building code of Pakistan

The seismic design of buildings and other similar structures shall be carried out in accordance with Building Code of Pakistan (Seismic Provisions 2007). The key requirements of the Code are described hereunder: 1.0 GENERAL 1.1 Scope 1.1.1 Seismic provisions are applicable to reinforced concrete, structural steel and masonry buildings and building-like structures. 1.1.2 Requirements for structures outside the scope of these provisions shall be established by the Departments/Autonomous Organizations supervising the construction of such structures. 1.2 Seismic Hazard 1.2.1 Unless otherwise required, buildings shall be designed for a level of earthquake ground motion that has a 10% probability of exceedance in 50 years. 1.2.2 For the purpose of seismic design of buildings, Pakistan has been divided into five zones based on ground accelerations as specified in 1.2.1 above. The seismic zones are designated as Zone 1, Zone 2A, Zone 2B, Zone 3 and Zone 4. The seismic zoning map of Pakistan is given in Annexure – A. 2 1.2.3 The requirements of the seismic zoning map shall be superseded if a specific hazard analysis, probabilistic, deterministic or both, is carried out for a building or structure. 1.3 Site Considerations 1.3.1 Important buildings shall not be located within 200 m on either side of an active fault. 1.3.2 Liquefaction potential of subsurface shall be studied through geotechnical investigations for all important buildings. 1.3.3 In mountainous terrain stability of slopes around the building shall be investigated and improved, if required. 1.3.4 For sites comprising sensitive clays mitigation measures shall be adopted on the basis of detailed geotechnical investigations. 1.4 Soils and Foundations 1.4.1 Soil conditions for a building to be constructed in seismic zones shall be categorized in five types of soil profiles namely SA, SB, SC, SD, SE and SF as specified in Table 1 of Annexure B. 1.4.2 In seismic Zone 3 and 4 the short term dynamic nature of loads may be taken into account in establishing soil properties used for foundation design. 1.4.3 Non-prestressed concrete piles in seismic Zone 3 and 4 shall have transverse reinforcement meeting requirements for “Special Moment Frame Members subjected to bending and axial load”. 1.4.4 Prestressed concrete piles in seismic Zone 3 and 4 shall have a minimum volumetric ratio of spiral reinforcement not less than 0.021 for 350 mm square and smaller piles unless a smaller value may be justified by rational analysis. Interpolation may be used between specified ratios for intermediate sizes. 3 1.5 Structural Design Requirements 1.5.1 A building shall be designed for all loads anticipated during its service life including but not limited to dead load, live load, snow load, wind load, earthquake load, temperature effects etc. 1.5.2 Until a detailed wind speed data is available, all the structures in land shall be designed to resist a minimum wind velocity of not less than 120 km per hour (75 miles per hour) and all structures along the coast shall be designed to resist a wind velocity of not less than 130 km/hr at a height of 10m. 1.5.3 For seismic analysis of simple buildings equivalent static load method shall be used whereas for complex and high-rise buildings dynamic analysis procedures should be adopted. 1.5.4 REQUIREMENTS FOR SEISMIC ZONE 1 (Low Seismic Risk) There are no special seismic requirements for Zone 1. 1.5.5 REQUIREMENTS FOR SEISMIC ZONE 2 (Moderate Seismic Risk) • There is no limit on height of buildings. • The reinforcing steel shall conform to ASTM A615. • The mortar used for masonry construction shall have an average crushing strength not less than 750 psi • The structural concrete shall have a minimum compressive strength of 17.25 MPa (2500 psi) at 28 days. • Concrete blocks (solid or hollow) shall meet the following strength requirements calculated on net area. Individual unit – Minimum 1700 psi Average of 3 units – Minimum 1900 psi 4 • Clay brick shall have following minimum strengths Individual unit – Minimum 1250 psi Average of five units – Minimum 1500 psi • For reinforced concrete structures Intermediate or special moment frames, or ordinary, intermediate, or special structural walls, shall be used to resist forces induced by earthquake motions. Where the design seismic loads are computed using provisions for special concrete system, the requirements for special system shall be satisfied. • For structural steel buildings at least Intermediate moment frames and Ordinary concentrically braced frames expected to withstand limited inelastic deformations, shall be provided. • Masonry constructed in seismic Zone 2 shall have at least one #4 vertical bar at each