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Wind load calculation
 

Wind load calculation

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REINFORCED CONCRETE DESIGN AND DETAILING II

REINFORCED CONCRETE DESIGN AND DETAILING II

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    Wind load calculation Wind load calculation Presentation Transcript

    • MBEYA UNIVERSITY OF SCIENCE AND TECHNOLOGY DEPARTMENT OF CIVIL ENGINEERING REINFORCED CONCRETE DESIGN AND DETAILING II (CEH7422) NTA LEVEL 7B– SECOND SEMESTER 2013/2014 ACADEMIC YEAR ENG. JULIUS J. NALITOLELA/PROF. J. MSAMBICHAKA
    • TOPIC 1: WIND LOAD CALCULATION CONTENT 1. Background 2. Codes of practice/guideline 3. Definitions 4. Wind speed areas in Tanzania 5. Design procedures 6. Examples 7. Load partial factor of safety and load combination CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 1. Background Wind represents masses of air moving mainly horizontally (parallel to the ground) from areas of high pressure to ones of low pressure. Wind generates pressures on external (and also internal) surfaces of structures The main effect of wind is a horizontal loading of buildings (especially high-rise). This effect of the wind on the structure (i.e. the response of the structure), depends on the size, shape and dynamic properties of the structure
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 1. Background When the wind enters the building from the windward side and leeward side is relative closed, internal pressure is developed that acts like negative pressure Similarly, when high speed wind passes by a building, it produces a vacuum on the leeward side, this vacuum results in internal suction producing negative pressure from the structure Keeping the movements in the upper levels of the building to acceptable human tolerances is the goal of the structural engineer.
    • 1. Background CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 2. Codes of practice & Guideline  CP3: Chapter V: Part 2  BRU Technical Guideline no. 2 – LOADS FOR STRUCTURAL DESIGN  BS 6399-2:1997  Reynolds C.E and Steednam J.E (1981)
    • 3. Definitions  Design strength -characteristic strength divided by material strength coefficient  Material coefficient- partial coefficient of material (the coefficient takes account of the unpredictable variations of the properties, inaccuracy of calculation models, geometrical data etc.)  Limit state - a particular state which a structure or a component has attained due to loads acting on it when it is at the point of no longer fulfilling the particular requirement it was designed for. CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • 3. Definitions  Ultimate limit state- is the state corresponds to the requirement governing structurally safety against complete collapse due to excessive loading  Serviceability limit state- the serviceability limit state corresponds to requirements governing normal use and durability state.  basic wind speed the hourly mean wind speed with an annual probability risk of being exceeded of 0.02, irrespective of wind direction, at a height of 10 m over completely flat terrain at sea level that would occur if the roughness of the terrain was uniform everywhere  site wind speed the basic wind speed modified to account for the altitude of the site and the direction of the wind being considered CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • 3. Definitions  effective wind speed the site wind speed modified to a gust speed by taking account of the effective height, size of the building  Gusts are variations in the local winds, which are of a smaller character  dynamic pressure the potential pressure available from the kinetic energy of the effective wind speed  pressure coefficient the ratio of the pressure acting on a surface to the dynamic pressure  external pressure the pressure acting on an external surface of a building caused by the direct action of the wind CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • 3. Definitions  internal pressure the pressure acting on an internal surface of a building caused by the action of the external pressures through porosity and openings in the external surfaces of the building  net pressure the pressure difference between opposite faces of a surface  building height the height of a building or part of a building above its base  reference height the reference height for a part of a structure is the datum height above ground for the