two-hour fire rating” is referred to commonly in commercial buildings.

Question 12

What is the concept of return period?

Answer 12

Return period is a concept 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 known as the 50-year return period - equivalent to an event that has a 2% chance of occurring in any one year). Earthquake loads are based on 2500 year return period, which is equivalent to an event that will have a 2% chance of occurring in a 50 year period.

Question 13

What is Elastic Material Theory?

Answer 13

Theory that is based on the simple observation that all loads cause materials to deform. Elastic materials, within a certain load range, are expected to return to their original shape once the load has been removed (e.g. an elastic band).

Question 14

What are the categories in which forces interact with structural elements?

Answer 14

1. Compression
2. Tension
3. Shear
4. Torsion
5. Bending

Question 15

What are the three most important performance goals to consider when choosing a building superstructure?

Answer 15

1. Economy of construction
2. Serviceability: occupant comfort and use of building
3. Durability: expected lifespan of the building and maintenance costs

Question 16

What are load paths?

Answer 16

Path in which loads are carried into the substructure and ultimately into the ground.

Question 17

What are the key architectural elements?

Answer 17

1. Style or the distinct architectural impression of the building. (i.e. modern, federal, greek revival, victorian, art deco, art modern, themed etc.)
2. Form - refers to building massing or the scale, height, shape and configuration of a building.
3. Finishing materials are the visible elements use to clad the interior and exterior of a building. Most buildings will have a variety of materials for interior and exterior application. The combination and application of these materials will help define the architecture of the building.

Question 18

What are the different types of superstructure materials?

Answer 18

1. Wood
2. Steel
3. Concrete
4. Masonry

Question 19

What are the advantages of using wood for building superstructures?

Answer 19

1. Very high strength in resisting compression in comparison to its weight.
2. 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.
3. 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.
4. Small wood members are stronger per unit area then large wood members - this is a key advantage of wood frame construction since the wood elements are easy to handle and frame.

Question 20

What is more desirable in wood building material; softwood or hardwood?

Answer 20

Softwood - such as pine, because it has a more regular cell structure and longer wood fibers compared to hardwoods translating into more consistent compressive, shear and tensile strength.

Question 21

What is hygroscopicity?

Answer 21

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

Question 22

What are some advantages of using engineered wood products?

Answer 22

1. These products are typically stronger and more dimensionally stable than sawn lumber products made from solid wood.
2. High strength to weight ratio and seismic and fire resistance means that taller and larger commercial wood buildings can be built.
3. Improved acoustic and vibration dampening and thermal insulation
4. Lower carbon footprint and better sustainability relative to steel and concrete.

Question 23

What are the two broad types of conventional wood superstructures?

Answer 23

1. Platform framing, also known as light frame or stick built construction.
2. Post and beam framing

Question 24

What are the advantages of using a wood frame superstructure?

Answer 24

1. Building materials are light and easy to handle
2. Building materials are widely available in standard or custom dimensions
3. Wood lumber is easy to attach to other wood through nailing, use of steel hangers or bolting the elements together
4. A wood frame structure is considerable lighter than concrete structures - an advantage where underlying soils are weak and the overall weight is a concern
5. Wood frame structures can generally be erected faster than reinforced concrete frame structures and more easily modified “on the fly” compared to concrete or steel
6. Wood elements can be combined with light steel to create very strong, rigid and light structural elements for roof and roof assemblies

Question 25

What are the disadvantages of using wood frame superstructure?

Answer 25

1. Lower fire resistance relative to steel and concrete - factor has historically limited the height of wood frame buildings
2. Member spans are limited. Consider steel, where very long beams can be created by welding or bolting together shorter members. Same with concrete
3. Potential shrinkage or swelling associated with changes in moisture content in wood frame elements
4. Wood is relatively weak compared to steel and concrete and this becomes a factor in tall structures. 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 26

What are the advantages of using steel for modern commercial building designs?

Answer 26

1. Steel is isotropic - meaning it has the same physical properties in all dimensions
2. Steel elements have predictable consistency and a high strength-to-weight ratio.
3. 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.
4. 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. Think of the empire state building that could build a floor a day compared to concrete which takes time to cure and can only build a floor a week.
5. 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 enhanced fire resistance over steel alone.
6. Long spans are possible with steel framing, allowing large, open office and retail floor plates with maximum flexibility for occupants.
7. Steel can be also 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 27

What are some disadvantages of using steel in superstructure?

