(SEM-VII) THEORY EXAMINATION 2018-19 DESIGN OF STEEL STRUCTURES

DESIGN OF STEEL STRUCTURES (NCE-701 / ECE-701)

B.Tech – Semester VII

SECTION A

(Attempt all questions)

(a) Difference between factor of safety and partial safety factor

The factor of safety is a traditional design concept used in working stress method, where a single factor is applied to material strength to ensure safety against failure. It does not distinguish between different uncertainties.
Partial safety factors are used in limit state design and are applied separately to loads and material strengths. This approach accounts for variability in loads, material properties, and construction quality, making the design more rational, economical, and reliable.

(b) Disadvantages of steel as a structural material

Although steel has high strength and durability, it also has certain disadvantages. Steel structures are prone to corrosion when exposed to moisture and aggressive environments, requiring regular maintenance and protective coatings. Steel loses strength rapidly at high temperatures, making it vulnerable during fire unless fire protection is provided. Additionally, steel structures may suffer from buckling under compressive loads if not properly designed.

(c) Meaning of 4 and 6 in bolt grade 4.6

In bolt grade 4.6, the first digit represents one-hundredth of the ultimate tensile strength in MPa, while the second digit represents the ratio of yield strength to ultimate strength. Thus, a grade 4.6 bolt has an ultimate tensile strength of 400 MPa and a yield strength equal to 0.6 times the ultimate strength, i.e., 240 MPa.

(d) Preference of fillet welds over butt welds

Fillet welds are preferred over butt welds because they are easier to execute, require less edge preparation, and are more economical. Fillet welds can accommodate minor inaccuracies in fabrication and alignment. They are widely used in structural connections where simplicity and speed of construction are important.

(e) Shear lag effect

Shear lag is a phenomenon in tension members where stress distribution is non-uniform across the cross-section due to the manner of load transfer through connections. Portions of the section farther from the connection experience lower stress. This effect reduces the effective area of the member and must be considered while designing tension members.

(f) Lug angle

A lug angle is an auxiliary angle section connected to the main tension member to provide additional load-carrying capacity or to reduce the length of connection. It helps in improving stress distribution and enhancing the strength of bolted or welded connections.

(g) Inelastic buckling

Inelastic buckling occurs when a column buckles at stress levels beyond the elastic limit of the material. This type of buckling is common in short and intermediate columns where yielding begins before buckling failure. It is accounted for using design curves in steel codes.

(h) Effective length of a column

The effective length of a column is the length between points of zero bending moment, depending on end support conditions. It represents the length over which the column is assumed to buckle. Effective length plays a crucial role in determining slenderness ratio and buckling strength.

(i) Use of I-sections as beams despite low lateral buckling strength

I-sections are commonly used as beams because they efficiently resist bending by placing most material away from the neutral axis. Although their lateral buckling strength is low, this drawback is overcome by providing lateral restraints, making I-sections economical and structurally efficient.

(j) Purpose of a gantry girder

A gantry girder is used to support overhead cranes in industrial buildings. It transfers crane loads, including vertical loads, horizontal forces, and impact loads, safely to the columns and foundation. Proper design ensures smooth and safe crane operation.

SECTION B

(Attempt any three)

(a) Loads acting on steel structures

Steel structures are subjected to various loads such as dead loads, live loads, wind loads, seismic loads, and temperature effects. Dead loads include self-weight of structural members, while live loads arise due to occupancy or moving loads. Wind and seismic loads are environmental loads that cause lateral forces and vibrations. Proper load assessment ensures safety and serviceability of structures.

(b) Working stress method, ultimate strength method, and limit state design

The working stress method limits stresses within elastic range using a factor of safety. The ultimate strength method considers failure loads but lacks serviceability control. Limit state design combines safety and serviceability by checking both ultimate and serviceability limit states using partial safety factors. It is the most widely adopted modern design method.

(c) Design philosophy of steel structures as per IS 800:2007

IS 800:2007 follows the limit state design philosophy, ensuring safety against collapse and satisfactory performance under service conditions. The code specifies design criteria, load combinations, material properties, and safety factors. It provides a comprehensive and rational framework for steel structure design.

SECTION C

(Attempt any one)

(a) Loads acting on steel structures with emphasis on environmental loads

Environmental loads include wind, earthquake, snow, and temperature effects. Wind loads cause lateral forces and uplift, earthquake loads induce dynamic forces, and temperature variations cause expansion and contraction. These loads significantly influence structural design, especially for tall and flexible steel structures, and must be carefully evaluated.

(b) Design of welded tension member and block shear failure

A welded tension member must be designed to resist yielding, rupture, and block shear failure. Block shear occurs when a portion of the member fails due to combined tension and shear along a defined failure path. The design strength is calculated by considering both shear and tensile resistances as per IS 800 provisions, ensuring safe and ductile behavior.

(c) Design of steel columns and slab base

Steel columns are designed based on axial load, slenderness ratio, and buckling strength. A slab base distributes column load uniformly to the concrete foundation. The design involves checking bearing pressure, base plate thickness, and anchor bolts to ensure stability and safety.