Roof Truss MCQ Quiz - Objective Question with Answer for Roof Truss - Download Free PDF

Explanation:

Angle Sections:

1. Lightweight: Angle sections are often used for lightly loaded beams like roof purlins, making them cost-effective for smaller, less demanding structures.

2. Versatile: They can be used in various configurations, including horizontal, vertical, or sloped, providing flexibility in design.

3. Economical: Due to their shape and material efficiency, angle sections are cheaper compared to more complex beam designs, making them ideal for non-heavy duty applications.

Additional Information Castellated Beams:

1. Lightweight with High Strength: Castellated beams are designed with a web that is cut and rejoined, making them lighter while maintaining strength, ideal for longer spans.

2. Aesthetic Appeal: The design is often chosen for its aesthetic value, as it allows for architectural uniqueness in larger buildings or structures.

3. Not Ideal for Light Loads: Castellated beams are typically used for medium to heavy loads, making them unsuitable for lightly loaded applications like roof purlins.

Box Girders:

1. High Load-Bearing Capacity: Box girders are typically used for bridges or heavy-duty structures due to their ability to bear significant loads.

2. Complex Design: They are more complex in design and fabrication, making them costlier and less suited for light applications like roof purlins.

3. Durable: Box girders offer good resistance to torsional forces, making them suitable for situations where rotational forces are present, but not for lightly loaded beams.

Plate Girders:

1. Strong Load-Bearing Capacity: Plate girders are used for supporting large loads and are common in bridges, large buildings, and industrial structures.

2. Costly and Heavy: Due to their large size and the heavy material used in their construction, plate girders are more expensive and unsuitable for light-load applications.

3. Complex Fabrication: Plate girders require specialized fabrication, which makes them less efficient and practical for simpler structures like roof purlins and sheeting rails.

Explanation:

• A collar beam in a roof structure is placed horizontally between opposing rafters, typically at a higher point of the roof.
• Its primary function is to prevent the rafters from bending or buckling under load by tying the rafters together and providing additional support.
• This helps to maintain the roof’s structural integrity.

 Additional InformationCoupled Roof:

• Structure: A simple roof type where rafters are paired to form a basic triangular shape.

• Function: The rafters support the roof covering and are typically connected at the ridge.

• Design: Commonly used in small buildings or low-cost structures.

• Collar Beam: Not typically used in coupled roofs; rafters act directly to support the load.

• Application: Suitable for smaller span buildings, such as houses with simple roof designs.

Scissors Roof:

• Structure: A roof design that uses intersecting rafters to create an angled structure resembling a scissors mechanism.

• Function: Provides more headroom and creates an open, vaulted ceiling space within the building.

• Design: More complex than a coupled roof, offering aesthetic value along with functionality.

• Collar Beam: May be used in scissor roofs to support the rafters and prevent them from bending under load.

• Application: Common in buildings requiring high ceilings or an open interior space, such as churches or large homes.

Explanation

Collar roof:

• When the span increases or when the load is more, the rafter of the couple close roof have the tendency to bend.
• This is avoided by raising the tie beam and fixing it at one-third to one-half of the vertical height from wall plate to the ridge. This raised beam is known as a collar beam or collar tie.
• Suitable for span up to 5 m.

Important Points

Coupled roof:

• Formed by pair of rafters which slope to both sides of the ridges of the roof.
• Upper ends of rafters nailed to a common ridge and lower ends nailed to the wooden wall plates.
• Applicable for span upto 3.6 m.
• It has a tendency to spread out at the feet ( wall plate level ) and thrust out the walls supporting the wall plates.

Scissors roof:

• similar to collar roof except that two collar beams crossing each other to have an appearance of scissors is provided.

Explanation:

Purlins: These are the members which are spanning on the roof frames to support the roof coverings and runs parallel to ridge to connect different trusses situated in the longitudinal direction.

Rafters: These are a series of sloped structural members that extend from the ridge to the downslope perimeter or to the bottom chords and are designed to support the roof deck and its associated loads.

The principal rafter is the top chord member of the truss and is subjected to compressive forces from loads transferred by purlins at the nodes. The rafters act as simply supported beams between the purlins.

Explanation:

Concept:

Purlin:

It is a member of truss which are supported on the principle rafter and which transverse loads to the truss. These are some important properties of purlin- 

• It is a biaxial bending member.
• The span of purlin is center to center of truss, purlin is located at the panel point of the truss.
• Maximum spacing between purlins is less than 1.4 m. 
• Angle, Channel, I-sections and Z-sections are used for purlins and girders to support the cladding. 

