EASWARI ENGINEERING COLLEGE, RAMAPURAM of Steel... · ENGINEERING COLLEGE ... Gantry girders are...

06 Semester Civil CE2352 Design of steel structures. by M.Alaguraj AP / Civil

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI - 621213.

QUESTION BANK WITH ANSWER

DEPARTMENT: CIVIL SEMESTER: 06

SUBJECT CODE /NAME: CE 2352/DESIGN OF STEEL STRUCTURES YEAR: III

UNIT V – ROOFTRUSSES AND INDUSTRIALSTRUCTURES

PART - A (2 marks)

1. What are the loads to be considered for the design of gantry girder? (NOV/DEC 11)

The reaction from the crane girder, acting vertically downwards.

The longitudinal thrust, due to starting or stopping of crane, acting in the longitudinal

direction.

The lateral thrust, due to starting of the crab acting horizontally, normal to the gantry

girder.

2. Give general guidelines for fixing spacing of roof trusses. (NOV/DEC 11)

The spacing between trusses is determined by the required column spacing and by

considerations of minimum cost for the structure as a whole. The usual economical spacing

ranges between 4 and 8m, with the lower limit for short truss spans and the higher limit for long

spans of about 30m or over.

3. With a neat sketch show the various components in a steel roof truss. (MAY/JUNE 12)

4. State the purpose of gantry girders. (MAY/JUNE 12)

Gantry girders are designed as laterally unsupported beams unless the compression flange

is laterally supported by either a catwalk or by additional member.


5. What are the considerations for selection of a suitable type of truss? (MAY/JUNE 12)

The followings are the considerations for selection of a suitable roof truss

Roof coverings

Fabrication and transportation

Aesthetic

climate

.

6. Name the commonly used roof coverings. (MAY/JUNE 12)

The two materials are used as roof coverings:

GI sheet

A.C. sheet

7. What are the types of roof trusses? (NOV/DEC 12)

Kingpost truss

Queen post truss

French truss

North light truss

8. Name the components of steel roof truss. (NOV/DEC 12)

Strut

Principal rafter

Sling

Principle tie

Tie

Purlins

GI or A.C. sheet

9. How to calculate the deflection of trusses?

The deflection of a pin jointed truss can be calculated using the strain energy method. The

deflection at a node is given by.

Δ=ΣPuL/AE

Where,

P=force in a truss member due to applied load

u=force in a truss member due to unit load applied at the node

L=Length of the truss member

A=Area of a truss member

E=young’s modulus of elasticity

10. What are main benefits of using composite floors with profiled steel decking? i. Savings in steel weight are typically 30% to 50% over non-composite Construction ii. Greater stiffness of composite beams results in shallower depths for the same span. Hence lower storey heights are adequate resulting in savings in cladding costs, reduction in wind loading and savings in foundation costs. iii. Faster rate of construction.


11. Explain about the importance of steel decking. • It supports loads during construction and acts as a working platform • It develops adequate composite action with concrete to resist the imposed loading Steel • It transfers in-plane loading by diaphragm action to vertical bracing or shear walls • It stabilizes the compression flanges of the beams against lateral buckling, until concrete hardens. • It reduces the volume of concrete in tension zone • It distributes shrinkage strains, thus preventing serious cracking of concrete 12. What is meant by Composite Beam Stage?

The composite beam formed by employing the profiled steel sheeting is different from the one with a normal solid slab, as the profiling would influence its strength and stiffness. This is termed ‘composite beam stage’. In this case, the profiled deck, which is fixed transverse to the beam, results in voids within the depth of the associated slab. 13. Write short notes on composite slab stage

The structural behavior of the composite slab is similar to that of a reinforced concrete beam with no shear reinforcement. Steel sheeting provides adequate tensile capacity in order to act with the concrete in bending. However, the shear between the steel and concrete must be carried by friction and bond between the two materials. The mechanical keying action of the indents is important. This is especially so in open trapezoidal profiles, where the indents must also provide resistance to vertical separation.

