Chapter 9

9.1 Basis for Design

9.2.1 For the purpose of designing any element, member or a structure, the following loads (actions) and their effects shall be taken into account, where applicable, with partial safety factors and combinations (see 11.3.3).

a) Dead loads;

b) Imposed loads (live load, crane load, snow load, dust load, wave load, earth pressures, etc);

c) Wind loads;

d) Earthquake loads;

e) Erection loads;

f) Accidental loads such as those due to blast, impact of vehicles, etc; and

g) Secondary effects due to contraction or expansion resulting from temperature changes, differential settlements of the structure as a whole or of its components, eccentric connections, rigidity of joints differing from design assumptions.

9.2.1.1 Dead loads should be assumed in design as specified in 2 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code.

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$^{1)}$ Stress at 0.2 percent non-proportional elongation, Min.

9.2.1.2 Imposed loads for different types of occupancy and function of structures shall be taken as recommended in 3 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code. Imposed loads arising from equipment, such as cranes and machines should be assumed in design as per manufacturers/suppliers data (see 9.5.4). Snow load shall be taken as per in 6 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code.

9.2.1.3 Wind loads on structures shall be taken as per the provisions of 4 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code.

9.2.1.3.1 Wind pressure

In calculating the effective wind pressure on exposed circular tube members of a structure, the effective area

shall be taken as 0.6 times the projected area of the member (see 4 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’).

9.2.1.4 Earthquake loads shall be assumed as per the recommendations of 5 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code.

9.2.1.5 The erection loads and temperature effects shall be considered as specified in 9.3 and 9.4, respectively.

All loads required to be carried by the structure or any part of it due to storage or positioning of construction material and erection equipment, including all loads due to operation of such equipment shall be considered as erection loads. Proper provision shall be made,

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including temporary bracings, to take care of all stresses developed during erection. Dead load, wind load and also such parts of the imposed load as would be imposed on the structure during the period of erection shall be taken as acting together with the erection loads. The structure as a whole and all parts of the structure in conjunction with the temporary bracings shall be capable of sustaining these loads during erection.

9.4.1 Expansion and contraction due to changes in temperature of the members and elements of a structure shall be considered and adequate provision made for such effect.

9.4.2 The temperature range varies for different localities and under different diurnal and seasonal conditions. The absolute maximum and minimum temperatures, which may be expected in different localities of the country, may be obtained from the India Meteorological Department and used in assessing the maximum variations of temperature for which provision for expansion and contraction has to be made in the structure.

9.4.3 The range of variation in temperature of the building materials may be appreciably greater or lesser than the variation of air temperature and is influenced by the condition of exposure and the rate at which the materials composing the structure absorb or radiate heat. This difference in temperature variations of the material and air shall be given due consideration. The effect of differential temperature within an element or member, due to part exposure to direct sunlight shall also be considered.

9.4.4 The coefficient of thermal expansion for steel is as given in 8.2.4.1(e).

9.5.1 Load combinations for design purposes shall be those that produce maximum forces and effects and consequently maximum stresses and deformations. The following combination of loads with appropriate partial safety factors (see Table 4) may be considered:

a) Dead load + Imposed load,

b) Dead load + Imposed load + Wind or Earthquake load,

c) Dead load + Wind or Earthquake load, and

d) Dead load + Erection load.

NOTE — In the case of structures supporting cranes, imposed loads shall include the crane effects as given in 9.5.4.

9.5.2 Wind load and earthquake loads shall not be assumed to act simultaneously. The effect of each shall be considered separately.

9.5.3 The effect of cranes to be considered under imposed

loads shall include the vertical loads, eccentricity effects induced by the vertical loads, impact factors, lateral (surge) and the longitudinal (horizontal) thrusts, not acting simultaneously, across and along the crane rail, respectively [see 3 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code].

9.5.4 The crane loads and their combinations to be considered shall be as indicated by the customer. In the absence of any specific indications, the load combinations shall be in accordance with the provisions in 3 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code or as given below:

a) Vertical loads with full impact from one loaded crane or two cranes in case of tandem operation, together with vertical loads without impact from as many loaded cranes as may be positioned for maximum effect, along with maximum horizontal thrust from one crane only or two in case of tandem operation;

b) Loads as specified in 9.5.4 (a), subject to cranes in maximum of any two bays of the building cross-section shall be considered for multi-bay multi-crane gantries;

c) The longitudinal thrust on a crane track rail shall be considered for a maximum of two loaded cranes on the track; and

d) Lateral thrust (surge) and longitudinal thrust acting across and along the crane rail, respectively shall be assumed not to act simultaneously. The effect of each force, shall however be investigated separately.

9.5.5 While investigating the effect of earthquake forces, the resulting effect from dead loads of all cranes parked in each bay, positioned to cause maximum effect shall be considered.

9.5.6 The crane runway girders supporting bumpers shall be checked for bumper impact loads also, as specified by the manufacturers.

