Loads on Structures. Dead Load / Fixed Load Live Load / Imposed Load Earthquake Load Wind Load Snow Load

Transcription

1 Loads on Structures Dead Load / Fixed Load Live Load / Imposed Load Earthquake Load Wind Load Snow Load

2 Characteristics of Wind Load Depends upon - velocity and density of the air height above ground level shape and aspect ratio of the building topography of the surrounding ground surface angle of wind attack solidity ratio or openings in the building

3 Determination of Wind Loads as per IS 875 (Part 3) Design wind speed V z = V b k 1 k 2 k 3 V b = Basic wind speed k 1 = Probability factor or risk coefficient k 2 = Terrain and height factor k 3 = Topography factor

4 Basic wind speed IS 875 (Part 3) gives the basic wind speeds having a return period of 50 years and at a height of 10 m above ground level. Entire country is divided into six wind zones.

5 Basic wind speeds in m/s (Based on 50yr return period) For some important cities, basic wind speed is given in Appendix A of the code

6 Probability factor / Risk Coefficient (k 1 ) Basic wind speed is based on a 50yr return period. There is always a probability (howsoever small) that basic wind speed may be exceeded in a storm of exceptional violence; the greater the number of years over which there will be exposure to wind, the greater is the probability. The factor k 1 is based on statistical concepts, which take account of the degree of reliability required, and period of time during which there will be exposure to wind i.e. life of the structure. IS 875 gives values of k 1 for different classes of buildings.

7 For some important structures (nuclear power plants, satellite communication towers etc.) code gives a formula to calculate the value of k 1.

8 Terrain and Height Factor (k 2 ) Four terrain categories have been considered by the code depending on the surroundings of structure. Category 1 : Exposed open terrain with few or no obstructions Avg. height of surrounding objects is 1.5 m. Eg. Open sea coasts, flat treeless plains. Category 2 : Open terrain with well scattered obstructions with height b/w m. Eg. includes airfields, open parklands etc. Category 3 : Terrain with numerous closely spaced obstructions having the size of building-structures up to 10 m Eg. Towns and industrial areas, full or partially developed Category 4 : Terrain with numerous large high closely spaced obstructions. Eg. Large city centres

9 Buildings have been divided into 3 classes Class A : Structures having maximum dimension (greatest horizontal or vertical dimension) less than 20 m. Class B : Maximum dimension b/w m. Class C : Maximum dimension greater than 50 m. IS 875 gives the values of k 2 at different heights for the above four categories and different classes of buildings.

10 Wind profile does not develop fully at the start of the terrain. Height of development increases with the upward distance or fetch distance. For structures of height greater than the developed height velocity profile can be determined from the method described in Appendix B of the code.

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12 Topography Factor (k 3 ) V b does not take into account the local topography features such as hills,valleys etc. Topography features affect the wind speeds. Accelerated near the summits and decelerated in the valleys

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14 Value of k 3 level ground or when upwind slope is less than 3 0 is equal to 1.0 Otherwise k 3 = 1 + Cs where C = 1.2 Z / L for θ = 3 o 17 o = 0.36 for θ > 17 o Z = height of the crest or hill L = length of the upward slope θ = upwind slope of ground s is the factor obtained from figures.

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16 Design Wind Pressure Design wind pressure p d = 0.6 V z 2 where p d = design wind pressure V z = design wind speed Note : In cyclone prone regions, design wind speed is increased by a certain factor to account for drastic increase in wind speed during cyclone.

17 Wind Pressure on Roofs Pressure acts normal to the element where F = (C pe -C pi ) A p d F = net wind force on the element A = surface area of the element p d = design wind pressure C pe = external pressure coefficient C pi = internal pressure coefficient

18 External Pressure Coefficients

19 Y = h or 0.15 W whichever is less

20 Internal Pressure Coefficients Depends upon permeability of the building and the direction of wind. Type of building C pi Low permeability (less than 5% openings) 0.2 Medium permeability (5 20% openings) 0.5 Large permeability (openings > 20%) 0.7 Different coefficients for buildings with large openings on one side.

21 Internal Pressure coefficients for buildings with large openings on one side and top closed.

22 Positive pressure coefficients - Pressure Negative pressure coefficients - Suction

23 Mini-Design Project #1 Problem Statement An industrial shed of 32 m 16 m is to be built for a manufacturing unit. The frames are spaced at 4 m centers and the ceiling height is 8 m measured at the roof truss bottom from the shop floor. The shed is located in a fully developed industrial area. Analyse and design the building considering various load combinations (DL+ LL+ WL).

24 Sectional Elevation

25 Side Elevation Elevation at Centre

26 Roof Bracing in Top Chord Purlins Top Chord Level (Roof Plan) Eaves Level (Roof Plan)

27 Basic Design Data Roof span Bay width Column height Total roof dead load on plan (due to CGI sheeting, insulation & lighting, purlins) Total roof imposed load on plan Roof slope with horizontal 16 m 4 m 8 m 0.5 kpa 0.75 kpa 3 o

28 Required Work Calculate the wind load acting on the roof (as per IS 875: Part 3). Ignore the frictional drag and dynamic effects due to wind. Also calculate total dead and live load as per IS 875 (pt 1 and 2). These loads are transferred to the truss via purlins (i.e. a concentrated load will be transferred on the truss at the purlin points.) Model and analyze the truss in SAP 2000 with the loads calculated in the part (a) and determine the forces in the members of the truss for applicable load cases as per IS 800 with suitable load factors. Analyze the 2D truss only. Determine the member which is in maximum tension and size the section for economical design. You can use either a double angle or a pipe section.

29 Project Report The report will be graded for its technical accuracy and presentation, which include the following: Lightest section where appropriate Correct numerical calculations Appropriate solution procedure and Appropriate documentation of work.

30 Marks distribution Items Weight Estimation of loads 25 % Analysis of frame for design forces showing five most 20 % stressed members in all considered load combinations in Table with proper identification of members Design of tension member 25 % Documentation Neat sketches showing the loads and forces in the truss Neat sketches showing the details of the design of the tension member Appropriate and sequential approach to solve the problem. 30 %