Overview of IS 16700:2023 (by priyansh verma)

This ppt gives an overview of the Indian standard code 16700:2023 in an efficient manner.

1. UNDERSTANDING IS 16700-2023 Submitted By PRIYANSH VERMA (2300520735011) Structural Engineering IET Lucknow CRITERIA FOR STRUCTURAL SAFETY OF TALL CONCRETE BUILDINGS Institute of Engineering and Technology (IET) College Lucknow, Uttar Pradesh Submitted To Prof. Abhishek Mishra  Codal Provisions for design & Structural Safety Assessments(MAST-022)

2. OUTLINE WHAT’S NEW IN IS 16700:2023 SCOPE TERMINOLOGIES GENERAL REQUIREMENTS LOADS STRUCTURAL ANALYSIS(MODELLING) STRUCTURAL DESIGN FOUNDATIONS NON STRUCTURAL ELEMENTS(NSEs) MONITORING DEFORMATIONS IN BUILDING

3. What's new in IS 1670 -2023? Recent Changes in the Revision (First Revision) : A new expression for estimating the approximate fundamental natural period of buildings over 50 meters has been introduced. Load combinations now consider P-Δ effects. An expression for the inter storey drift stability coefficient θ has been added. The minimum requirement for transverse reinforcement in structural walls has been revised The minimum requirement for transverse reinforcement in structural walls has been revised; and The procedure for approval of buildings that do not conform to the prescriptive requirements of this standard has been revised in Annex B.

4. SCOPE What it's about : Making sure tall buildings made of reinforced concrete (RC) are safe and strong, between 50 m to 250 m tall. How it works : It tells us exactly how to design these buildings, including picking the right structure, making sure it's the right size, and ensuring it can handle things like wind and earthquakes. What's not included : Buildings near earthquake-prone areas or with more than 20,000 people, and ones taller than 250 m. Different types of structures : Like walls, frames, and tube systems. Rules to follow : We have to follow Indian Standards, and if there's a conflict, this standard wins. When things don't fit the rules : There's a special plan in Annex B. Options for extra safety : Owners or authorities can ask for even stricter rules if they want.

5. Building Height - What it means: How tall a building is, from where it sits on the ground to the top of the roof. Connecting Structure :Types of connections: There are two kinds - some only handle the weight of the building, while others help the building stay sturdy during strong winds or earthquakes. Core -What it does : A group of strong walls and links that keep the building from moving too much side to side. Coupled Structural Wall Building : How it's built: Walls connected by beams, making them strong against both up-and-down and side-to-side forces. Frame Building :Type of structure: Buildings made with strong beams and columns that resist both up-and-down and side-to-side forces. Gravity Columns : Their job: these are the columns that hold up the building from above, not side-to-side. Key Elements -Important parts: These are the pieces that, if they break, could make the whole building fall down, so we try to keep them really strong. Linking Beams : Connections between walls- Horizontal bars connecting two vertical walls to make them stronger together. TERMINOLOGIES

6. GENERAL REQUIREMENTS ELEVATION 1.Height Limit for Structural Systems : It sets the maximum height a building can be based on its structural design.Different types of structural systems have different height limits, listed in Table 1. 2.Slenderness Ratio : It's about the ratio between a building's height and its minimum base width (B).There's a maximum allowed ratio to ensure buildings aren't too slender, listed in Table 2. 3.Aerodynamic Effects : Buildings should be designed to minimize wind drag.Factors like the shape of the building's elevation, facade features, and plan shape should be considered.Features like sharp corners and balconies sticking out can affect wind drag and need to be considered during design.

7. PLAN 1. Plan Geometry : - Prefer rectangular or elliptical plans for efficient structural resistance to lateral loads. - Avoid re-entrant corners to prevent additional structural complexities. 2. Plan Aspect Ratio :- The ratio between length and width of the building's plan shouldn't exceed 5.0. - For L-shaped buildings, the ratio applies to each leg separately. 3. Storey Stiffness and Strength : - Each storey should maintain a minimum lateral stiffness and strength relative to the one above it. the lateral translational stiffness of any storey should not be < 70% of that of the storey above lateral strength not <90% above. DEFORMATION - Limits are set for inter-storey lateral drift under various loads to ensure structural integrity. - Drift ratios should be within specified limits based on different load scenarios.

