Elements Of Structure

Question 1

List the potential signs of collapse

Answer 1

(i) Dropping of arch’s and/or lintels above openings.

(ii) Sagging of floors or floors deflecting from walls.

(iii) Cracks in walls.

(iv) Displaced columns

(v) Bulging walls

(vi) Buckling columns or beams

(vii) Water or smoke penetration what appears to be a solid wall.

(viii) Unusual noises coming from the building.

Question 2

Explain the function of beams and gives examples of types of beams.

Answer 2

Transmit forces perpendicular to the points of support.

When loaded the beam deflects or bends downward.

Types:
(i) Simple beam - supported at each end.

(ii) Continuous beam - supported at 3 or more points.

(iii) Fixed beam - supported at 2 points and rigidly held in position at both ends.

(iv) Joists - wood, steel or precast concrete directly supporting a floor.

(v) Girder - beam of any material which supports other beams.

(vi) Lintel - beams spanning opening in masonry walls.

(vii) Cantilever- rigidly fixed at one end.
- Tension in top compression in bottom.
- very likely to be unstable in serious fire and fire may destroy method of fixing.

Question 3

Explain the distribution of load for beams

Answer 3

Assume a simple beam can carry 8 units of distributed load.

If load is concentrated at the centre it can only carry 4 units.

If the beam is cantilevered and load is distributed can only carry 2 units.

If load is at the unsupported end of the cantilevered, only 1 unit can be carried.

Question 4

Describe the structure of concrete beams and how they are produced.

Answer 4

REINFORCED
- High tensile steel rods encased in concrete.

• held in place by a system of steel latticework, designed to meet strength requirements.
• used due to
  (i) ability to withstand high tensile stress
  (ii) expansion rates of steel and concrete almost the same.
  (iii) adhesion between the two surfaces results in efficient bonding.

PRE-TENSIONED
- Cables stretched between two anchored points

• framewirk then built around the cables and concrete poured around the cables.
• once concrete has cured to a sufficient point the cable tension is released.
• on releasing of the tension the cables try to return to their original stated and in doing so compress the concrete.

POST-TENSIONED
- cables are fixed to anchor points and concrete formed around them.

• once concrete has mature to an adequate strength, using a special jack at one end the cables are stretched until the right stress is reached at the opposite end.
• this induces concrete in the tensile region to be in compression.
• when load is applied there remain sufficient compressive strength in the tensile zone to neutralise the tension.

Question 5

Describe how steel columns can be protected from the effects of fire.

Answer 5

SOLID Protection
- via concrete encasement or application of mineral fibres sprayed on to the column in the form of vermiculite, concrete or magnesium oxychloride, the length of fire resistance would be linked to the density of the aggregate used and the thickness of the encasement surrounding the steel column.

• via intumescent paints, either sprayed or brushed onto the steel, during a fire these expand and char sealing the steel underneath off from oxygen and protect them from the effects of the heat of the fire for a specified amount of time linked to type of protection.

HOLLOW
- via encasement with fire resistance boards, such as gypsum or vermiculite, care must be taken to make sure the entire column is surrounded and attention paid to sealing joints with the appropriate protection inline with the fire resistance of the boards used.

• via application of fire resistant mesh which is then sprayed with fire resistant compounds or filled with additional thermal protection such as mineral wool fibres.

Again care must be taken to finish joints carefully especially at floors and walls.

• Any column must have the minimum fire resistance for any element it carries.
• If it forms part of a compartment or more than one building it must comply with the maximum fire resistance for those areas.
• Light steel columns require more protection than heavy steel columns.

Question 6

What are the factors affecting the fire resistance of timber floors?

Answer 6

• Thickness of the floor
• Type of wood (hard/soft)
• Any fire resistance applied? (Intumescent paints/impregnation)
• Type of joist supporting the floor
• How the floor structure (including joists) are fixed to the buildings structure.