CSR CRR FS ,
where CSR is as estimated in Step 5 and CRR in Step 6. When the design ground motion is conservative, earthquake-related permanent ground deformation is generally small, if FS1.2.
Table F1 Recommended Standardized SPT Equipment
Element Standard Specification
Sampler Standard split-spoon sampler with: Outside diameter OD = 51 mm, and Inside Diameter ID = 35 mm
(constant, that is, no room for liners in the barrel)
Drill Rods A or AW type for depths less than 15.2 m; N or NW type for greater depths
Hammer Standard (safety) hammer with (a) weight = 63.5 kg; and
(b) drop height = 762 mm (delivers 60 of theoretical free fall energy) Rope Two wraps of rope around the pulley
Borehole 100-130mm diameter rotary borehole with bentonite mud for borehole stability (hollow stem augers where SPT is taken through the stem)
Drill Bit Upward deflection of drilling mud (tricone or baffled drag bit) Blow Count Rate 30 to 40 blows per minute
Penetration Resistant Count
Table F2 Correction Factors for Non-Standard SPT Procedures and Equipment
Correction for Correction Factor
Nonstandard Hammer Type
/auto trip with DH for . pulley and rope with DH for . 33 1 75 0 CHT and ER = 80, where
DH= doughnut hammer, and ER = energy ratio.
Nonstandard Hammer Weight or Height of fall 387 48 HW CHW , where
H = height of fall (mm), and W = hammer weight (kg) Nonstandard Sampler Setup (standard
samples with room for liners, but used
without liners sand dense for . sand loose for . 2 1 1 1 CSS Nonstandard Sampler Setup (standard samples with room for liners, but liners
are used) sand dense for . sand loose for . 8 0 9 0 CSS
Short Rod Length CRL =0.75 for rod length 0-3 m
Nonstandard Borehole Diameter
mm 200 of Diameter Hole Bore for . mm 150 of Diameter Hole Bore for . mm 115 - 65 of Diamter Hole Bore for . 15 1 05 1 00 1 CBD Notes
N = Uncorrected SPT Blow Count
BD RL SS HW HT 60 C C C C C C 60 60 NC N
FIG. F1 MAGNITUDE SCALING FACTORMSF
FIG. F3 INITIAL STATIC SCALING FACTOR k
FIG. F5 RELATION BETWEEN CRR AND
N1 60 FOR SAND FOR M 7.5 w EARTHQUAKESANNEX G ( Foreword )
PERFORMANCE-BASED DESIGN (For Information Only) G-1 DEFINITIONS
The following definitions are applicable for this informative Annex. G-1.1 Performance-Based Seismic Design
Framework for seismic design of structures to ensure the targeted performance under specified hazard level(s). The performance is indicator of the damage expected in various structural and non-structural components and is usually quantified in terms of plastic deformation in different structural components, drift (maximum inter-storey drift in case of buildings), and acceleration at the level of connection of the non-structural component. In this methodology, the inelastic response of considered structure under specified hazard level is explicitly estimated. G-1.2 Force Based Design
The earthquake resistant design methodology, followed by most of the current design codes, worldwide, in which the different components of the structure are designed to have specified minimum strength. It is accompanied by prescribed detailing of structural configuration, reinforcement and connections, and specified hierarchy of strength in different components and modes of failure. The inelastic energy dissipation by the structure during earthquake is accounted for approximately by specifying ‘Response Reduction Factors’ for different classes of structures.
G-1.3 Displacement Based Design
In this approach the structure is designed to achieve a specified deformation state under the considered hazard level. Contrary to force based design, in this approach the members are proportioned for local deformation demand and not the strength. G-1.4 Performance Objective
A desired level of seismic performance, expressed in terms of acceptable structural and non-structural damage for a specified level of seismic hazard (for example, DBE, MCE).
G-1.5 Performance Level
A limiting damage state or condition described by the physical damage to different structural and non-structural components, threat to life safety of occupants/users, and post-earthquake serviceability of the structure.
G-1.6 ‘Operational’ Building Performance Level
Buildings meeting this target Building Performance Level are expected to sustain minimal or no damage to their structural and non-structural components. The building is suitable for its normal occupancy and use, although possibly in a slightly impaired mode, with power, water, and other required utilities provided from emergency sources, and possibly with some nonessential systems not functioning.
Buildings meeting this target Building Performance Level pose an extremely low Life Safety risk.
