Lateral load-resisting systems in buildings.pptx

Lateral load-resisting systems in buildings

1. Lateral load-resisting systems in buildings 6/20/2024 1

2. Introduction Lateral load resisting system are intended to transmit lateral loads safely to the foundation , like any other structural systems that’s exposed to different types of loads (Gravity loads, live load, impact loads, etc.) Primary Lateral Loads includes: Load generated by Wind Pressure Load generated due to Seismic Excitation/Earthquake Other Lateral Loads Load generated due to horizontal component of Gravity Loads in Inclined Systems and in Un-symmetrical structures Load due to lateral soil pressure, liquid and material retention 6/20/2024 2

3. Moment resisting frames These are generally composed of columns and beams. Their ability to resist lateral loads is entirely due to the rigidities of the beam-column connections and the moment-resisting capacities of the individual members. 6/20/2024 3

4. Shear walls These are solid walls, which usually extend over the full height of the building. Specially designed reinforced concrete walls parallel to the directions of load are used to resist a large part of the lateral loads caused by wind or earthquakes by acting as deep cantilever beams fixed at foundation. They are very rigid in their own plane and hence are effective in limiting deflections . They act as fire compartment walls . 6/20/2024 4 Shear wall

5. Cont’d… Shear wall Classification and behavior The behavior of shear wall is influenced by shape of the wall(s), the height to width ratio and their position in floor plan and we can classify them based on their height/width ratio as short, squat and cantilever. 6/20/2024 5

6. Cont’d… Shear walls has a different response to lateral loads than frame system and if we use frame and shear wall in combination (Dual system) to resist lateral load, then we will have a much stiffer structure than the individual systems. 6/20/2024 6

7. Cont’d… Opening in Shear Walls 6/20/2024 7

8. Cont’d… Basic shapes of shear wall and cores Based on the architectural shape and structural configuration requirements we provide different shapes of walls and we make sure those walls we are selected are the best fit for lateral load resistance in our structures. If a wall forms a channel or closed loop we call it a core . 6/20/2024 8

9. Cont’d… Different shear walls plan Configuration and their effects 6/20/2024 9

10. Braced Frames The X-bracing system works well for 20 to 60 story height, but it does not give room for openings such as doors and windows . The lateral load is primarily resisted by the Axial Force in the braces, columns and beams in the braced zone. The frame away from the braced zone does not have significant moments. Bracing does not have to be provided in every bay , but in every story. 6/20/2024 10

11. Tubes The structural systems discussed so far are not efficient for buildings taller than 60 stories. Framed -Tube Structures The framed tube is one of the most significant modern developments in high rise structural form. These are systems in which closely-spaced columns 2 - 4 m between centers are located along the periphery of a building. Deep spandrel beams , located on the exterior surface of the building, interconnect these columns. The idea is to create a tube that will act like a continuous perforated chimney or stack. 6/20/2024 11

12. Cont’d… Even though framed tube is a structurally efficient form , flange frames tend to suffer from shear lag. This results in the mid face flange columns being less stressed than the corner columns and therefore not contributing to their full potential lateral strength. 6/20/2024 12

13. Cont’d… Braced tube structures Further improvements of the tubular system can be made by cross bracing the frame with X-bracing over many stories, as illustrated in Figure below. This arrangement was first used in a steel structure, in Chicago's John Hancock Building, in 1969. 6/20/2024 13 they virtually eliminate the effects of shear lag in both the flange and web frames. The spacing of the columns can be increased and the depth of the girders will be less, thereby allowing large size windows than in the conventional framed tube structures.

