Wind load calculation

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


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


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



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.


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


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


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


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


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


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



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.


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)



STEP 3: DETERMINATION OF CHARACTERISTIC WIND PRESSURE

Wk = 0.616*Vs 2


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




Pressure to structure








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


5. DESIGN PROCEDURE


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


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