G-1.7 ‘Immediate Occupancy’ Structural Performance Level
The post-earthquake damage state in which only very limited structural damage has occurred. The basic vertical- and lateral-force-resisting systems of the building retain almost all of their pre-earthquake strength and stiffness. The risk of life-threatening injury as a result of structural damage is very low, and although some minor structural repairs might be appropriate, these repairs would generally not be required before re-occupancy.
G-1.8 ‘Life Safety’ Structural Performance Level
The post-earthquake damage state in which a structure has damaged components but retains a margin against the onset of partial or total collapse. The damage has not resulted in large falling debris hazards, either inside or outside the building. Injuries might occur during the earthquake; however, the overall risk of life- threatening injury as a result of structural damage is expected to be low.
G-1.9 ‘Collapse Prevention’ Structural Performance Level
The post-earthquake damage state in which a structure has components damaged to an extent that it has no margin against collapse but it continues to support gravity loads. The building is on the verge of partial or total collapse. Substantial damage to the structure has occurred, potentially including significant degradation in the stiffness and strength of the lateral-force-resisting system and large permanent lateral deformation of the structure.
G-2 Performance Based Design procedures can be used for design of structural systems not covered by this code and for design of structures for a better seismic performance objective than that intended in this code. Structures, including structural and non-structural components and their connections shall be demonstrated using appropriate analysis or experimental testing or by a combination of analysis and structural testing to provide a seismic performance equivalent or better than that intended in this code. Due consideration shall be given to the uncertainties associated with demand (loading) and capacity (resistance).
Minimum performance objective for buildings with Importance Factor, I=1.0 shall be ‘Collapse Prevention’ for the Maximum Considered Earthquake (MCE). For Important buildings (I=1.5), the minimum performance objective shall be ‘Immediate Occupancy’ for the ‘Design Basis Earthquake (DBE)’ and ‘Life Safety’ for the MCE. For hospital and other lifeline buildings, the non-structural components shall be demonstrated to have ‘Operational’ performance level for DBE. Enhanced performance objectives can be formulated in consultation with the owner, occupants and authority having jurisdiction. In no case, the prescriptive design criteria for regularity of structural configuration, and requirements for stiffness, strength and ductility laid down in this code, shall be diluted.
Nonlinear static or nonlinear dynamic analysis procedure, with appropriate assumptions regarding stiffness, damping, strength and hysteretic behaviour can be used. The assumptions shall be based on approved test data and/or referenced standards. Use of nonlinear static procedure shall be permitted for buildings having
regular configuration and negligible higher mode effects. For buildings having configurational irregularities or significant higher mode effects, nonlinear dynamic procedure (Nonlinear Time History Analysis) shall be required. Appropriate acceptance criteria in terms of plastic deformations in members, interstorey drift, and strain limits in different materials, for different performance levels shall be used based on approved test data and/or referenced standards. Till such standards are developed for Indian conditions, the analysis procedures, modelling assumptions and acceptance criteria of ASCE 41 can be used.
Independent peer review is an essential part of the performance-based design framework. The reviewer must have appropriate expertise and understanding of the structural system, loading, analysis method and testing procedure used.
G-3 COMMENTARY
A number of new structural systems and materials are gradually becoming popular in the Indian construction industry. Further, the intended performance objective of ‘No Collapse’ in case of a major earthquake may not be acceptable for some owners and users of ordinary buildings and for important buildings. Therefore, it is important to allow for alternative design methods to promote innovation and enhanced performance objectives. Performance-based design is a promising tool, gaining popularity world-over, and it is timely that it is adopted in Indian code. However, at the same time, it should be safeguarded against misuse and diluting the requirements of the current code of practice. The prescriptive guidelines of the existing codes in terms of selection of structural configuration, minimum design force levels, and detailing for ductility enhancement are time tested and cannot be compromised with in any case. It is to be noted that ASCE 41 (and other related documents) have been developed for seismic performance evaluation of existing buildings. The codes for new construction are intended to regulate the design and construction of new structures such that to encourage or require features for good seismic performance. Many existing buildings are designed and constructed without such features and therefore, ASCE 41 caters to much wider range of structures, which cannot be encouraged for new construction.
