Adv & Disadv of Steel as Roof Trusses

ADVANTAGES and

DISADVANTAGESof Using

STEELas

ROOF TRUSSES

INTRODUCTION

• Loading system of a roof

• Types of trusses commonly used

• Materials commonly used

• Factors leading to the use of steel

Loading System of A Roof

• Types of loadings

– Gravity• self-weight and weight of the finishes

– Climatic loads • Primary loads: wind and snow• Induced loads: rain and icing

Loading System of A Roof

• Load Transfer

• From the tiles and the roof itself to the purlins

• From purlins to truss

• From truss to columns or beams to finally be transferred to the ground

Types of Roof Trusses

• A roof truss is a structural member which supports the purlins in roof construction.


• Span up to a distance of 30m – used where large, clear, unobstructed areas are required.

• The ability to span large distances arises from two factors:– The high strength-to-weight ratio– The design of the truss in the form of a series of

triangles.


• Roof trusses save material and on-site labor costs. It is estimated that a material savings of about 30 percent is made on roof members and ceiling joists.

• Roof trusses come in a variety of shapes.

Types of Roof Trusses - Examples

Attic Bowstring

Double Howe Double Fink


Fan Dual Pitch

Fink Flat


Gambrel Hip

Howe Inverted


Kingpost Modified Queenpost

Monopitch Piggyback


Scissors Sloping Flat

Queenpost Polynesion


Stub Studio

Queenpost Truss

Materials Commonly Used

• CAST IRON and ALUMINIUM

• TIMBER– light timber roof trusses - used mainly for single family

dwellings– heavy timber roof trusses - usually used to create large

clear areas like cathedral ceilings.

• STEEL

Use of Steel in Roof Trusses

• High Yield's steel (yield strength 450 - 550 N / mm2), or known as cold formed steel and only suitable to be used to support spans less than 13m.

• Mild steel is usually used for longer spans. The yield strength is 275 N/mm2 and usually it is thicker. The extension used are welding or bolt and nut..

• Common use in factory building, hangar and long truss that spans a large distance and is high.

Factors Leading to the Use of Steel

• Availability: Steel service centre Availability of shape

• Maintenance: Soap or mild detergent and water (Preferably

warm) Soap and warm water or organic solvent (eg:

acetone, alcohol, methylated spirits) Mild cleaning solutions (eg: Jif, specialty stainless

steel cleaners)

Factors Leading to the Use of Steel

• Cost of Production: A cost-push comes from raw material suppliers The extent of demand emerging from BRIC

countries Increased capacity to produce steel signals lower

prices in the future The price of steel is highly dependent on global

economic cycle

British Standards

• BS 449:

The use of structural steel in building

• BS 4/ Bs 4848:

Specification for Hot-Rolled Structural Steel Sections

ADVANTAGES

and

DISADVANTAGES

ADVANTAGES

• High strength-to-weight ratio

• High strength - providing design flexibility and superior engineering provide excellent spanning capabilities.

• readily accommodate innovate renewable energy technology.

• Resistant to catastrophes such as earthquakes.• Weight of structures are smaller, thus easy to transport to

the site of construction.

ADVANTAGES

• Uniformity– The properties of steel do not change over time.

• Elasticity– It is very highly performing under both tension

and compression, up to fairly high stresses. – Behaves closer to design assumptions.

ADVANTAGES

• High Ductility– Steel deforms to a great extent before ultimate

failure.– Visible evidence of impending failure. Precaution

steps can be taken.

• Toughness– Steel has the ability to absorb energy in large

amounts before being deformed.

ADVANTAGES

• Straight and true– Steel members do not wrap, twist, sag or shrink.

They will remain straight and true minimizing ongoing maintenances.

• Permanence– Steel structures can last indefinitely, if properly

maintained. They can far exceed the useful life of buildings with conventional wood trusses.

ADVANTAGES

• Energy saving characteristic– They span an area and provide space for a full

depth of insulation in the attic. For example: Raised heel trusses

• Possible re-use (ultimate recyclable material)– Steel is 100% recyclable– Generate minimal waste products

ADVANTAGES

• 100% Termite-proof.– Total structural protection

• Air-vapor barrier– Reduces mold, condensation, and rot.

Other Advantages

• Ability to be fastened together by several connection devices

• Steel will not ignite or burn

• Adaptation to pre-fabrication

• Speed of erection

• Addition to existing structures

• Ability to be rolled into a wide variety of sizes and shapes

DISADVANTAGES

DISADVANTAGES

• Corrosion– Main disadvantage– When freely exposed to air and water– Needs to be painted regularly

• Susceptibility to Buckling– Danger of buckling increases with increase in

length and slenderness.

DISADVANTAGES• Fatigue

– In tension only– The strength decreases when subjected to a high

stress or a large number of variations of tensile stress

• Fire-Proofing costs– Steel is an excellent heat conductor– Can propagate heat very fast– Require insulation or thermal breaks

DISADVANTAGES

• Financial Restrictions:– Only a few types of trusses are financially feasible

• An increase in manufacturing cost;• Because it allows for maximum insulation

space, it will take more insulation to fill the attic.

DISADVANTAGES

• The Effect of Temperature

– Cold-drawn steel cables can totally fail at 800º F.

– Above 1000oF , expanding steel exert lateral thrust forces on wall, causing collapse.

– Expansion within the truss cause the bottom chord to buckle or fail

– Low temperatures may cause the steel to lose its ductility, thus becoming brittle.

DISADVANTAGES

• Skills Requirement

– Because light-gauge steel frame construction is relatively new and innovative within the residential construction industry, it is not only unfamiliar to craftsmen but it is also unfamiliar to engineers and code officials.

DISADVANTAGES

• Tools Requirement

In addition to the labor training required to convert to the use of steel, builders and laborers must obtain some new tools not presently used by carpenters. The combination of additional training and tools translate to increased costs of construction for a short period of time.

CONCLUSION

CONCLUSION

• Roof trusses are of various types• Dependant on the design requirements and

aesthetics view• Steel is a widely used material• Advantageous as compared to timber• New technologies and innovations are being

discovered

THANK YOU

Documents

Adv & Disadv of Steel as Roof Trusses