8. NATURAL MODES OF VIBRATION    - The natural vibration periods of torsional and translational modes should meet certain criteria to prevent excessive movement. FLOOR SYSTEMS    - Specifies requirements for materials, openings, and natural frequencies of floor systems. Material for Floor Slabs : Use in-situ casting for all floor slabs.For precast floor systems in seismic zones III, IV, and V, a minimum 75 mm structural topping of concrete with reinforcing mesh is required, which can be reduced to 50 mm in seismic zone II. Openings in Floor Diaphragm ; Avoid openings along any edge of the floor diaphragm unless perimeter members are stable and strong.Limit the total area of openings in any floor diaphragm to 30% of the plan area.Ensure lateral forces transfer from the diaphragm to vertical elements using collector elements if necessary. Minimum Width of Floor Slab : At any storey, the minimum width of the floor slab along any section, after deducting openings, should not be less than 5 m if there's no perimeter beam.Ideally, the cumulative width of the slab at any location should not be less than 50% of the floor width. Natural Frequency of Floor System ): Ensure that the natural vertical vibration frequency of any floor system is not less than 3 Hz, unless demonstrated acceptable using rational procedures

9. MATERIAL Concrete : The minimum grade allowed is M 30 & maximum grade permitted is M 70.For higher grades minimum crushing strain in compression of 0.002,ENSURED THROUGH TESTING. Reinforcing Steel : Reinforcing steel must meet the provisions outlined in IS 13920.Avoid lapping longitudinal bars in reinforced concrete (RC) columns and walls that are part of the lateral load resisting system, especially when the diameter of bars is 20 mm or higher . Use mechanical couplers as per IS 16172 to extend bars instead of lapping, where necessary. PROGRESSIVE COLLAPSE Precluding Progressive Collapse : Choose appropriate structural systems to maintain integrity,conduct rigorous investigations, ensure structural behavior, even if key members fail.Provide adequate redundancy and integrity to the structure. Key Element Requirement: Key elements are vital components whose failure could cause significant building deterioration.Enhance vertical and lateral resistance of key elements using higher safety factors for loads and materials.Design adjacent elements to provide alternative load transfer paths if key elements fail.

10. LOADS and LOAD COMBINATIONS 1.WIND EFFECTS Section 6 of the Code focuses on loads and load combinations for building analysis, with an emphasis on wind and seismic loads. Wind tunnel testing is mandated under certain conditions, such as building height exceeding 150m, plan or elevation asymmetry, complex topography, interference effects, or a natural period exceeding 5s. Wind tunnel tests are conducted for a 10-year return period and can assess torsional motion and to bring torsional velocity below 0.003 rad/sec. For RC tall buildings, the damping ratio is limited to 2%, unlike the uniform 5% damping ratio prescribed in IS1893 (Part1): 2016.

11. 2.SEISMIC EFFECTS Section 6.3 focuses on seismic analysis of tall buildings. The code recommends using response spectra from IS 1893 (Part 1): 2016, supplemented by site-specific hazard spectra in seismic zones IV and V. Seismic analysis in zone V must consider all three components of building shaking. Vertical shaking can increase the magnitude of gravity load on tall buildings, where gravity loads are already high and vertical load carrying capacity has a relatively low margin of safety. Failure of vertical load carrying members may lead to progressive collapse of the entire building.

12. STRUCTURAL ANALYSIS(MODELLING) Section 7.3 of the Code addresses modelling considerations in structural analysis software for buildings. Both lump mass and distributed modelling approaches can be adopted. Mass and stiffness properties are crucial for seismic analysis to reflect appropriate seismic behavior. Unreinforced masonry (URM) infill panels should be modelled to reflect their significant stiffness contribution to a storey. URM infill is modelled as diagonal struts, as per IS 1893 (Part 1): 2016. Irregularities in building configuration should be captured in the analytical model. Cracked section stiffness should be used, with prescribed factors for un-factored and factored load cases. Modelling of soil parameters is recommended based on geotechnical investigations. Secondary effects like P-Δ, shrinkage, creep, temperature, and foundation settlement must be considered. The code limits the inter-storey drift stability coefficient θ ( PuΔ /H) to 0.2 to restrict building flexibility.

13. CRACK SECTION Section 7.2 of the Code addresses additional considerations in modelling tall buildings, including rigid offset, floor diaphragm flexibility, crack section, and P-Δ effect. Crack section properties of the IS code are similar to the ACI code, as shown in Table 1. Crack formation under gravity loads and secondary effects like shrinkage, temperature, and hydration of concrete are generally negligible. Seismic effects may result in flexural cracks in structural elements due to stress reversals. Crack width varies along the section length, leading to reduced stiffness and strength parameters, and a reduction in moment of inertia. Cracked reinforced concrete section properties by Paulay and Priestly 1992 are provided in Table 2. Kumar and Singh (2010) proposed simplified formulas for calculating frame element stiffness based on axial load ratio, shown in Table 3.

14. Structural design Section 8 of the Code covers design criteria for various structural systems: frame buildings, structural wall systems, moment frame + structural wall systems, flat slab + structural wall systems, and structural wall + framed tube systems. Section 8.1.2 emphasizes the inclusion of staircases not confined by structural walls in modelling, as they can affect dynamic characteristics and structural performance. Section 8.1.3.2 discusses sensitivity analyses, recommending lower and upper bound analyses in addition to cracked section analysis. Lower bound sensitivity analyses involve reducing section properties of floor diaphragms and stiffness parameters of perimeter walls at the podium and backstay levels. Upper bound sensitivity analyses reduce area and moment of inertia properties of sections to 50% of gross parameters, while lower bound analyses reduce them to 15%. Section 8.2 underscores ensuring ductility in tall buildings, emphasizing various aspects of ductility.