14. What are economical considerations for industrial truss? • Method of fabrication and erection to be followed, facility for shop fabrication available, transportation restrictions, field assembly facilities. Steel f Technology Madras • Preferred practices and past experience. • Availability of materials and sections to be used in fabrication. • Erection technique to be followed and erection stresses. • Method of connection preferred by the contractor and client (bolting, welding or riveting). • Choice of as rolled or fabricated sections. • Simple design with maximum repetition and minimum inventory of material. 15. Write about basics of plastic analysis?

In plastic analysis and design of a structure, the ultimate load of the structure as a whole is regarded as the design criterion. The term plastic has occurred due to the fact that the ultimate load is found from the strength of steel in the plastic range.

This method is rapid and provides a rational approach for the analysis of the structure. It also provides striking economy as regards the weight of steel since the sections required by this method are smaller in size than those required by the method of elastic analysis. 16. What is meant by first yield moment?

As W is increased gradually, the bending moment at every section increases and the stresses also increase. At a section close to the support where the bending moment is maximum, the stresses in the extreme fibers reach the yield stress. The moment corresponding to this state is called the first yield moment My, of the cross section. 17. Write about Principles of plastic analysis (i) Mechanism condition: The ultimate or collapse load is reached when a mechanism is formed. The number of plastic hinges developed should be just sufficient to form a mechanism.


(ii) Equilibrium condition: ΣFx = 0, ΣFy = 0, ΣMxy = 0 (iii) Plastic moment condition: The bending moment at any section of the structure should not be more than the fully plastic moment of the section. 18. State that Uniqueness theorem

If both the above criteria are satisfied, then the resulting load factor corresponds to its value at rigid plastic collapse (λp). 19. Explain about Crane gantry girders

The function of the crane girders is to support the rails on which the traveling cranes move. These are subjected to vertical loads from crane, horizontal lateral loads due to surge of the crane, that is, the effect of acceleration and braking of the loaded crab and swinging of the suspended load in the transverse direction, and longitudinal force due to acceleration and braking of the crane as a whole.

20. What are the different types of floors used in steel-framed buildings? a) Concrete slabs supported by open-web joists? b) One-way and two-way reinforced concrete slabs supported on steel beams c) Concrete slab and steel beam composite floors d) Profiled decking floors e) Pre-cast concrete slab floors. 21. What are the advantages of steel-decking floors? (i) They do not need form work (ii) The lightweight concrete is used resulting in reduced dead weight (iii) The decking distributes shrinkage strains, thus prevents serious cracking (iv) The decking stabilizes the beam against lateral buckling, until the concrete (v) The cells in decking are convenient for locating services. 22. Write about Pre-cast concrete floors

Pre-cast concrete floors offer speedy erection and require only minimal formwork. Light-weight aggregates are generally used in the concrete, making the elements light and easy to handle. Typical pre-cast concrete floor slab sections are shown in Fig. It is necessary to use cast in place mortar topping of 25 to 50 mm before installing other Floor coverings. Larger capacity cranes are required for this type of construction when compared with those required for profiled decking. Usually pre-stressing of the pre-cast an element is also done.

23. What are assumptions are made for arrangement of live load in the analysis of frames? a) Consideration is limited to combination of:

i) Design dead load on all spans with full design live load on two adjacent spans and


ii) Design dead load on all spans with full design live load on alternate pans. b) When design live load does not exceed three-fourths of the design dead load, the load arrangement of design dead load and design live load on all the spans can be used. 24. Explain about Drift Analysis

Drift in building frames is a result of flexural and shear mode contributions, due to the column axial deformations and to the diagonal and girder deformations, respectively. In low-rise braced structures, the shear mode displacements are the most significant and, will largely determine the lateral stiffness of the structure.

In medium to high-rise structures, the higher axial forces and deformations in the columns, and the accumulation of their effects over a greater height, cause the flexural component of displacement to become dominant.