corner, at each side of each opening and at a maximum spacing of 4’-0” c/c. Horizontal bars of same size shall be provided at bottom and top of wall openings, at roof or floor slab and at bottom of wall or top of foundation. The distance between horizontal bars shall not exceed 10’. • As an alternate to above requirements is a combination of horizontal seismic bands (plinth, lintel and roof level) and vertical bars. 1.5.6 REQUIREMENTS FOR SEISMIC ZONE 3 and 4 (High Seismic Risk) • The minimum compressive cylinder strength of concrete shall not be less than 21 MPa (3000 psi) at the age of 28 days for all structures constructed in seismic Zone 3 and Zone 4. • The reinforcement resisting earthquake forces in concrete frame members and structural walls lying in seismic Zone 3 and Zone 4 shall comply with ASTM A 706. 5 The reinforcement conforming to ASTM A 605 Grade 40 and Grade 60 shall be permitted in these members, if a) The actual yield strength based in mill tests does not exceed fy by more than 124 Mpa (18000 psi) – retests shall not exceed this value by more than an additional 21 MPa (3000 psi) and b) The ratio of actual tensile strength to the actual yield strength is not less than 1.25. • Special moment frames, special structural walls, and diaphragms and trusses shall be used to resist forces induced by earthquake motions. • The mortar used for masonry construction shall have an average crushing strength not less than 1800 psi for seismic Zone 3 and 4. • The value of fyt for transverse reinforcement shall not exceed 420 Mpa (60,000 psi) • The seismic provisions for structural steel shall apply in conjunction with AISC “Specifications for Structural Steel Buildings – ANSI/ AISC 360-5”. The provisions shall apply only when seismic response modification factor R is taken greater than 3, regardless of seismic design category. Structural system for steel structures may comprise Special moment frames, Special truss moment frames, Special concentrically braced frames, Eccentrically braced frames, Buckling – restrained braced frames, Special plate shear walls. All these systems are expected to withstand significant inelastic deformation. 6 • Concrete blocks (solid or hollow) shall meet the following strength requirements calculated on net area. Individual unit – Minimum 1700 psi Average of 3 units – Minimum 1900 psi • Clay brick shall have following minimum strengths Individual unit – Minimum 1250 psi Average of five units – Minimum 1500 psi • The reinforcement in brick masonry zone shall be calculated by one of three methods. I. Working stress design Two choices in design are allowed. A: Use of one half of allowable stresses B: Use of full allowable stresses Use of one half of allowable stress is required when § No special inspection is provided § No letter of certification of unit strength is produced § Masonry compressive strength is limited to 1500 psi Full allowable stresses may be used when § Special inspection is arranged § Masonry compressive strength f’m is between 1500 psi and 4000 psi § Letter of certification of unit strength is provided. II. Strength Design When designing under strength design method, special inspection of masonry during construction is required. In the strength design method the 7 minimum and maximum compressive strength of masonry shall be 1500 psi and 4000 psi respectively. III. Empirical design Instead of detailed analytical design empirical design may be used for simple low rise structures. The specified vertical reinforcement at corners, on each face of each opening and horizontal reinforcement concentrated in seismic bands at plinth, sill, lintel and roof level is required. Additional horizontal corner reinforcement is required in high seismic zones. 1.6 ARCHITECTURAL ELEMENTS Architectural components and their means of attachment shall be designed to withstand seismic forces calculated for the relevant seismic zone. The particular emphasis shall be laid in attachments for partition walls, false ceiling, false floors etc. 1.7 MECHANICAL AND ELECTRICAL SYSTEMS Mechanical and Electrical components and their supports shall be designed for seismic forces corresponding to the concerned seismic zone. Attachment for floor mounted equipment weighing less than 2 KN and furniture need not be designed. 1.8 STRUCTURAL TESTS AND INSPECTIONS The owner or the engineer or the architect incharge acting as the owner’s agent shall employ one or more special inspectors who shall inspect the works relating to concrete, reinforcing steel, structural steel, welding, 8 bolting, anchors, structural masonry, insulating concrete fill, spray applied fire resistive materials, shoetree, piles etc. Inspection may involve some non-destructive testing.