Answer 27

1. Steel elements in a building superstructure must be pre-ordered well in advance of planned construction; changes to architectural drawings will often set back projects back since steel must be re-ordered with additional cost
2. Specialized labour is required to erect steel frame buildings; the availability of skilled trades may be limited in some locations
3. Steel is not fire resistant; extreme heat will cause steel to lose its ability to resist compressive and tension forces. As a result, steps must be taken to fire-proof structural steel, either through protective coatings or encapsulation with fire-proof materials.
4. The cost of steel can increase significantly between the date of order and delivery; steel price escalation was a prominent factor in driving up the commercial building construction costs between 2004-2008
5. 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.

Question 28

Why is steel and concrete a good combination for building superstructure?

Answer 28

Steel is generally added to reinforce concrete structural elements because it is bendable (ductile) and equally strong in tension or compression. It can either augment compression load carrying capability or it can be relied upon to carry internal tension loads. Concrete’s very low tensile strength means it cannot be relied upon to carry tension loads alone. The combination of the two materials capitalizes on the advantages offered for each material in an efficient and cost effective way.

Question 29

What are pre-engineered buildings?

Answer 29

Pre-engineered buildings are mass produced steel frame structures that can be configured in simple geometric patterns. This design allows for duplication of components and ease of fabrication.

Question 30

What are some advantages of pre-engineered steel buildings?

Answer 30

1. Reduced requirements for engineering since only the foundation must be custom designed.
2. Ease of manufacture and transport, and fast accurate assembly
3. Lower weight in relation to conventional steel frame buildings while achieving acceptable structural integrity
4. Fast erection
5. Low cost in relation to conventional steel or concrete frame construction, assuming the purchaser acquires a standard building package with little customization
6. Large clear span is possible - ideal for shops, some big box retail stores and warehousing
7. Relatively easily expanded to meet growth requirements

Question 31

What are disadvantages of pre-engineered steel buildings?

Answer 31

1. Their generic designs means the buildings are generally unsuitable for custom applications.
2. The use of light steel components and construction results in less durable structure and higher maintenance costs over the building lifecycle
3. The walls are roof are often un-insulated - meaning poor energy efficiency
4. Building lifecycle is typically shorter than conventional concrete or steel frame buildings
5. Bearing load is typically less than engineered steel frame structures; this may be an issue where a heavy overhead crane will be installed
6. Usually requires additional finishing and fit-out to create usable commercial or industrial space
7. No secondary roof membrane
8. Difficult to modify, such as adding doors or mezzanines

Question 32

What are the advantages of concrete frames?

Answer 32

1. Concrete is relatively easy to obtain in relation to steel, which must be pre-ordered to exact specifications
2. Concrete offers low cost and high durability
3. The major components of concrete (aggregate and water) are widely and locally available
4. The requirement for specialized skilled labour for concrete forming and placement is lower in relation to steel framed buildings
5. Concrete is not combustible
6. Concrete can be reinforced with steel rods or pre-stressed cable - achieving the advantages of both materials to resist compression and tension
7. Concrete elements can be pre-cast in a remote location and assembled at the job site reducing construction expense and improving quality control.
8. Concrete can be custom designed for different applications by changing the type and amount of admixtures or chemical additives

Question 33

What are some disadvantages of using concrete?

Answer 33

1. The production of cement is very energy intensive and requires high level of carbon emissions
2. Concrete, if exposed to moisture, salinity, and high humidity will deteriorate over time. As a result a wide range of sealers and coatings have evolved to protect finished concrete surfaces
3. Adverse weather conditions during a concrete pour will result in uneven curing of concrete - leading to weakness and potential failure when concrete elements are put under stress
4. Concrete is a mix of water, lime mixture, gravel and other ingredients - if the water or concrete dry mix becomes contaminated the resulting concrete will be weakened
5. Concrete mix will be weakened if the proportions are incorrect. Too much water will lead to weaker concrete
6. Concrete quality can be highly variable.