Earthquake Tip 25: Load Paths in Buildings PDF

Learning Earthquake Design Earthquake Tip 25 and...

Learning Earthquake Design Earthquake Tip 25 and Construction Why are Load Paths Important in Buildings? What are Load Paths? Mass is present all through in a building - from Column Floor roof parapet to foundation. Earthquake ground Slab Beam-Column shaking induces inertia forces in a building where Joints mass is present. These inertia forces are transferred downwards through horizontally and vertically Floor aligned structural elements to foundations, which, in turn, transmit these forces to the soil underneath. The Beam Shear Wall paths along which these inertia forces are transferred Braced Frame Bay through building are Load Paths (Figure 1a). Buildings may have multiple load paths running between locations Z of mass and foundations. Load paths are as much a Y Foundation concern for transmitting vertical loads (e.g., self- X Soil weight, occupancy load, and snow; Figure 1b) as for (a) Inertia Force horizontal loads (e.g., earthquake and wind; Figure 1c). in Y-direction Structural elements in buildings that constitute load paths include: (a) Horizontal diaphragm elements laid in horizontal plane, i.e., roof slabs, floor slabs or trussed roofs and bracings; (b) Vertical elements spanning in vertical plane along height of building, i.e., planar frames (beams and columns interconnected at different levels), walls (usually made of RC or masonry), & planar trusses; Not loaded Loaded (c) Foundations and Soils, i.e., isolated and combined Loaded Ground movement footings, mats, piles, wells, soil layers and rock; and (b) in Y-direction (c) (d) Connections between the above elements. Figure 1: Load paths for different load actions – Importance of Load Paths (a) key structural elements constituting load Buildings perform best in earthquakes, when paths, (b) vertical load paths, and (c) lateral load inertia forces generated in them are transmitted to paths foundation by continuous and direct load paths without being bent or interrupted. When some structural elements are discontinued along a direct load path, This edge loads have to bend and take detours to other load elongates paths; buildings with discontinuous or indirect load paths are undesirable, because brittle damage can occur in structural elements at the interruptions or bends. Horizontal Diaphragms No edge shortens Floor and roof slabs are thin, wide structural or elongates elements laid in a horizontal plane at different levels. They transfer inertia forces induced by their own Wall masses, to vertical elements on which they rest. During This edge earthquake shaking, horizontal diaphragms act like shortens beams in their own horizontal plane and transmit inertia (b) forces to vertical elements, such as structural walls or planar frames. Slabs that are long in plan (i.e., flexible in their own plane), bend and undergo undesirable stretching along one edge and shortening along the (a) other (Figure 2); they perform best when relative Figure 2: In-plane deformation in horizontal diaphragms – (a) absent, if plan aspect ratio is deformations are minimal and in-plane stiffness and up to 3, and (b) present, if plan aspect ratio strength sufficiently large. In general, slabs should be exceeds 5 rectangular with plan length/plan width ratio less than 3. IITK-BMTPC Earthquake Tip 25 Why are Load Paths Important in Buildings? page 2 Horizontal floors can effectively resist and transfer (b) Load path geometry must be simple: Uninterrupted, earthquake forces through direct load paths, provided direct load paths should be provided at regular that they do not have significant openings. Large intervals along length and width of the building; openings or cut-outs in floors interrupt load paths and (c) Load paths must be symmetrical in plan: A building may prevent smooth, direct transfer of forces to will sway uniformly in two horizontal directions, vertical elements. Openings in floors are necessary, when structural elements constituting load paths e.g., to allow for elevator core or staircase to pass are placed symmetrically in plan. Otherwise, it may through. But, these should be as small as possible, and twist about a vertical axis, which is detrimental to as few as possible. Their locations should be carefully its earthquake performance. considered; the ideal location for openings is close to (d) Robust connections are needed between structural center of floor slabs in plan. elements along load paths: In an earthquake-resistant Vertical Elements structure, every connection is tested during strong Typical structural elements (present in vertical earthquake shaking. These connections should be planes) of buildings are columns, braces and structural stiff and strong to offer continuous load paths walls or a combination of these (Figure 3). They collect without being damaged during strong earthquake gravity and (horizontal and vertical) earthquake inertia shaking (Figure 4). forces from floor diaphragms at different levels, and bring them down to the foundations below. It is possible to design and construct earthquake- resistant buildings with various structural systems, including Moment Resisting Frames (MRFs), Frames with Brace Members (called Braced Frames (BFs)), Structural Walls (SWs; also called Shear Walls), or a combination of these. Some of these systems require more advanced knowledge of design and higher quality control during construction than others, as reflected by their relative performance during earthquakes. For instance, buildings with SWs are easy to design and construct, and generally perform better during earthquakes, than buildings with MRFs alone. Photo Courtesy: Sudhir K. Jain Figure 4: Deficient connection between slabs and vertical elements – collapse of an RC frame building during 2001 Bhuj (India) earthquake (a) (b) (c) Related - Earthquake Tip Figure 3: Structural systems in buildings that Tip 5: What are seismic effects on structures? Tip 6: How architectural features affect buildings during help resist lateral earthquake-induced inertia earthquakes? loads – (a) MRFs : moment-resisting frames, (b) BFs : braced frames, and (c) frame-wall dual systems Resource Material Arnold,C., and Reitherman,R., (1982), Building Configuration and Seismic Design, John Wiley, USA Key Requirements of Load Paths Ambrose,J., and Vergun,D., (1999), Design for Earthquakes, John Earthquake performances of buildings are Wiley & Sons, Inc., USA determined by soundness of their load paths, independent of the material with which buildings are built, e.g., masonry, RC or structural steel. Earthquake Authored by: codes require designers to ensure presence of adequate C.V.R.Murty lateral load paths in buildings in two horizontal plan Indian Institute of Technology Jodhpur, India directions. Salient requirements of load paths are: Sponsored by: (a) Load paths must exist in all directions of a building: Building Materials and Technology Promotion Earthquake shaking occurs in all directions, and Council, New Delhi, India can be expressed as a combination of shaking in one vertical and two (mutually perpendicular) This release is a property of IIT Kanpur and BMTPC. It may horizontal directions. Hence, adequate load paths be reproduced without changing its contents with due acknowledgement. Suggestions or comments may be sent are needed along the vertical and the two mutually to: [email protected]. To see all IITK-BMTPC Earthquake Tips, visit perpendicular horizontal directions. www.nicee.org or www.bmtpc.org.