When designing buildings and structures, the calculation of the wind load has to be done quite often. This indicator is calculated using special formulas. It is important to take into account such a load, for example, when drawing up the drawings of roof truss systems, 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 an aggregate:
- pressure acting on the outer surfaces of structures of a structure or element;
- force of friction 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 a building with permeable building envelopes or open openings.
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.
Average component: basic formula
If the wind load is not taken into account during the design, this will subsequently have an extremely negative impact on the performance of the building or structure. Its average component is calculated by the following formula:
W=Wok.
Here W is the calculated value of the wind load at a height z above the earth's surface, Wo is its standard value, k is the coefficient of pressure change with height. All initial data from this formula are determined from tables.
Sometimes the calculation also uses the parameter c - the aerodynamic coefficient. The formula in this case looks like this: W=Wokс.
Normative value
To find out what this parameter is, 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 little-studied areas of the country, as well as for mountainous regions, this SNiP parameter allows you to determine according to officially registered weather stations and based on the operating experience of existing buildings and structures. In this case, a special formula is used to determine the standard value of the wind load. It looks like this:
Wo=0.61 V2o.
Here V2o - wind speed in meters per second at a level of 10 m, corresponding to an averaging interval of 10minutes and exceeded 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 the construction of the structure or building is supposed to be taken into account. There are three of them:
- Type "A" - open flat areas: coasts of seas, lakes and rivers, steppes, deserts, tundra regions, forest-steppes.
- Type "B" - terrain covered with obstacles up to 10 meters high: urban area, forests, etc.
- Type "C" - urban areas with buildings over 25 m high.
The type of construction area is also determined taking into account the requirements of SNiP. This must be taken into account when designing. Any building is considered to be located in a locality 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 building height, the type of terrain is considered certain if it remains at least 2 km from the windward side.
How to calculate ripple load
According to SNiP wind load, as already mentioned, should be determined as the sum of the average standard and pulsation. The value of the last parameter depends on the type of structure itself and the features of its design. In this regard, they distinguish:
- structures with a natural frequency of oscillation exceeding the established limit value (chimneys,towers, masts, column-type apparatuses);
- structures or elements of their construction, which are a system with one degree of freedom (transverse frames of industrial 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:
Wp=WGV.
Here W is the standard load determined by the formula presented above, G is the pressure pulsation coefficient at height z, V is the pulsation correlation coefficient. The last two parameters are determined by the tables.
For structures with a natural oscillation frequency exceeding the established limit value, the following formula is used when determining the pulsating wind load:
Wp=WQG.
Here Q is the dynamic coefficient determined from the diagram (presented below) depending on the parameter E, calculated by the formula E=√RW/940f (R is the load safety factor, f is the frequency of natural oscillations) and the logarithmic decrement fluctuations. The last parameter is constant and accepted for:
- for steel frame buildings as 0.3;
- for masts, liners, etc. as 0.15.
For symmetrical buildings, the pulsating wind load is calculated by the formula:
-
Wp=mQNY.
Here Q is the coefficient of dynamism, m is the mass of the structure at the height z, Y is the horizontal vibrations of the structure at the level z according to the first form. N in this formula is a special coefficient, which can be determined by first dividing the structure into r, the number of sections within whose boundaries the wind load is constant, and using special formulas.
One more way
You can calculate the wind load using a slightly different method. In this case, you first need to determine the wind pressure using the formula:
(Psf)=.00256V^2.
Here V is the wind speed (in miles per hour).
Then you should calculate the drag coefficient. It will be equal to:
- 1.2 - for long vertical structures;
- 0.8 - for short vertical lines;
- 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:
F=APCd.
Here A is area area, P is wind pressure, Cd is drag coefficient.
You can also use a slightly more complicated formula:
F=APCdKzGh.
When applied, exposure factors Kz b and gust sensitivity Gh 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 from experts
To calculate the wind load, engineers often advise using the well-known MS Excel and OOo Calc programs from the Open Office package. The procedure for using this software, for example, could be:
- Excel is enabled on the "Wind Energy" sheet;
- wind speed is recorded in cell D3;
- time is in D5;
- air flow area - in D6;
- air density or specific gravity - in D7;
- Wind turbine efficiency - in D8.
There are other ways to use this software with other inputs. In any case, it is quite convenient to use MS Excel and OOo Calc to calculate the wind load on buildings and structures, as well as their individual structures.