pressure coefficients and is defined with the pressure coefficients for that part  obstruction height the average height above ground of buildings, structures or other permanent obstructions to the wind immediately upwind of the site CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • 3. Definitions  effective height the height used in the calculations of the effective wind speed determined from the reference height with allowance for the obstruction height  building length the longer horizontal dimension of a building or part of a building  building width the shorter horizontal dimension of a building or part of a building or structural element being considered and of permanent obstructions upwind CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • 4. Wind speed in Tanzania  Light wind areas  Inland, except areas mentioned below  Particularly protected areas ling within the heavy wind areas mentioned below  Strong wind areas  Coastal regions including the islands (from shores to approximately 50 km up country)  Lake regions (Lake Nyasa, Lake Tanganyika, Lake Victoria, from the shores to approximately 50 km up country  Mountain and other areas where experience shows that particularly strong winds CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • 5. DESIGN PROCEDURE– reynolds’ table 13 & 14  STEP 1: DETERMINATION OF SITE BASIC SPEED (Vb ) Refer to definition part of this presentation , the determination of basic wind speed; Vb is from annual wind studies at a particular place as related to probability of wind speed occurrence  STEP 2: DETERMINATION OF DESIGN WIND SPEED Vs = Vb x S1 x S2 x S3 Whereby, Vb – Basic wind speed , S1 – Multiplier related to topology S2 – multiplier related to height above ground and wind breaking, S3 – multiplying factor related to life of structure CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 5. DESIGN PROCEDURE–STANDARD METHOD  Values of S1 may generally always be taken as unity except in the following cases: On sites adversely affected by very exposed hill slopes and crests where wind acceleration is known to occur: S1 = 1.1, On sites in enclosed steep-sided valleys completely sheltered from winds: S1 = 0.9  Values of S3 is a probability factor relating the likelihood of the design wind speed being exceeded to the probable life of the structure. A value of unity is recommended for general use and corresponds to an excessive speed occurring once in fifty years.
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 5. DESIGN PROCEDURE–STANDARD METHOD
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 5. DESIGN PROCEDURE–STANDARD METHOD  Generally, BRU Guidelines no. 2, highlighted the design wind speed in Tanzania, which depend on the wind areas such as: o In Light wind areas Vs = (35/3)*(log (h)+ 2) o In strong wind area Vs = (45/3)*(log (h)+ 2)
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 5. DESIGN PROCEDURE–STANDARD METHOD  STEP 3: DETERMINATION OF CHARACTERISTIC WIND PRESSURE Wk = 0.616*Vs 2
    • 5. DESIGN PROCEDURE–STANDARD METHOD  STEP 4: DETERMINATION WIND PRESSURE TO THE STRUCTURE  External pressure pe = qs x Cpe x Ca Cpe is the external pressure coefficient Ca is the size effect factor = 1 CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 5. DESIGN PROCEDURE–STANDARD METHOD  Pressure to structure
    • 5. DESIGN PROCEDURE–STANDARD METHOD CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • 5. DESIGN PROCEDURE–STANDARD METHOD CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 5. DESIGN PROCEDURE–STANDARD METHOD
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 5. DESIGN PROCEDURE–STANDARD METHOD  Internal pressure pi = qs x Cpi x Ca Cpi is the internal pressure coefficient Ca is the size effect factor = 1
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 5. DESIGN PROCEDURE
    • CEH7422; TOPIC 1-WIND LOAD CALCULATION 5. DESIGN PROCEDURE–STANDARD  STEP 5: NET PRESSURE DETERMINATION p = pe – pi  STEP 6: CHARACTERISTIC WIND FORCE DETERMINATION W k= (pe – pi) *A
    • 6. EXAMPLES  Example 1 - One MUST building  Example 2 - Mkapa tower – Dar es Salaam  Example 3 - A small building in Kariakoo surrounded by higher buildings CEH7422; TOPIC 1-WIND LOAD CALCULATION
    • 7. PARTIAL FACTOR OF SAFETY AND LOAD COMBINATION Load combination Safety factors Dead load Imposed load Wind load 1. Dead 1.4 2. Dead + Imposed 1.4 1.6 - 3. Dead + Wind 0.9 or 1.4 - 1.4 4. Dead + Imposed + Wind 1.2 1.2 1.2 CEH7422; TOPIC 1-WIND LOAD CALCULATION