14. Cont’d… Tube-in-Tube Structures When the (outer) framed tube is combined with an ‘inner tube’ (or a central shear core), the system is called a tube-in-tube. 6/20/2024 14

15. Cont’d… Bundled Tube When the sectional plan of the building comprises several perforated tubular cells, the system is called a bundled tube or ‘multi-cell framed tube’. The increase in stiffness is apparent . The system allows for the greatest height and the most floor area. 6/20/2024 15

16. Selection of proper system and configuration for lateral loads Building (superstructure and non-structural components) should be light and avoid unnecessary masses. Building and its superstructure should be simple, symmetric, and regular in plan and elevation to prevent significant torsional forces, avoiding large height-width ratio and large plan area. Building and its superstructure should have a uniform and continuous distribution of mass, stiffness, strength and ductility, avoiding formation of soft and weak stories. Superstructure should have relatively shorter spans and avoid use of long cantilevers. The non-structural components should either be well separated so that they will not interact with the rest of the structure, or they should be integrated with the structure. 6/20/2024 16

17. Cont’d… Superstructure should be detailed so that the inelastic deformations can be constrained (controlled) to develop in desired regions and according to a desirable hierarchy. Superstructure should have the largest possible number of defense lines, that is, it should be composed of different tough structural subsystems which interact or are interconnected by very tough structural elements (structural fuses) whose inelastic behavior would permit the whole structure to find its way out from a critical stage of dynamic response. Superstructure should be provided with balanced stiffness and strength between its members, connections and supports. The stiffness and strength of the entire building should be compatible with the stiffness and strength of the soil foundation. 6/20/2024 17

18. Cont’d Soft story 6/20/2024 18

19. Structural Regularity For the purpose of seismic design, building structures are categorized into being regular or nonregular. This distinction has implications for the following aspects of the seismic design: The structural model, which can be either a simplified planar model or a spatial model; The method of analysis, which can be either a simplified response spectrum analysis (lateral force procedure) or a modal one; 6/20/2024 19 Regularity   Allowed Simplification Behavior factor Plan Elevation Model Linear-elastic Analysis (for linear analysis) Yes Yes No No Yes No Yes No Planar Planar Spatial Spatial Lateral force Modal Lateral force Modal Reference value Decreased value Reference value Decreased value

20. Criteria for regularity in plan With respect to the lateral stiffness and mass distribution , the building structure shall be approximately symmetrical in plan with respect to two orthogonal axes. The plan configuration shall be compact , i.e., each floor shall be delimited by a polygonal convex line. No set back or for each set-back, the area between the outline of the floor and a convex polygonal line enveloping the floor does not exceed 5 % of the floor area. The in-plan stiffness of the floors shall be sufficiently large in comparison with the lateral stiffness of the vertical structural elements , so that the deformation of the floor shall have a small effect on the distribution of the forces among the vertical structural elements. 6/20/2024 20

21. Cont’d… In this respect, the L, C, H, I, and X plan shapes should be carefully examined, notably as concerns the stiffness of the lateral branches , which should be comparable to that of the central part, in order to satisfy the rigid diaphragm condition . The application of this paragraph should be considered for the global behavior of the building. All lateral load resisting systems, such as cores, structural walls, or frames, run without interruption from the foundations to the top of the building. The deflected shapes of the individual systems under horizontal loads are not very different . This condition may be considered satisfied in the case of frame systems and wall systems . In general, this condition is not satisfied in dual systems. 6/20/2024 21

22. Criteria for regularity in elevation All lateral load resisting systems, such as cores, structural walls, or frames, shall run without interruption from their foundations to the top of the building, if setbacks at different heights are present, to the top of the relevant zone of the building. Both the lateral stiffness and the mass of the individual storey's shall remain constant or reduce gradually , without abrupt changes , from the base to the top of a particular building. In framed buildings the ratio of the actual storey resistance to the resistance required by the analysis should not vary disproportionately between adjacent storey's. 6/20/2024 22

23. Cont’d… When setbacks are present, the following additional conditions apply: 6/20/2024 23

24. To create a stable and stiff structure the following recommendation can be helpful 6/20/2024 24 Tall and slender buildings should be avoided (a) Concentration of masses at the top of the building should be avoided (b) Setbacks should be avoided (e) If required, adequate structural separation should be provided (f) Irregularities within the framing system should be avoided (g) Any connection (bridging) between two independent buildings should be avoided ( i ) Staggered floor arrangements should be avoided (k)

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