15. section 8.3 of the Code pertains to provisions for 'frame buildings', requiring tall buildings to have a minimum of 3 bays and 2 frames to resist seismic effects. The minimum number of bays is crucial for the frame mechanism during earthquakes. Moment frames must be detailed according to the 'special moment frame' system in seismic zone III, aligning with IS 13920:2016. Section 8.4 addresses the dual system (moment frame - structural Wall), necessitating the support of the structural wall on stiff and ductile elements and restricting discontinuity even in higher stories. Discontinuity of the structural wall, even in higher stories, leads to larger lateral drift in the top stories and uneven distribution of shear demand in the building. Section 8.5 focuses on the 'structural wall' system, recommending maintaining a minimum thickness based on inter storey height and providing additional reinforcement for openings. Openings smaller than 800 mm or 1/3rd length of the wall (whichever is smaller) may neglect influence on overall stiffness, with additional reinforcement required for larger openings. Large openings in the structural wall increase building flexibility, with coupling beams experiencing higher rotations. Diagonal reinforcements are recommended in coupling beams to resist rotations, with maximum reinforcement based on span-depth ratio. Structural walls with staggered openings exhibit higher rigidity and load-carrying capacity compared to ordered openings. Beams or columns with high vertical loads should not be supported on coupling beams.

16. Section 8.6 of the Code addresses the 'flat slab + Structural wall' system, stipulating that the structural wall bears the entire lateral load of the building, with the column strip of the flat slab system excluded from the lateral load-resisting system. Section 8.7 focuses on the 'framed tube system', 'tube-in-tube system', and 'multiple tube system', where tubular systems act as hollow cylinders perpendicular to the ground to resist lateral loads. Structural walls/columns must be closely spaced to create a 3D cylindrical member, with restrictions on length-to-width ratio and spacing between columns/walls. Re-entrant corners and sharp changes should be avoided to maintain cylindrical action, with outer columns resisting lateral loads and internal columns supporting gravity loads. Corner column area should be at least twice that of internal columns, with recommendations for minimum diameters of longitudinal beams and stirrups based on seismic zones. Stirrup spacing should be limited to specific values depending on the seismic zone.

17. FOUNDATION Section 9 of the Code addresses factors of safety for buildings, geotechnical investigations, minimum foundation depth, soil modeling, and permissible settlement. A uniform factor of safety of 1.5 is recommended for overturning and sliding of buildings under various loads. Geotechnical investigations, including liquefaction potential analysis, soil spring constants, and modulus of subgrade reaction, are mandated for tall building sites. The code recommends a minimum of 3 borehole tests per tower, spaced at 30m intervals in the plan area. Minimum foundation depths for raft and pile foundations are suggested as 1/15 and 1/20 of the building's height, respectively. Soil modeling in software can be achieved through spring constants, zoned spring constants, or modulus of subgrade reaction. Soil-structure interactions are recommended for buildings over 150m tall, considering actual column loads and positions on the foundation. Permissible foundation settlement should adhere to IS 1904 and IS 12070 requirements.

18. NON STRUCTURAL ELEMENTS Section 10 of the code addresses the design of non-structural elements (NSEs) in tall buildings. Designing NSEs is crucial due to their significant cost in the total project budget and their observed poor performance during past earthquakes. Failure of NSEs can disrupt building functionality, impacting occupants even with minor disruptions like water or power supply issues. Achieving operational or immediate occupancy seismic performance levels requires appropriately designed non-structural elements. When NSEs significantly affect structural response, they should be considered in building design and modeling. Section 10.1 classifies NSEs into three categories based on their seismic behavior: acceleration sensitive, deformation sensitive, and acceleration-and-deformation sensitive. Section 10.2 provides design guidelines for acceleration-sensitive NSEs, including computation of design lateral force based on seismic zone and equations for calculating these forces, considering factors like amplification and response modification.

19. Section 10.3 addresses design guidelines for deformation sensitive NSEs. These elements are impacted by building deformation or inter-story drift. Performance can be ensured by either limiting inter-story drift or designing for expected drift. Guidelines are formulated based on building displacements.

20. MONITORING DEFORMATION IN BUILDING Section 11 of the code focuses on recommended monitoring practices for buildings during earthquakes. It suggests instrumentation with tri-axial accelerometers for buildings in seismic zone V and those taller than 150 m in seismic zone III, IV, to monitor translational and twisting behavior during strong earthquakes. For buildings taller than 150 m, the code recommends installing anemometers and accelerometers at the top to measure wind speed, acceleration, and direction. Permanent settlement markers are required to record foundation settlements during construction and the building's lifespan. Data from these instruments are valuable for validating design loads and checking settlement limits specified by the code. Settlement records help observe soil behavior during construction.

21. REFERENCES (PDF) ISSUES IN DESIGN OF TALL CONCRETE BUILDINGS IN INDIA WITH REFERENCE TO IS 16700: 2017 CODE (researchgate.net) Tall Building Design Code (bis.gov.in)

22. THANK YOU !!!