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SEISMIC BUILDING CODE OF PAKISTAN

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SEISMIC BUILDING CODE OF PAKISTAN

NED UNIVERSITY OF ENGINEERING & TECHNOLOGY

SITE CONSIDERATIONS

- Chapter 3 highlights different types of soil hazards that can damage a structure, in case of an earthquake.
- In conjunction some outlines are provided in order to select a site as to avoid maximum damage from these hazards.
- These hazards are listed as ;
- Fault rupture hazard
- Liquefaction
- Landslide and Slope instability
- Sensitive clays

SOILS AND FOUNDATIONS

- Chapter 4 emphasizes on the component where the SSI (Soil-Structure-Interaction) takes place.
- Sections 4.1 – 4.3 define the different terminologies and terms used in the Chapter.
- However, the core information is divided into the rest of the sections which forms the backbone of the chapter.
- 4.4 – Soil Profiles
- 4.5 – Requirements for Foundation
- 4.6 – Seismic Soil Pressures and Soil Retaining Structures

- Soil profile development procedures are identified here.
- Vs Method (Average shear wave velocity method)
- N Method (Average field penetration resistance method)
- Su Method (Average undrained shear strength method)

- Foundation requirements in different conditions are presented here, as to make certain that the underlying soil does not impose significant damage on the superstructure.
- Rules given in this Chapter for foundations are applicable to the foundations of reinforced concrete, structural steel, timber and masonry buildings.
- Some of the topics discussed are:
- Foundation Construction in Areas in Seismic Zones 2, 3, 4
- Superstructure-to-Foundation Connection
- Piles, Caps and Caissons
- Foundation Tie Beams
- Wall Foundations of Masonry and Timber Buildings
- Footings on or adjacent to Slopes

- This section presents different soil pressure coefficients (and distribution) at rest and incase of an earthquake, on retaining structures for design and analysis purposes. Such as:
- Total Active and Passive Pressure Coefficients
- Dynamic Active and Passive Soil Pressures
- Dynamic Soil Pressures in Layered Soils

- Factor of safety against sliding (F.S. = 1.1)
- Factor of safety against over-turning (F.S. = 1.3)
- Reduction factor to convert the dynamic internal forces applicable for section design of RCC (RZA = 1.5) and Steel sheet piles (RZA = 2.5).

STRUCTURAL DESIGN REQUIREMENTS

- Chapter 5 is divided into five sub divisions
- Division I – General Design Requirements
- Division II – Snow Loads
- Division III – Wind Design
- Division IV – Earthquake Design
- Division V – Soil Profile Types

- This division provides the general design requirements applicable to all structures.
- Sections 5.1 to 5.4 present a general description of the terminologies used in the division.
- Section 5.5 presents the requirements to achieve a stable structure, discussing issues such as; complete load path, overturning, distribution of horizontal shear force, anchorage, etc.
- Section 5.6 defines the partition loads on buildings and access floor system as 21 psf & 10.5 psf, respectively.
- Section 5.7 defines the live loads and their distribution on the floors according to different occupancies, enlisted in Table 5-A.
- Along side it also discusses the cases for live load reduction as given by the following equation:

TABLE 5-A – UNIFORM AND CONCENTRATED LOADS

- Section 5.11 (Other Minimum Loads) provides description of other loads and some other guidelines, such as;
- Impact loads
- Interior wall loads
- Retaining walls
- Water accumulation
- Heliport and heli-stop landings

- 1.4 D (5.12-1)
- 1.2 D + 1.6 L + 0.5 (Lr or S) (5.12-2)
- 1.2 D + 1.6 (Lror S) + (f1L or 0.8 W) (5.12-3)
- 1.2 D + 1.3 W + f1L + 0.5 (Lr or S) (5.12-4)
- 1.2 D + 1.0 E + (f1L + f2S) (5.12-5)
- 0.9 D ± (1.0 E or 1.3 W) (5.12-6)

- The allowable design combinations being;
- D (5.12-7)
- D + L + (Lrr or S) (5.12-8)
- D + (W or E / 1.4) (5.12-9)
- 0.9 D ± E / 1.4 (5.12-10)
- D + 0.75 [L+ (Lror S) + (W or E / 1.4)] (5.12-11)

- 1.2 D + f1 L+ 1.0 Em (5.12-17)
- 0.9 D ± 1.0 Em (5.12-18)

- UBC-97 is referred for calculating the minimum design load:
- Pf= Ce I Pg(40-1-1)

- Where:
- Ce = snow exposure factor (see Table A-16-A).
- I = importance factor (see Table A-16-B).
- Pg =basic ground snow load, psf (N/m2) – (For 50-year mean recurrence interval maps)

- Rs = S/40-0.024

- Rs = S/40-1/2

- Rs = snow load reduction in kilo-Newton per square meter (lb/ft2) per degree of pitch over 20 degrees.
- S = total snow load in kilo-Newton per square meter (lb/ft2).

- 5.20 defines the wind pressure on a surface as:
P = CeCqqsIw(5.20-1)

- Where
- Ce = combined height, exposure and gust factor coefficient as given in Table 5-G.
- Cq = pressure coefficient for the structure or portion of structure under consideration as given in Table 5-H.
- Iw = importance factor as set forth in Table 5-K.
- P = design wind pressure.
- qs = wind stagnation pressure at the standard height of 10 meters (33 feet) as set forth in Table 5-F.