9.5.7 Stresses developed due to secondary effects such as handling; erection, temperature and settlement of foundations, if any, shall be appropriately added to the stresses calculated from the combination of loads stated in 9.5.1, with appropriate partial safety factors.

9.6.1 General

The geometrical properties of the gross and the effective cross-sections of a member or part thereof, shall be calculated on the following basis:

a) The properties of the gross cross-section shall be calculated from the specified size of the

member or part thereof or read from appropriate table.

b) The properties of the effective cross-section shall be calculated by deducting from the area of the gross cross-section, the following:

1. The sectional area in excess of effective plate width, in case of slender sections (see 9.7.2).

2. The sectional areas of all holes in the section except for parts in compression. In case of punched holes, hole size 2 mm in excess of the actual diameter may be deducted.

9.7.1 Plate elements of a cross-section may buckle locally due to compressive stresses. The local buckling can be avoided before the limit state is achieved by limiting the width to thickness ratio of each element of a cross-section subjected to compression due to axial force, moment or shear.

9.7.1.1 When plastic analysis is used, the members shall be capable of forming plastic hinges with sufficient rotation capacity (ductility) without local buckling, to enable the redistribution of bending moment required before formation of the failure mechanism.

9.7.1.2 When elastic analysis is used, the member shall be capable of developing the yield stress under compression without local buckling.

9.7.2 On basis of the above, four classes of sections are defined as follows:

a) Class 1 (Plastic) — Cross-sections, which can develop plastic hinges and have the rotation capacity required for failure of the structure by formation of plastic mechanism. The width to thickness ratio of plate elements shall be less than that specified under Class 1 (Plastic), in Table 2.

b) Class 2 (Compact) — Cross-sections, which can develop plastic moment of resistance, but have inadequate plastic hinge rotation capacity for formation of plastic mechanism, due to local buckling. The width to thickness ratio of plate elements shall be less than that specified under Class 2 (Compact), but greater than that specified under Class 1 (Plastic), in Table 2.

c) Class 3 (Semi-compact) — Cross-sections, in which the extreme fiber in compression can reach yield stress, but cannot develop the plastic moment of resistance, due to local buckling. The width to thickness ratio of plate elements shall be less than that specified under Class 3 (Semi-compact), but greater than that specified under Class 2 (Compact), in Table 2.

d) Class 4 (Slender) — Cross-sections in which the elements buckle locally even before reaching yield stress. The width to thickness ratio of plate elements shall be greater than that specified under Class 3 (Semi-compact), in Table 2. In such cases, the effective sections for design shall be calculated either by following the provisions of good practice [6-6(34)] to account for the post-local-buckling strength or by conservatively deducting width of the compression plate element in excess of the semi-compact section limit (see Fig. 2B and Fig. 2C).

The design of slender web elements in flexural members may be done as given in 14.2.1.1 for flexure and 14.4.2.2 for shear.

When different elements of a cross-section fall under different classes, the section shall be classified as governed by the most critical element.

The maximum value of limiting width to thickness ratios of elements for different classifications of sections are given in Fig. 2A. The effective section in case of class 4 (slender elements) may be conservatively taken as shown in Fig. 2B and Fig. 2C.

9.7.3 Types of Elements

a) Internal elements — These are elements attached along both longitudinal edges to other elements or to longitudinal stiffeners connected at suitable intervals to transverse stiffeners; for example web of I-section and flanges and web of box section.

b) Outside elements or outstands — These are elements attached along only one of the longitudinal edges to an adjacent element, the other edge being free to displace out of plane, for example flange overhang of an I-section, stem of T-section and legs of an angle section.

c) Tapered elements — These may be treated as flat elements having average thickness as defined in accepted standard [6-6(35)].

The design of slender compression element (Class 4) considering the strength beyond elastic local buckling of element is outside the scope of this section. Reference may be made to in good practice [6-6(34)] for such design provisions. The design of slender web elements may be made as given in 14.2.1.1 for flexure and 14.4.2.2 for shear.

9.7.4 Compound Elements in Built-up Section (see Fig. 2)

In case of compound elements consisting of two or more elements bolted or welded together, the limiting width

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to thickness ratios as in Table 2 should be considered on basis of the following:

a) Outstanding width of compound element $(b_{\mathrm{c}})$ to its own thickness.

b) The internal width of each added plate between the lines of welds or fasteners connecting it to the original section to its own thickness.

c) Any outstand of the added plates beyond the line of welds or fasteners connecting it to original section to its own thickness.

9.8.1 The maximum effective slenderness ratio, $KL / r$; values of a beam, strut or tension member shall not exceed those given in Table 3. 'KL' is the effective length of the member and 'r' is appropriate radius of

FIG. 2 CLASSIFICATION OF SECTIONS — (Continued)

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2B EFFECTIVE SECTION OF SLENDER MEMBERS UNDER COMPRESSION, PURE

2C EFFECTIVE SECTION OF SLENDER MEMBERS UNDER COMPRESSION, FLEXURAL

FIG. 2 CLASSIFICATION OF SECTIONS

gyration based on the effective section as defined in 9.6.1.