This set of Tough Design of Steel Structures Questions and Answers focuses on “Gantry Girders & Design of Industrial Buildings”.
1. Which of the following is correct regarding gantry girders?
a) It is laterally supported except at the columns
b) It is subjected to impact load
c) It should not be analysed for unsymmetrical bending
d) It is not subjected to longitudinal load
View Answer
Explanation: Gantry girder are different from beams in buildings. It is generally laterally unsupported except at the columns. It is subjected to impact load. It must be analysed for unsymmetrical bending because of lateral thrust from the starting and stopping of the crab. It is subjected to longitudinal load due to starting and stopping of crane bridge itself. They are always simply supported.
2. Which of the following loads are not considered in the design of gantry girders?
a) longitudinal loads
b) gravity loads
c) lateral loads
d) wind loads
View Answer
Explanation: The loads considered in the design of gantry girders are vertical loads or gravity loads, longitudinal loads, lateral loads and impact loads. The vertical force is the reaction from crane girder, acting vertically downward. The longitudinal thrust is due to starting and stopping of crane acting in longitudinal direction. The lateral thrust is due to starting and stopping of the crab acting horizontally normal to the gantry girder.
3. The wheel load transferred from trolley to gantry girder is given by
a) W1 = [Wt(Lc+L1)]/(2Lc)
b) W1 = [Wt(Lc-L1)](2Lc)
c) W1 = [Wt(Lc-L1)]/(2Lc)
d) W1 = [Wt(Lc+L1)]/( Lc)
View Answer
Explanation: Since trolley moves on the crane girder along the span of truss, its weight is transferred to the crane wheels as the axle load and finally to gantry girder. The wheel load transferred from trolley to gantry girder is given by W1 = [Wt(Lc-L1)]/(2Lc), where W1 is load of each wheel on gantry girder, Wt is weight of trolley or crab car, Lc is distance between gantry giders, L1 is distance between centre of gravity of trolley and gantry.
4. For gantry girders carrying electrically operated overhead travelling cranes, the lateral forces are increased by ____ for impact allowance.
a) 10% of weight of crab and weight lifted on the crane
b) 20% of weight of crab and weight lifted on the crane
c) 25% of maximum static wheel load
d) 50% of maximum static wheel load
View Answer
Explanation: For gantry girders carrying electrically operated overhead travelling cranes, the lateral forces are increased by 10% of weight of crab for impact allowance and weight lifted on the crane. The vertical forces can be increased by 25% of maximum static wheel load.
5. For gantry girders carrying hand operated cranes, the vertical forces are increased by____ for impact allowance
a) 10% of maximum static wheel load
b) 25% of maximum static wheel load
c) 10% of weight of crab and weight lifted on the crane
d) 20% of weight of crab and weight lifted on the crane
View Answer
Explanation: For gantry girders carrying hand operated cranes, the vertical forces are increased by 10% of maximum static wheel load for impact allowance. The lateral forces can be increased by 5% of weight of crab and weight lifted on the crane.
6. Fatigue effect for light and medium duty cranes need not be checked if
a) Nsc > 10×106 [(27/γmft)/γmt].
b) Nsc < 5×106 [(27/γmft)/γmt]3
c) Nsc > 5×106 [(27/γmft)/γmt]2
d) Nsc < 5×106 [(27/γmft)γmt].
View Answer
Explanation: Fatigue effect for light and medium duty cranes need not be checked if normal and shear design stress ranges f ≤ (27/γmft) or if actual number of stress cycles, Nsc < 5×106 [(27/γmft)/γmt]3, where f = actual fatigue stress range, γmft = partial safety factor for strength, γmf = partial safety factor for material = 1.10.
7. The maximum wheel load is obtained when
a) crane crab is farthest to gantry girder
b) crane crab is closest to gantry girder
c) crane crab is not attached
d) crane crab is at mid span
View Answer
Explanation: The maximum wheel load is obtained when crane crab is closest to gantry girder. The crab in such position on the crane girder gives maximum reaction on the gantry girder. The vertical reaction of crane girder is transferred through its two wheels on to the gantry girder. Therefore, the maximum wheel load is half of this reaction. This maximum wheel load is then increased for impact and used for design of gantry girder.
8. The bending moment due to dead load of girder is maximum at
a) one-third distance at span
b) two-third distance at span
c) end of span
d) centre of span
View Answer
Explanation: The bending moment considered in the design of gantry girder are the bending moment due to maximum wheel loads (with impact) and the bending moment due to dead load of the girder and rails. The bending moment due to dead loads is maximum at the centre of span.
9. What is the maximum vertical deflection allowed for a gantry girder where the cranes are manually operated?
a) L/500
b) L/700
c) L/600
d) L/800
View Answer
Explanation: The vertical deflection of gantry girder where the cranes are manually operated should not exceed L/500, where L is the span of gantry girder. The maximum vertical deflection allowed for a gantry girder where the cranes are travelling overhead and operated electrically upto 500kN is L/750 and operated electrically over 500kN is L/1000. When gantry girders carry moving loads such as charging cars, the deflection should not exceed L/600.
10. The minimum recommended rise of trusses with Galvanised Iron sheets is
a) 1 in 12
b) 1 in 6
c) 1 in 10
d) 1 in 18
View Answer
Explanation: The pitch of truss depends upon the roofing material. The minimum recommended rise of trusses with galvanised iron sheets is 1 in 6 and with asbestos cement sheets is 1 in 12.
11. The economic spacing of roof truss depends on
a) cost of purlins only
b) cost of purlins and cost of roof covering
c) dead loads
d) cost of roof covering and dead loads
View Answer
Explanation: The economic spacing of the truss is the spacing that makes the overall cost of trusses, purlins, roof coverings, columns, etc. the minimum. It depends upon the relative cost of trusses, purlins, roof coverings, spacing of columns, etc. If the spacing is large, the cost of these trusses per unit area decreases but the cost of purlin increases. But if the spacing of trusses is small, the cost of trusses per unit area increases. Roof coverings cost more if the spacing of trusses is large.
12. Which of the following is true for economic spacing?
a) cost of trusses should be equal to twice the cost of purlins
b) cost of trusses should be equal to twice the cost of purlins minus cost of roof coverings
c) cost of trusses should be equal to the cost of purlins plus cost of roof coverings
d) cost of trusses should be equal to twice the cost of purlins plus cost of roof coverings
View Answer
Explanation: For economic spacing of roof trusses, the cost of trusses should be equal to twice the cost of purlins plus cost of roof coverings. This equation is used for checking the spacing of trusses and not for design of trusses.
13. Which of the following load combination is not considered for design of roof trusses?
a) Dead load + crane load
b) Dead load + wind load
c) Dead load + earthquake load
d) Dead load + live load + wind load
View Answer
Explanation: Earthquake loads are not significant for roof trusses because of the small self weights. The following load combinations can be considered : (i) Dead load + snow load, (ii) Dead load + partial/full live load, (iii) dead load + live load + internal positive air pressure, (iv) dead load + live load + internal suction air pressure, (v) dead load + live load + wind load.
14. Live load for roof truss should not be less than
a) 0.4kN/m2
b) 0.2kN/m2
c) 0.75kN/m2
d) 0.8kN/m2
View Answer
Explanation: The live load for roof truss should not be less than 0.4kN/m2. For roof slopes ≤ 10o and access provided, the live load to be taken is 1.5kN/m2 of plan area. For roof slopes > 10o and access is not provided , the live load to be taken is 0.75kN/m2 of plan area.
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