- Where
- Unless detailed wind data is available;
- All the structures inland shall be designed to resist a wind velocity of not less than 144 km per hour (90 mph) at a height of 10 meters (33 ft)
- All the structures along the coast shall be designed to resist a wind velocity of not less than 180 km per hour (109 mph) at a height of 10 meters (33 ft).

- 5.21: The primary frames or load-resisting system of every structure shall be designed for the pressures calculated using Formula (5.20-1) and the pressure coefficient, Cq, of either Method 1 (Normal Force Method) or Method 2 (Projected Area Method)

- Table 5-G

- Table 5-H

- Table 5-K

- Table 5-F

- Sections 5.26 to 5.28 provide some basic definitions and notations used in the chapter. Some of them being:
- Design Basis Ground Motion is that ground motion that has a 10 percent chance of beingexceeded in 50 years as determined by a site-specific hazard analysis or may be determined from a hazard map.
- Design Response Spectrum is an elastic response spectrum for 5 percent equivalent viscous damping used to represent the dynamic effects of the Design Basis Ground Motion for the design of structures in accordance with Sections 5.30 and 5.31.
- Soft Storey is one in which the lateral stiffness is less than 70 percent of the stiffness of the storey above.
- Weak Storey is one in which the storey strength is less than 80 percent of the storey above.

- The procedures and the limitations for the design of structures are described here considering seismic zoning, site characteristics, occupancy, configuration, structural system and height.
- 5.29.6 - Structural systems
- Bearing wall system. A structural system without a complete vertical load-carrying space frame.
- Building frame system. A structural system with an essentially complete space frame providing support for gravity loads. Resistance to lateral load is provided by shear walls or braced frames.
- Moment-resisting frame system. A structural system with an essentially complete space frame providing support for gravity loads. Moment-resisting frames provide resistance to lateral load primarily by flexural action of members.
- Dual system. Resistance to lateral load is provided by shear walls or braced frames and moment resisting frames (SMRF, IMRF, MMRWF or steel OMRF). The moment-resisting frames shall be designed to independently resist at least 25 percent of the design base shear.

- Section 5.29.8 provides the criteria to choose the procedure of lateral force analysis.
- Simplified Static :
- Buildings of any occupancy (including single-family dwellings) not more than three storeys in height excluding basements that use light-frame construction. And other buildings not more than two storeys in height excluding basements.

- Static:
- All structures, regular or irregular, in Seismic Zone 1 and in Occupancy Categories 4 and 5 in Seismic Zone 2.
- Regular structures under 73.0 meters (240 feet) in height with lateral force resistance provided by systems listed in Table 5-N. And irregular structures not more than five storeys or 20 meters (65 feet) in height.
- Structures having a flexible upper portion supported on a rigid lower portion where both portions of the structure considered separately can be classified as being regular.

- Dynamic:
- The dynamic lateral-force procedure of Section 5.31 shall be used for all other structures.

- Simplified Static :

- Design Base Shear – (Equation 5.30-4)

(Seismic Coefficient, depending on seismic zone and soil profile type)(Table 5-R)

(Importance factor, depending on the use of the building)(Table 5-K)

(The total Seismic Dead Load) (Section 5.30.1.1)

≤

≤

(Design Base Shear)

(Response modification factor, representing Over Strength and global ductility capacity of lateral force-resisting systems)(Table 5-N)

(Elastic fundamental Time Period of the structure) To be calculated as stated in Section 5.30.2.2

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- 5.30.2.2 Structure Period:The fundamental time period T shall be determined from equation 5.30-8 or from equation 5.30-9

Method A

(5.30-8)

Method B

(5.30-9)

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Seismic dead load, W, is the total dead load and applicable portions of other loads listed below.

- 1. In storage and warehouse occupancies, a minimum of 25 percent of the floor live load shall be applicable.
- 2. Where a partition load is used in the floor design, a load of not less than 0.48 kN/m2 (10 psf) shall be included.
- 3. Design snow loads of 1.44 kN/m2 (30 psf) or less need not be included. Where design snow loads exceed 1.44 kN/m2 (30 psf), the design snow load shall be included, but may be reduced up to 75 percent where consideration of siting, configuration and load duration warrant when approved by the building official.
- 4. Total weight of permanent equipment shall be included.

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- 5.30.5 – Vertical distribution of force:

Where Ftis the concentrated force at the top: Ft = 0.07 TV

- There fore the force at a level x is:

(5.30-15)

- 5.30.6 – Horizontal distribution of force:
- The design storey shear, Vx, shall be distributed to the various elements of the vertical lateral-force-resisting system in proportion to their rigidities

- The maximum inelastic drift is to be calculated by:
∆M = 0.7 R ∆S (5.30-17)

- Where∆S is the drift computed from the elastic analysis of the frame, using load combinations in section 5.12

- 5.31.2-Ground Motions: The ground motion representation shall be one
- having a 10-percent probability of being exceeded in 50 years,
- shall not be reduced by the quantity R
- and may be one of the following:
- An elastic design response spectrum constructed using the values of Caand Cv consistent with the specific site.
- A site-specific elastic design response spectrum based on the geologic, tectonic, seismologic and soil characteristics associated with the specific site. (for a damping ratio of 0.05)
- Ground motion time histories developed for the specific site shall be representative of actual earthquake motions.
- The vertical component of ground motion may be defined by scaling corresponding horizontal accelerations by a factor of two-thirds.

- The basic soil profile types are the same as defined in section 4.4
- 5.36.2.1 - Average shear wave velocity may be computed as:

(5.36-1)

- 5.36.2.2 - Average Field Penetration resistance may be computed as:

- 5.36.2.3 - Average Un-drained Shear Strength may be computed as: