Roof Truss MCQ Quiz in తెలుగు - Objective Question with Answer for Roof Truss - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Concept:

As per IS 800: 2007(C- 4.3.6)

Notional Horizontal Loads

• To analyze a frame subjected to gravity loads, considering the sway stability of the frame, notional horizontal forces should be applied.
• These notional horizontal forces account for practical imperfections and should be taken at each level as being equal to 0.5 percent of factored dead load plus vertical imposed loads applied at that level.
• The notional load should not be applied along with other lateral loads such as wind and earthquake loads in the analysis.

C- 4.3.6.1

• The notional forces should be applied on the whole structure, in both orthogonal directions, in one direction at a time, at roof and all floor levels or their equivalent,
• They should be taken as acting simultaneously with factored gravity loads.

C- 4.3.6.2

The notional force should not be,

1. applied when considering overturning or overall instability;
2. combined with other horizontal (lateral) loads;
3. combined with temperature effects; and
4. taken to contribute to the net shear on the foundation.

C- 4.3.6.3

• The sway effect using notional load under gantry load case need not be considered if the ratio of height to the lateral width of the building is less than unity.

Explanation:

Sag rod or Sag bar

• The sag bar is provided in purlins to support the section in bending about the minor axis.
• They provide lateral support to purlin to prevent sag in the direction parallel to the sloping roof due to vertically applied loads and reduce their tendency to bend laterally and twist.
• They are designed to support the component of roof loads parallel to the roof.

Purlins

• These are horizontal beam members that run parallel to the ridge and connect the trusses along the length of the ridge.
• Purlins are subjected to vertical loads due to dead and live loads and to loads normal to roof covering due to wind pressure. Therefore, purlins are subjected to biaxial bending.

• Bending Moment in the purlin is given by, M = wl2/10

Principal rafter

• It 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.

Concept: 

Loads on roof truss:

(i) Dead load: If the spacing of truss is 4 m and pitch of truss is 1:4 then self weight of the truss is taken as 

w = (L/3 + 6) kg/m3 of plan area

(ii) Live loads: when no access is provided

For θ ≤ 10° , live load = 0.75 kN/m2

If slope of truss is more than 10°, live load is taken as 

live load = 0.75 kN/m2 - 0.02 kN/m2 (For every degree increases in slope over 10°)

(iii) Snow load: Snow load is taken as 2.5 kN/m2 for every 1 m depth of snow. If slope of truss is more than 50°, snow load need not be considered.

Calculation:

Given, slope of roof = 15° 

Live load on roof (when access is not provided and slope of roof > 10°)

Live load = 0.75 - 0.02 × (θ - 10) kN/m2

Live load = 0.75 - 0.02 × (15 - 10) = 0.65 kN/m2

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.

Confusion Points Minimum pitch for GI sheet is 1/6 and for AC sheet is 1/12, but the question is not asking minimum pitch, but it is asking normal pitch

Explanation

The weight of trusses cannot be determined or calculated until the stresses in members are determined and the truss design.

Exact calculations of stresses are not possible until the weights are known. Hence the weight of the truss has to be estimated. The weight of the truss varies with the span, and the rise of the truss, the spacing of trusses, and the type of roof covering material. It is not possible to have an exact formula that includes all these factors. The self-weight of the truss may be assumed as 0.090 to 0.150 kN per square meter of the plan area.

Important Points

• As the weight of trusses cannot be determined or calculated until the stresses in members are determined and the truss design.
• When calculating the weight of a steel structure, there is a general method to calculate the weight of all of the various members and plates first and then add an additional percentage to account for the weight of nuts and bolts. This percentage is usually taken as 5 % of the total weight.

Additional Information

The self-weight of truss is found by empirical formula as self-weight of truss in kN/sqm of the plan are

A) Dead loads:

The simple formula for estimation of the approximate deadweight of roof truss in N/m2 is

\({\rm{Dead\;weight}} = \left( {\frac{{\rm{L}}}{3} + 5} \right) \times 10\)

where L = Span of the truss

Concept:

For economic spacing of roof trusses, the cost of truss should be equal to twice the cost of purlins + the cost of roof covering.

As a guide the spacing of the roof trusses can be kept:

1/4 of span up to 15.0m.

Concept:

Effective Length of Column:

The effective column length can be defined as the length of an equivalent pin-ended column having the same load-carrying capacity as the member under consideration. The smaller the effective length of a particular column, the smaller its danger of lateral buckling and the greater its load-carrying capacity. It must be recognized that column ends in practice are neither perfectly fixed nor perfectly hinged. The designer may have to interpolate between the theoretical values given, to obtain a sensible approximation to actual restraint conditions.

As per IS 800-2007, The Equivalent Length of Columns for Various End Conditions:

Calculation:

Given That,

Height of the shed = 6 m

Effectively held in position at both ends restrained against rotation at one end = 0.80 L

= 0.80 X 6

=4.8 m

So the correct answer is Option 2

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.

Concept: 

Loads on roof truss:

(i) Dead load:

If the spacing of truss is 4 m and pitch of truss is 1:4 then self weight of the truss is taken as 

w = (L/3 + 6) kg/m3 of plan area

(ii) Live loads: 

when no access is provided

For θ ≤ 10° , live load = 0.75 kN/m2

If slope of truss is more than 10°, live load is taken as 

live load = 0.75 kN/m2 - 0.02 kN/m2 (For every degree increases in slope over 10°)

(iii) Snow load: 

Snow load is taken as 2.5 N/m² for every 1 m depth of snow. If slope of truss is more than 50°, snow load need not be considered.

Explanation:

​According to IS: 800 Purlin should be designed as a continuous beam.

Purlins are subjected to vertical loads due to dead and live loads and to loads normal to roof covering due to wind pressure. Therefore, purlins are subjected to biaxial bending.

As per IS 800 – 2007 (clause 8.9), the maximum bending moment of purlins is ​WL/10.