Concept:

Live load: 

As per IS: 875, Part 2 specifies the following Live loads:

For roof membrane sheets or purlins: 0.75 kN/m2 less 0.02 kN/m2 for every degree increase in slope over 10° subject to a minimum of 0.4 kN/m2.

Calculation:

Given data, 

Access is not provided to the roof and the slope = 15°

So, in this case 

The live load for the sloping roof is

LL = 0.75 – (15 – 10) × 0.02 = 0.65 kN/m2

Concept:

Bracings: When a strong wind flows parallel to ridge, the sloping of pitched industrial roof truss deforms and instability arises parallel to ridge. To arrest this deformation and instability the top and bottom chords of the roof truss are joined by cross diagonal members which are called bracing. This bracings helps to resist the wind load parallel to ridge.

Purlins: These are horizontal beam members runs parallel to ridge and connects the trusses along the length of ridge.

Truss: These are combined arrangement of several structural members to transfer loads from top to bottom, and usually occurs at regular interval.

Columns: Columns are the vertical beam members which take load from the trusses and transfer it to the foundation.

Explanation:

(i) A queen post is a tension member in a truss that can span longer openings than a king post truss.

(ii) A king post uses one central supporting post, whereas the queen post truss uses two. Even though it is a tension member, rather than a compression member, it is commonly still called a post.

(iii) King Post Truss - Span length is in the range of 5 to 8 meters.

(iv) Queen Post Truss - Span length is in between 8 to 12 meters.

(v) Pratt Truss - Span length is in between 6 to 10 meters.

(vi) Howe Truss - Span length is in between 6 to 30 meters.

The Correct Answer is Option 2

Explanation:

Given,

Dead Load + Live Load = 3 kN/m²,  Wind Load = 5 kN/m²

Plan Area = 4 × 1.212 = 4.849 m²

Vertical Load will be due to Deal Load & Live Load 

Vertical Load = 3 × 4.849 = 14.55 kN

For Wind Load 

Area = 1.4 × 4 = 5.6 m²

Inclined Point Load = 5.6 × 5 = 28 kN

∴ Vertical point loads is 14.54 kN and inclined point loads is 28 kN

King Post Truss - Span length up to 5 meter (i.e ≤ 5 meter )

Fink Truss - Span length is in between 5 to 10 meter

Queen Post Truss - Span length is in between 8 to 12 meter.

Howe Truss - Span length is in between 6 to 30 meter.

Concept:

According to IS 875(part 2) - Imposed load

Hence, The live load taken for the design of the roof of an industrial building using a truss that is accessible is 1.5 kN/m2

Components of roof trusses are:

1. Upper chord members: The uppermost line of members which extend from one support to the other through the apex is called the upper chord.

2. Bottom chord members: The lowermost line of members which extend from one support to the other through the apex is called the lower chord.

3. Web members or diagonals: The top and the bottom chord members are connected by vertical or diagonal members called web members.

In trusses simply supported at the ends, the members in the top chord are subjected to compression and the members of the bottom chord are subjected to tension. Tension members are called ties while compression members are called struts.

A roof truss will slightly deflect under a vertical or moving load. The upper chord will be under compression while the lower chord is under tension. Diagonals can either be compression or tension members depending on their inclination. Therefore, diagonals are truss members the stress depends upon whether the load is moving on the top chord or bottom chord.

Concepts:

Pitch or slope refers to the amount of vertical measurement (rise) compared to horizontal measurement (run).

Specifications of roof Truss:

1. The pitch of a roof truss should be 1/4 to 1/6 of its span for proper drainage.
2. The spacing of roof trusses is kept 1/3 to 1/5 of the span.
3. The gusset plate should be at least 6 mm thick.
4. The bolts or riveted used to connect the various members of roof truss should be of minimum 2 numbers.
5. A minimum angles section of ISA 50 × 50 × 6 should be used.

Calculation:

Given data 

Span = 6m 

Pitch is 1/4 to 1/6

\(Pitch = \frac{Rise}{span}\)

\(\frac{1}{4}= \frac{Rise}{9m}\)

Rise = 2.25m

Concept:

For economical spacing of truss,

The cost of the truss should be equal to twice the cost of purlins plus the cost of roof covering.

t = 2p + r

where,

t = cost of truss, p = cost of purlin, r = cost of the roof.

However, the above expression is used to check the spacing of the roof truss.

It can not be used to design the spacing as spacing does not occur in the equation.

As a guide, the spacing of roof trusses can be kept 1 / 4 of the span up to 15 m and 1 / 5 of span for 15 - 30 m spans of roof trusses.

∴ Option 4 is most appropriate