Chapter 18Design and Detailing for Earthquake Loads

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 1038

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18.1 General

Steel frames shall be so designed and detailed as to give them adequate strength, stability and ductility to resist severe earthquakes in all zones classified in 5 of Part 6 'Structural Design, Section 1 Loads, Forces and Effects' of the Code without collapse. Ductility requirements in 18.4 to 18.11 and 18.13 need not be satisfied in case of non-ductile frames as defined in 18.12. Frames, which form a part of the gravity load resisting system but are not intended to resist the lateral earthquake loads, need not satisfy the requirements of this section, provided they can accommodate the resulting deformation without premature failure.

18.2Load and Load Combinations

18.2.1 Earthquake loads shall be calculated as per 5 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code, except that the reduction factors recommended in 18.3 may be used.

18.2.2 In the limit state design of frames resisting earthquake loads, the load combinations shall conform to Table 4.

18.2.3 In addition the following load combination shall be considered as required in 18.5.1.1, 18.7.3.1 and 18.11.2.2:

a) 1.2 Dead Load (DL)+ 0.5 Imposed Load (IL) ± 2.5 Earthquake Load (EL); and

b) 0.9 Dead Load (DL) ± 2.5 Earthquake Load (EL).

NOTE — The above combinations need to be considered for only the design axial forces of members, ignoring simultaneous bending moments.

18.3Response Reduction Factor

For structures designed and detailed as per the provision of this section, the response reduction factors specified in Table 23 may be used in conjunction with the provision in 5 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code for calculating the design earthquake forces.

Table 23Response Reduction Factor, R, for Building System

Sl No.(1)	Lateral Load Resisting System(2)	R(3)
i)	Braced frame systems:
	a) Ordinary concentrically braced frame (OCBF)	4.0
	b) Special concentrically braced frame (SCBF)	4.5
	c) Ordinary eccentrically braced frame (OEBF)	4.5
	d) Special eccentrically braced frame (SEBF)	5.0
ii)	Moment frame system:
	a) Ordinary moment resisting frame (OMRF)	4.0
	b) Special moment resisting frame (SMRF)	5.0
iii)	Non-ductile frames	1.5

The braced and moment resisting frames can be designed as non-ductile frames, if their overall height to overall width ratio does not exceed 1.0, such as in industrial buildings. Such frames need not meet the ductility requirements of other types of frames as specified in 18.4 to 18.11 and 18.13.

18.4Connections, Joints and Fasteners

18.4.1 All bolts used in critical zones of frames designed to resist earthquake loads shall be fully tensioned High Strength Friction Grip (HSFG) bolts or turned and fitted bolts. Critical zones of frames are zones of plastic hinges of moment resisting frames and beam to column as well as bracing end connections of braced frames. The plastic hinge zones shall be taken as a distance equal to 2 times the depth of the member from the section of maximum bending moment.

18.4.2 All weld connections used in the critical zones of frames designed to resist earthquake loads shall be complete penetration butt welds, except in column splices, which shall conform to 18.5.2.

18.4.3 Bolted joints shall be designed not to share load in combination with welds on the same faying surface.

PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL

18.5Columns

18.5.1 Column Strength

When $P_{\mathrm{r}} / P_{\mathrm{d}}$ is greater than 0.4, the requirements in 18.5.1.1 and 18.5.1.2 shall be met,

where

$P_{r}$ = required compressive strength of the member as evaluated for load combination in Table 4, and

$P_{d}$ = design strength in axial compression as obtained from 13.1.2.

18.5.1.1 The required axial compressive and axial tensile strength, shall be determined from the load combination in 18.2.3, without considering the simultaneous moments as obtained from the analysis.

18.5.1.2 The required axial strength determined in 18.5.1.1 need not exceed the maximum load transferred to the column considering 1.2 times the design strength of the connecting beam or brace element, or the resistance of the foundation to uplift.

18.5.2 Splices

18.5.2.1 A partial-joint penetration groove weld may be provided in axially loaded member splice, such that the design strength of the joints shall be at least equal to 200 percent of the required strength.

18.5.2.2 The minimum required strength for each flange splice shall be 1.2 times $f_{y}A_{f}$ as shown in Fig. 20, where $A_{f}$ is the smaller of the area of each flange in the connected columns, and 1.2 times $f_{y}A_{w}$ as minimum required strength in cases where web splices are provided.

18.5.2.3 The bolted column splice shall meet the requirements of 18.4 and 18.5.1.

18.6 Storey Drift

The storey drift limits shall conform to 5 of Part 6 'Structural Design, Section 1 Loads, Forces and Effects' of the Code. The deformation compatibility of members not designed to resist seismic lateral load shall also

FIG. 20 PARTIAL PENETRATION GROOVE WELD IN COLUMN SPLICE

conform to 5 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code.

18.7Ordinary Concentrically Braced Frames (OCBF)

18.7.1 Ordinary concentrically braced frames (OCBF) should be shown to withstand inelastic deformation corresponding to a joint rotation of at least 0.02 radians without degradation in strength and stiffness below the full yield value. Ordinary concentrically braced frames meeting the requirements of this section shall be deemed to satisfy the required inelastic deformation.

18.7.1.1 Ordinary concentrically braced frames shall not be used in seismic zones IV and V and for buildings with importance factor greater than unity (I >1.0) in Seismic Zone III.

18.7.1.2 The provision in this Section apply for diagonal and X-bracing only. Specialist literature may be consulted for V and inverted V-type bracing. K-bracing shall not be permitted in systems to resist earthquake.

18.7.2 Bracing Members

18.7.2.1 The slenderness of bracing members, columns and beams shall not exceed 120.

18.7.2.2 The required compressive strength of bracing member shall not exceed 0.8 times $P_{d}$ , where $P_{d}$ is the design strength in axial compression (see 13.1.2)

18.7.2.3 Along any line of bracing, braces at all levels shall be provided such that for lateral loading in either direction, the tension braces will have to resist between 30 and 70 percent of the total floor shear at that level.

18.7.2.4 Bracing cross-section can be plastic, compact or semi-compact, but not slender, as defined in 9.7.2.

18.7.2.5 For all built-up braces, the spacing of tack fasteners shall be such that the unfavourable slenderness ratio of individual element, between such fasteners, shall not exceed 0.4 times the governing slenderness ratio of the brace itself. Bolted connections shall be avoided within the middle one-fourth of the clear brace length (0.25 times the length in the middle) in such built-up braces.

18.7.2.6 The bracing members shall be designed so that gross area yielding (see 12.2) and not the net area rupture (see 12.3) would govern the design tensile strength.

18.7.3 Bracing Connections

18.7.3.1 End connections in bracings shall be designed to withstand the minimum of the following:

a) A tensile force in the bracing equal to $1.2f_{\mathrm{y}}A_{\mathrm{g}}$

b) Force in the brace due to load combinations in 18.2.3; and

c) Maximum force that is transferred to the brace by the system, based on analysis.

18.7.3.2 The connection should be checked for tension rupture and block shear under the load determined in 18.7.3.1.

18.7.3.3 The connection shall be designed to withstand a moment of 1.2 times the full plastic moment of the braced section about the buckling axis.

18.7.3.4 Gusset plates shall be checked for buckling out of their plane.