PART B (16 MARKS)

1. Explain detail the steps involved in the design of channel purlin of a steel roof truss.

(MAY/JUNE 12)


2. i) list out the various elements of the roof truss and mark all its significance. (MAY/JUNE 12)

Principal rafter

Purlins

Sheeting’s

Principal tie

Struts Principal rafter Principal rafter is the top chord member of the truss and is subjected to compressive forces when the loads from purlins are transferred at the nodes. Purlins As far as possible purlins should be located on panel points of top chord members. However it depends upon the type of roofing materials also. Generally the spacing of purlins varies from 1.35 to 2.0m. Sheeting’s Commonly used sheeting’s are GI and A.C. GI sheets Corrugated iron sheets are galvanized for protection against corrosion and are used as roof coverings. The common sizes of GI sheets are:

8 corrugations, 75mm wide and 19mm deep which have overall width of 660mm.

10 corrugations, 75mm wide and 19mm deep, which have overall width of 810mm.


A.C.sheets Asbestos cement sheets are better insulators for sun’s heat compared to GI sheet. They are used commonly in the factories and godowns. They are available in two common shapes, corrugated and traffold. Principal tie The lower chord of the truss is principal tie and carries tension. The long middle tie member of the truss may sag because of its self weight and to check this sag tie may be provided. Struts Struts are the members of the truss which are under compression while those bearing tension are called ties. ii) Explain the design principles of gantry girder. (MAY/JUNE 12) The step by step design procedure of a gantry girder is as follows:

The maximum wheel load is determined.

The maximum bending moment in the gantry girder due to vertical loads.

The maximum shear force is computed. This consists of shear force due to wheel load and dead loads from the gantry girder and rails. The shear due to the wheel load is maximum when one of the wheels is at the support.

The lateral forces on the girder and the maximum bending moments and shear due to these are calculated. The position of the wheels should be same as that in step 1 and step3.

The plastic section modulus of the trial section is determined by Mp=fyx2XA/2Xy=ZPfy ZP,REQ=(1.4 to 1.5)Mu/fy

Section is classified.

The girder is checked for moment capacity.

Local moment carrying capacity of the girder is checked.

The girder is checked for buckling resistance.

The section is checked for shear capacity.

Buckling of the web under wheel load is checked.

The girder is checked for bearing.

Rivets/bolts or welds connecting the channel to the I section are designed.

Deflection of the gantry girder is checked.

The girder is checked for fatigue strength.

The bracket and the connection with column are designed. 3. Design an I section purlin for an industrial building to support a galvanized corrugated iron sheet roof. Given: Spacing of the trusses=5.0m Spacing of purlins=1.5m Inclination of main rafter to horizontal=300 Weight of galvanized sheet taking into account laps and connecting bolts=130N/m2 Imposed load=1.5kN/m2

Wind load= 1.0kN/m2


4. Design angle purlin for the following data by simplified method: Spacing of trusses=4m Spacing of purlins=1.6m Weight of A.C sheets including laps and fixtures=0.205kN/m2 Live load=0.6 kN/m2 Wind load=1 kN/m2 Inclination of main rafter of truss=210


5. Design a welded plate girder of span 30m to carry on superimposed load of 35kN/m. avoid use of bearing and intermediate stiffeners. Use Fe415 steel.


6. Design a welded plate girder of span 30m to carry on superimposed load of 35kN/m. use intermediate stiffeners. Use Fe415 steel.


7. design a simply supported gantry girder to carry one electric overhead travelling crane, given Span of the gantry crane=6.5m Span of the crane girder=16m Crane capacity=250kN Self weight of crane excluding trolley=280 kN Self weight of the trolley=50 kN Minimum hook approach=1.0m Distance between wheels=3.5m Self weight of rails=0.3 kN/m


8. A roof truss shed is to be built in lucknow for an industry. The size of shed is 24mx40m.the height of building is 12m at the eves. Determine the basic wind pressure.


9. A power plant structure having maximum dimension more than 60m is proposed to build on downhill side near dehradun. The height of the hill is 400m with a slope of 1 in 3.the location is 250m from the crest of the hill on downward slope, and its eve board is at a height of 9m, determine the design wind pressure.


10. Design a truss of span 15m spacing 4m to be built near Visakhapatnam with the following details: Class of building=general with life of 50 years Terrain: category 2 Max.dimension:40m Width of building: 15m Height at eve level: 8m Topography: less than 300

EASWARI ENGINEERING COLLEGE, RAMAPURAM of Steel... · ENGINEERING COLLEGE ... Gantry girders are...

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