Performance-based design is primarily a displacement-based design methodology, which requires realistic (or conservative) estimates of displacement. This necessitates the use of reasonable effective stiffness estimates for cracked sections in case of RC members. In case of the current force-based design approach of
IS 1893, an overestimation of stiffness may be conservative for evaluating the design base shear, but in case of displacement-based approach an overestimation of stiffness will be non-conservative. Therefore, proper modelling of stiffness is a crucial step in performance-based design. Effective stiffness of RC members depends on a number of factors, including level of axial force in the member, reinforcement ratio, and grades of concrete and steel. In absence of more reliable test results for Indian conditions, the guidelines of ASCE 41 can be used for this purpose.
Another important deviation in displacement-based design approach, as compared with the conventional force-based design approach, is use of ‘Expected’ strength of components in place of the Characteristic (or ‘Nominal’, or ‘Specified’ or ‘Lower- bound’) strength. The relationship between expected and characteristic strength, depends on the manufacturing process and quality control. It is to be estimated
considering the variance of test results. There is need to conduct a large number of tests on different materials being used in India in different regions and under different conditions of manufacturing. It is to be noted that different standards/documents have significant variation in this regard, and the values specified in a different national standard, e.g. ASCE 41 are to be used with caution.
Significant variation exists among different documents/standards available on Performance-/Displacement-Based design, regarding the definition and acceptance criteria for different performance levels. Similar uncertainty also exists about the modelling of inelastic (nonlinear cyclic) behaviour of different components. The structural designer should refer to more than one standard/document and use more than one performance acceptance criteria to take into account the uncertainty. The performance acceptance criteria should also be supported by test results, as far as possible. The relevant documents/standards in this regard are listed below:
ASCE 41-13, (2013), Seismic Evaluation and Retrofit of Existing Buildings, American Society of Civil Engineers, Reston, USA.
Dubai Municipality, (2013), Seismic Design Code for Dubai
Calvi G.M., and Sullivan, T.J., (2009), A Model Code for Displacement-Based Seismic Design of Structures, IUSS Press, Pavia, Italy.
Fib Bulletin 65, (2012), Model Code 2010, final draft, International Federation for Structural Concrete, Lusanne, Switzerland.
Priestley, M.J.N., Calvi, M., and Cowalsky, M.J., (2007), Displacement-Based Seismic Design of Structures, IUSS Press, Pavia, Italy.
PEER / ATC-72-1 (2010). Modelling and Acceptance Criteria for Seismic Design and Analysis of Tall Buildings, Applied Technology Council, Redwood City, California.
ANNEX H ( Foreword )
DESIGN OF SLAB-COLUMN SYSTEMS (For Information Only)
H-1 DEFINITION
The following definitions are applicable for this informative Annex. H-1.1 Slab-column Frame System
A building structural system also known as flat-slab or flat plate system, having RC slabs directly supported on columns, without beams. The system is known to have brittle punching shear failure around the slab-column connections and is not recommended to be used as primary lateral load resisting system in seismic regions. H-1.2 Lateral Force Resisting System (LFRS)
A structural system having adequate stiffness, strength and ductility in lateral direction, which can be used as primary system to resist earthquake forces. The common LRFS include RC shear walls, RC/Steel moment resisting (rigid jointed) frames, and steel braced frames.
H-1.3 Gravity Shear Ratio (VR)
Fraction of the punching shear capacity consumed by gravity loads (Dead Load, Live Load and Snow load, if applicable). It is defined as the ratio of the two-way (punching) shear stress at critical section due to gravity loads, to the punching shear strength, calculated in accordance with Cl. 31.6 of IS 456.
H-2 The slab-column systems, commonly known as flat-slab or flat-plate systems cannot be used as lateral force resisting systems in seismic regions. However, these can be used as non-seismic load resisting systems along with other primary lateral force resisting systems (LFRS), such as shear walls or moment resisting frames. When used along with these LFRS, the compatibility of the slab-column system with the LFRS in lateral drift is crucial. Mere designing of the LFRS for the full design base shear is not adequate, and the slab-column system shall be demonstrated to retain its vertical load carrying capacity subjected to the resulting inter-storey drifts. This requires a performance (displacement) based design approach considering all potential failure modes including flexure, shear, shear-moment transfer and reinforcement development at any section. The analytical model shall consider effective stiffness of cracked sections of slab and other RC elements including shear walls, shall have explicit modelling of torsional stiffness of slab-column joint, and shall capture the potential for both slab yielding and connection failure due to punching. ASCE 41 and ACI 318 guidelines for modelling and acceptance criteria for slab-column building systems can be used, till detailed guidelines are provided in the relevant Indian codes.
