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.

METHOD

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)

METHOD
 STEP 3: DETERMINATION OF CHARACTERISTIC
WIND PRESSURE
Wk = 0.616*Vs
2

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

METHOD
 Pressure to structure

METHOD

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

 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

Wind load calculation

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