Q.
a) State the differences between softwood and hardwood based on the botanical terms. (10 marks)
b) Discuss the advantages and disadvantages of wood as a building material compared to concrete and steel. (15 marks)
(25 marks, 2017 Q5)
A.
a) Botanical classification and difference between hard and soft wood
Ref:
Earlier post.
b) Adv and disadv of wood as material vs concrete and steel
See Timber vs Concrete vs Steel here.
Wood
Read Jastin McGar here on Timber vs Steel vs Concrete Structures.
TIMBER VS STEEL
THERMAL
Steel - The coefficient of expansion of steel is very high as compare to wood so they cause failure of structures when increases. Failure may occur due to deflection structural member which are subjected to high temperature.
Timber - The coefficient of expansion of wood is very low i.e. it does not provide increase in length when temperature is raised so this may provide safety to structures and make to cool enough as compare to steel. However contraction may be danger full to timber structures because below 0 C its starts contract rapidly.
ACOUSTICS
Steel - structures may produce sound i.e. echo because they have less properties of absorption of sound waves so sound proof material may be used in these structures.
Timber - don’t reflect sound waves rather it has greater intensity to absorb sound waves so no need of providing sound proof materials in buildings which reduce cost .
COST
Steel - structures are very much costly than the timber structures because of their metallic behavior and used for high structural variations such as loads, span and strength.
Timber - The cost of timber structures is very low because these structures are design for low loads, strength and span.
DURABILITY
Steel - structures are more durable than timber because these structures got extra strength which against natural agents such as wind , rain, earthquake etc. the life time steel structures may be up to 50 years.
Timber - structures are less durable than steel structures because they have properties to against natural phenomenon. Life time may be up to 15 to 20 years.
DETERIORATION
Steel - structures are liable to be corroded this may reduce life span but if steel coated with paints then corrosion is reduced at great extent.
Timber - deteriorates more than steel because termites badly affect the timber which makes tunnel through it and make it weak enough. It can be reduced by painting.
VARIETIES
Steel - Varieties of steel is limited, they can be made variable by industrial agents i.e. by looking adding different metals, alloys etc which may get its variation.
Timber - More than 5000 kinds of wood are available in the world with different eternal structures and properties. So they can be used in engineering for various purpose.
TIMBER VS CONCRETE
1. Concrete is a man made material whereas timber is a piece of wood.
2. Concrete buildings last for a long time and need low maintenance as compared to wooden or timber structures.
3. Concrete is environment friendly and has high thermal efficiency as compared to timber.
4. Timber is largely used as a construction material in the earthquake zones.
5. Timber is tensile and resists bending whereas concrete has low tensile and hard in nature.
Read more: Difference Between Concrete and Timber | Difference Between http://www.differencebetween.net/miscellaneous/difference-between-concrete-and-timber/#ixzz4t7hGmEHP
Key setback with Wood are:
- Fire
- Durability
- Pests
Key advantages with Wood are:
- Timber has higher structural efficiency as carried load per unit weight compared to reinforced concrete and steel structures
- A common stud used in house construction has similar compressive strength to general purpose concrete
- Many timbers are either naturally durable or can be easily treated to make very durable.
- Timber used internally provides a healthier environment for occupants as it helps maintain a better relative humidity
- The vast majority of structural timber is sourced from sustainably managed forests and plantations.
Concrete
Key advantages of Concrete are:
3 key criteria: speed and efficiency; the reduction of on-site risks; and sustainability and waste reduction.
Speed and efficiency
- Computer modelling before fabrication on computer numerically controlled (CNC) equipment means the components are produced right first time and there is minimal rework
- Earlier construction provides faster completion and payback commencement
- Faster construction means the builder’s staff can be released earlier to start the next project
- Steel is a structurally efficient building material and so buildings are lighter and often foundations can be smaller as a result
- Steel is fabricated in controlled conditions driven by 3D modelling and CNC equipment increasing safety and reducing MTIs and LTIs
- A test certificate is available for all steel used and this can be made traceable through to the finished product
- Design for standardised bolted connections and repetitive floor plates can increase speed of construction
Reducing onsite risks
- Steel use reduces the number of workers onsite (approximately 10 to 20 per cent of the labour needed for concrete construction), reducing accident liability for builders
- Preassembled steel packages can be lifted straight from the truck in sequence
- Offsite fabrication relieves congested and hard to access sites
- Reduced noise and dust and construction times ease disruption to current occupants and neighbourhoods
- For domestic housing, steel framing, roofing and cladding offer more termite-free and fire resistant abodes than with other commonly used building materials
Sustainability and waste reduction
- Waste removal is significantly less than for a concrete building
- More than 95 per cent of all structural steel is recovered and reused or recycled
- Steel buildings inherently lend themselves to structural addition and modification easily
- Independently certified steel fabricator members of the ASI’s Environmental Sustainability Charter attract an extra Green Star point for projects
- Savings in freight and materials through not having to use and remove temporary formwork
- The design can be future proofed for longer life such as provision of large beam penetrations to accommodate future services