The primary LFRS used in slab-column buildings shall be designed to provide adequate stiffness, strength and torsional rigidity to the building. In case of shear walls being used as the primary LFRS, the walls shall be provided symmetrically in both the directions and as far away from the center of the building as possible,
preferably along the perimeter. The LFRS shall be proportioned to restrict the maximum interstorey drift DR (%) at DBE (R times the interstorey drift at design load), as
DR= 3.5 - 5×VR, for VR<0.6, and (1a)
DR= 0.5, for VR>0.6 (1b)
Where, VR is the gravity shear ratio (ratio of factored two-way shear force at the critical section due to dead and live loads to the shear strength of the slab-column joint).
Use of column or shear capitals, drop panels, and shear reinforcement in the form of stirrups and shear heads/ shear studs shall be permitted to enhance the shear strength of the slab-column joint. A response reduction factor of 4 shall be used for design of shear walls and shear walls shall be designed and detailed following the provisions of IS 13920. The columns shall also be detailed as per IS 13920. For ductile detailing of beam-column joints, the provisions of ACI 318 shall be followed. In case of shear wall-flat slab buildings without masonry infills and up to 20 storeys tall, use of the following simplified procedure shall be permitted:
1. Modelling of shear wall-flat slab system, using equivalent frame approach, with cracked section stiffness, equivalent width of slab, and explicit modelling of joint torsional rigidity.
2. Analysis using multi-mode superposition method, considering adequate number of modes contributing more than 90% seismic mass or using time history analysis method. Use response reduction factor of 4 and scale the design base shear as per the requirements of Cl. 7.7.3.
3. Check for adequacy of torsional rigidity, according to the following conditions: (i) The slenderness λ = Lmax/Lmin of the building in plan shall not be higher
than 4, where, Lmax and Lmin are respectively the larger and smaller dimensions in plan of the building, measured in orthogonal directions. (ii) At each level and for each direction of analysis x and y, the structural
eccentricity eo and the torsional radius r shall be in accordance with the two conditions below, which are expressed for the direction of analysis y:
eox ≤ 0.30 rx (2) and
rx≥ ls (3) where,
eox is the distance between the centre of stiffness and the centre of mass, measured along the x direction, which is normal to the direction of analysis considered;
rx is the square root of the ratio of the torsional stiffness to the lateral stiffness in the y direction (“torsional radius”); and
ls is the radius of gyration of the floor mass in plan (square root of the ratio of the polar moment of inertia of the floor mass in plan with respect to the centre of mass of the floor to the floor mass).
4. Design slab, columns, and slab-column joints, using the provisions of IS 456, for different load combinations as per IS 1893.
5. Design of shear walls using the provisions of IS 1893. Detail the reinforcement in all the components according to IS 13920 provisions.
6. Check that R times the maximum inter-storey drift DR is less than the specified limit (Eqn. 1). If the interstorey drift is more than the limit, provide shear reinforcement in the slab as per ACI 318-15 guidelines or revise the design with increased size of shear walls and/or increased thickness of slab/shear capitals.
7. Detail the slab reinforcement as per guidelines of ACI 318, with continuity of at least 50% of the bottom bars in column strip and minimum 2 bars passing through the cage of the column reinforcement.
H-3 COMMENTARY
Flat slab buildings provide some advantages as compared to the conventional beam- column frame systems, such as use of relatively simple formwork and shorter storey heights and are becoming popular in Indian construction industry. However, these buildings are known for their poor seismic performance due to their flexibility and brittle punching shear failure. In absence of adequate design guidelines for seismic loads, these are being designed and constructed without due care. Therefore, it is important that detailed seismic design guidelines are provided in the code for these buildings.
Due to their shortcomings in terms of lack of stiffness and ductility, these buildings cannot be used as primary lateral load resisting system and another lateral force resisting system (LFRS) is required to support these buildings. The LFRS should have adequate strength, stiffness and inelastic energy dissipation capacity. RC shear walls are an ideal LFRS to be used in conjunction with flat slab buildings. However, it is to be noted that these buildings also suffer from lack of torsional rigidity and shear walls should be so configured and placed that the building as a whole has adequate torsional rigidity. The EC8 guidelines are the most elaborate in this context and can be used for this purpose.