Framed Structure Q3

Thomas Sim PEA2066

Q.
a) Explain the factors that influence the choice of steel and concrete as materials for framed structures. (15 marks)

b) Outline the advantages of the following frame structures:-

i) In-situ cast reinforced concrete structures
ii) Steel frame structures (10 marks)

(25 marks, 2013 Q3)

A.
(a) Factors influencing choice of steel and concrete as material for framed structure.

Similar question was asked in 2011 Q4a.

(b) Advantages of framed structures.

i) In-situ cast reinforced concrete structures

Advantages are:

  1. Low cost
  2. Speed in construction
  3. Lack of seams and joints (solid construction)
  4. Any shape, any size or load (weight)
  5. Good job of monolithic structures to dynamic and alternating loads
More detailed explanation below:

• QUICKER START-UP TIMES: A reinforced concrete framing system does not require extensive preordering of materials and fabrication lead time. Construction can begin on the foundations and lower floors prior to the structural design of the upper floors being finalized.

• REDUCED TOTAL CONSTRUCTION TIME: Reinforced concrete buildings can be constructed at a rate of one floor per week (above the first few floors) and other sub-trades can begin work on completed floors earlier. COST SAVINGS

• FAVOURABLE CASH FLOW: Materials and labour are expensed to the project as they are completed, unlike structural steel, where substantial down payments are required months before the material arrives on-site.

• STANDARD FLOOR LAYOUTS: Repetitive flooring systems which employ flying forms, uniform forming layouts and standard reinforcing steel details lead to significant cost savings.

• FASTER FORMING REUSE: Performance Rated Concrete (PRC) allows for faster form stripping and reuse.

• LOWER FLOOR TO FLOOR HEIGHTS: Reinforced concrete framing systems allow for the lowest floor to floor heights, minimizing exterior cladding and vertical servicing costs.

• ZONING HEIGHT RESTRICTIONS: Reinforced concrete framing systems allow for a greater number of floors within a given building height restriction, due to lower floor to floor heights.

• THERMAL RESISTANCE: The thermal mass of a reinforced concrete structure offers a lower rate of building heat gain or loss resulting in reduced building cooling/heating costs. In addition, lower floor to floor heights result in a reduced interior volume of air that must be heated or cooled by the HVAC system.

• FIRE RESISTANCE: Reinforced concrete structures are inherently fire resistant and do not require the expensive secondary application of coatings in order to obtain the necessary fire rating values.

• MORE FLOOR SPACE: High Performance Concrete (HPC) means smaller column sizes and more rentable floor space.

• MINIMAL MAINTENANCE: Concrete provides a hard, durable wearing surface that resists weathering extremely well.

• ARCHITECTURAL FINISHES: Reinforced concrete can act both as a structural member and an architectural finish with the use of coloured concrete and special texturing techniques

Ref:
The Advantages of Reinforced Concrete Building Framing System. Power of Concrete Vol 1. Available at,
http://3qcake159kca33qhh23so84g.wpengine.netdna-cdn.com/wp-content/uploads/2014/09/Power-of-Concrete-Volume-1.pdf

ii) Steel frame structures

 

Steel structures have the following advantages:

  1. They are super-quick to build at site, as a lot of work can be pre-fabbed at the factory.
  2. They are flexible, which makes them very good at resisting dynamic (changing) forces such as wind or earthquake forces.
  3. A wide range of ready-made structural sections are available, such as I, C, and angle sections
  4. They can be made to take any kind of shape, and clad with any type of material
  5. A wide range of joining methods is available, such as bolting, welding, and riveting
Advantages of steel frame structure are like:

Its immense strength is of great advantage to buildings. The other important feature of steel framing is its flexibility. It can bend without cracking, which is another great advantage, as a steel building can flex when it is pushed to one side by say, wind, or an earthquake. The third characteristic of steel is its plasticity or ductility. This means that when subjected to great force, it will not suddenly crack like glass, but slowly bend out of shape. This property allows steel buildings to bend out of shape, or deform, thus giving warning to inhabitants to escape. Failure in steel frames is not sudden - a steel structure rarely collapses. Steel in most cases performs far better in earthquake than most other materials because of these properties.

However one important property of steel is that it quickly loses its strength in a fire. At 500 degrees celsius (930 degrees F), mild steel can lose almost half its strength. This is what happened at the collapse of the World Trade Towers in 2001. Therefore, steel in buildings must be protected from fire or high temperature; this is usually done by wrapping it with boards or spray-on material called fire protection.

Ref:Steel frame structure. Understand Building Construction. Available at,
http://www.understandconstruction.com/steel-frame-structures.html