When designing buildings and structures, the calculation of the wind load has to be done quite often. This indicator is calculated by special formulas. It is important to take into account such a load, for example, when drawing up rafters of roof systems of houses, choosing the location and design of billboards, etc.
SNiP standards
Actually, the very definition of this parameter gives SNiP 2.01. 07-85. According to this document, the wind load should be considered as a combination of:
pressure acting on the external surfaces of the structures of the structure or element;
friction forces directed tangentially to the surface of the structure, referred to the area of its vertical or horizontal projection;
normal pressure applied to the inner surface of the building with permeable building envelopes or open apertures.
How to determine
When calculating the wind load, two main parameters are taken into account:
average component;
pulsating.
The load is defined as the sum of these two parameters.
Middle component: basic formula
If the wind load is not taken into account during design, this will subsequently have an extremely negative impact on the operational characteristics of the building or structure. Its average component is calculated by the following formula :
W = Wo * k.
Here W is the calculated value of the wind load at a height z above the surface of the earth, Wo is its standard value, and k is the coefficient of pressure change in height. All initial data from this formula is determined by the tables.
Sometimes the calculations also use the parameter c - aerodynamic coefficient. The formula in this case looks as follows: W = Wo * k s.
Normative value
To find out what this parameter is equal to, you need to use the table of regions for the wind load of the Russian Federation. There are only eight of them. The table of wind loads (the dependence of Wo values on a particular region of Russia) is presented below.
For poorly studied areas of the country, as well as for mountainous regions, this SNiP parameter can be determined from officially registered weather stations and based on experience in operating existing buildings and structures. In this case , a special formula is used to determine the normative value of the wind load . It looks like this:
Wo = 0.61 V 2 o.
Here V 2 o is the wind speed in meters per second at the level of 10 m, corresponding to the averaging interval of 10 minutes and exceeded once every 5 years.
How is the coefficient k determined?
There is also a special table for this parameter. When determining it, the type of the area where construction of the structure or building is supposed to be taken into account is taken into account. In total there are three:
Type "A" - open flat areas: the coasts of the seas, lakes and rivers, steppes, deserts, tundra areas, forest-steppes.
Type "B" - terrain covered with obstacles up to 10 meters high: urban area, forests, etc.
Type "C" - urban areas with buildings with a height of more than 25 m.
The type of construction site is also determined taking into account the requirements of SNiP. When designing this must be taken into account. Any building is considered to be located in an area of a certain type if the latter is located on the windward side of it at a distance of 30h. Here h is the design height of the structure up to 60 m. With a higher height of the building, the type of terrain is considered certain if it remains at least 2 km from the windward side.
How to calculate the ripple load
According to SNiP, the wind load, as already mentioned, should be determined as the sum of the average normative and pulsating. The value of the last parameter depends on the type of structure itself and the features of its design. In this regard, distinguish:
structures with their own frequency of oscillations exceeding the established limit value (chimneys, towers, masts, column-type apparatuses);
constructions or structural elements representing a system with one degree of freedom (transverse frames of production one-story buildings, water towers, etc.);
- symmetrical in terms of the building.
Formulas for different types of structures
For the first type of structures, when determining the pulsating wind load, the formula is used:
W p = WGV.
Here W is the normative load, determined by the formula presented above, G is the pressure pulsation coefficient at height z, and V is the pulsation correlation coefficient. The last two parameters are determined by the tables.
For structures with a natural frequency of oscillations exceeding the established limit value, when determining the pulsating wind load , the following formula is used:
W p = WQG.
Here Q is the dynamic coefficient determined by the diagram (presented below) depending on the parameter E calculated by the formula E = √ R W / 940f ( R is the reliability coefficient for the load, f is the natural frequency of oscillations) and the logarithmic decrement of oscillations. The last parameter is constant and is accepted for:
for buildings with steel frame as 0.3;
for masts, lining pipes, etc. as 0.15.
For buildings that are symmetrical in terms of buildings, the ripple load is calculated in accordance with the formula:
Here Q is the dynamic coefficient, m is the mass of the structure at a height of z, Y is the horizontal vibrations of the structure at level z in the first form. N in this formula is a special coefficient, which can be determined by first dividing the structure by r the number of sections within which the wind load is constant, and using special formulas.
Another way
It is possible to calculate the wind load using a slightly different technique. In this case, you first need to determine the wind pressure by the formula:
Here V is the wind speed (in miles / h).
Then you should calculate the drag coefficient. It will be equal to:
1.2 - for long vertical structures;
0.8 - for short vertical ones;
2.0 - for long horizontal structures;
1.4 - for short ones (for example, the facade of a building).
Next, you need to use the general formula for the wind load on a building or structure :
Here A is the area of the region , P is the wind pressure, Cd is the drag coefficient.
You can also use a slightly more complicated formula:
When applying it, the exposure factors K z b and the gust sensitivity G h are additionally taken into account . The first is calculated as z / 33] ^ (2/7, the second - 65 + 60 / (h / 33) ^ (1/7). In these formulas, z is the height from the ground to the middle of the structure, h is the total height of the latter.
Recommendations of specialists
To calculate the wind load, engineers are often advised to use the well-known many programs MS Excel and OOo Calc from the Open Office suite. The procedure for using this software, for example, may be as follows:
- Excel is included on the "Wind Power" sheet;
- wind speed is recorded in cell D3;
- time in D5;
- the cross-sectional area of the air flow is in D6;
- air density or specific gravity - in D7;
- The efficiency of the wind turbine is in D8.
There are other ways to use this software with other input data. In any case, using MS Excel and OOo Calc to calculate the wind load on buildings and structures, as well as their individual structures, is quite convenient.