Calculation of snow load on a pitched roof. Independent calculation of the snow load on the roof - how accurate should the calculation be?
During roof construction special attention should be given to calculating it bearing capacity, since the structure is constantly affected huge amount strength One of the forces that acts on the roof is the snow load, accordingly with which the roof is built. It is this that determines how thick the load-bearing elements will be and how to build the rafter system. Its value is calculated using a special formula, according to SNiP.
Snow load and its negative impact
Typically, up to 5% of the snow cover is removed from a pitched roof within 24 hours. It is blown away by the wind, slides off or becomes covered with crust. But the remaining amount negatively affects not only the structure, but also the person:
- The weight of snow may increase during a sharp frost after warming. In this case, deformations of the rafter system, waterproofing and thermal insulation are possible.
- Snow load on roofs that have complex design, as a rule, is distributed unevenly.
- Snow sliding towards the eaves can be dangerous for people nearby, so the installation of snow guards is mandatory.
- In addition to being dangerous to humans, sliding snow can cause harm drainage system. That is why it is necessary to clean it in time or install snow guards.
Cleaning the roof from snow mass
Most in an efficient way removing snow from the roof is manual cleaning. But it is very dangerous to carry out independently without preliminary preparation. That is why a correctly calculated snow load can help avoid constantly removing snow.
The angle of the roof slope has a positive effect on snow melting. Most the best option roofing for regions where there is a high probability of large amounts of snow is from 45 to 60 degrees.
In order to reduce ice build-up and prevent the formation of icicles, you can install cable heating around the perimeter of the roof. It can be automated or manually controlled.
Calculation of snow load on the roof
Even at the stage of designing the roof, to avoid damage to its structure during heavy rainfall, calculation measures are carried out. The average weight of snow is 100 kg per cubic meter. meter, and wet precipitation weighs even more, which is 300 kg per 1 cubic meter. meter. Knowing these approximate values, you can quite simply calculate the permissible snow load.
But this will also require knowledge of the thickness of the falling snow layer. This indicator can be measured on a flat area, and the resulting number multiplied by a coefficient that assumes a reserve and is equal to 1.5. In order to take into account the regional indicator, you can use a special map. It became the basis for obtaining SNiP rules and other regulations. In general, the indicator is determined by the following formula:
S=S calc. * μ
In accordance with this formula, its components are deciphered as follows:
- S calculated - weight value at square meter horizontal platform.
- μ - roof slope coefficient.
Usually, as mentioned earlier, calculations are made using the snow load map, which is presented below:
In accordance with SNiP, there are the following indicators of the roof slope coefficient:
- If the roof slope is less than 25 degrees, then the coefficient is 1.
- If the roof slope is in the range from 25 to 60 degrees, then the coefficient will be equal to 0.7.
- If the slope is more than 60 degrees, the coefficient may not be taken into account at all.
In this case, the side from which the wind blows is also taken into account. This is necessary, since there will in any case be less snow on the windward side than on the leeward side.
In order to better understand how the snow load is calculated, let’s imagine clear example for the Moscow region. The calculated roof has a slope of 30 degrees. So, according to the requirements of SNiP, we make the calculation:
- In the map we find the location of the Moscow region and reveal that it belongs to the third climatic region. Here the roof load value is 180 kg per 1 sq. meter.
- According to the formula, we calculate general indicator weight of snow. To do this, we multiply 180 by a coefficient equal to 0.7. We get the number 126 kg per square meter. meter.
- Already based on this indicator, a rafter system is created, which is calculated based on the maximum numbers.
In addition to this option, there is a complete calculation, which is also presented in SNiP and has a corresponding table there. The calculation is carried out using the following formula:
Q1 = m*Q
Here the coefficient indicator is m, which is calculated using the interpolation method. With a roof slope of 30 degrees it is 1, and at 60 degrees it is 0.
Q is the snow load that is indicated in the SNiP table.A standard indicator can be calculated. To do this, you need to use an atlas that records changes in SNiP or calculate the indicator using the formula: Q2 = 0.7* Q* m. If the calculation is made for a structure that is installed in areas with constant winds blowing snow off the roof, then it is necessary to add coefficient C to the formula. It is equal to 0.85. But there are a number of conditions for adding this indicator. This is a wind speed of at least 4 m/s, average monthly temperature in the winter months no higher than -5 degrees, and the slope should be between 12 and 20 degrees.
Important! If it is not clear how to calculate the load yourself, then it is better to turn to specialists.
Features of installing snow guards
If the roof structure is done correctly, taking into account the calculations, then there is no need to remove snow from the roof. And in order to prevent severe slipping, snow guards are required to be installed. Such designs are very convenient and help not to remove snow from the roof during heavy rainfall.
Typically, tubular-type snow retainers are installed, which can be used with a snow load of no more than 180 kg per 1 square meter. meter. If the weight of the snow cover is greater, then the structures are installed in several rows. SNiP regulates cases and rules when the installation of snow guards is necessary:
- The slope is more than 5%, and there is also an external drain.
- There should be a minimum distance of 0.6 m from the edge of the roof to the installed snow guard.
- If installed tubular structures, then only continuous sheathing is provided under them.
Features of calculating snow load for flat roofs
On the roof flat type accumulates enough large number snow, therefore all requirements for calculating the snow load must be met so that the roof can withstand such weight for a long time.
In most of Russia, flat roofs are not created, since a layer of snow can create an excessive load on the rafter structure. But, if, nevertheless, the house design provides for just such a reinforced concrete or other roof and it cannot be replaced, then during installation it is necessary to provide a heating system to ensure high-quality drainage of water from it.
Important! Flat roof must have minimum slope, which is equal to 2 degrees, so that water from the entire surface can drain without problems.
Conclusion
Calculating the snow load on the roof will help create optimal design rafter system, and will also keep the roofing in good condition. The correctness of the calculation depends on theoretical knowledge in this area, which can be obtained by reading this article.
At the calculation stage truss structure When choosing a covering and installing all roof elements, take into account the climate characteristics of the area where the building is located. This applies not only industrial facilities And apartment buildings, but also private cottages with pitched roofs. Given the unpredictability of Russian winters, it is important snow load calculation.
“Cap” on one of the roofs in the Moscow region, creating a snow load
Why are snow loads dangerous?
Atmospheric precipitation, especially snow accumulating on the roof, puts significant pressure on it. As it may seem, the further north the house is, the larger it is. This is only partly true. The fact is that due to frequent temperature changes from positive to negative, ice also forms on the roof. Such blocks are significantly heavier. Besides, The weight of wet snow can be three times the weight of regular snow! It is not difficult to guess that under its influence the roof structure may be deformed.
Consequences of leaks due to incorrect calculation and installation of the roof
In addition, large volumes of snow and ice can damage gutters and pose a danger to property, health and even human life. Especially for this purpose, the roof safety system includes systems that promote uniform outflow of water from the roof surface.
Map and formula for calculating snow load
To determine the value of the snow load, you need to know 2 indicators: the region of Russia where the house is located (determined from the map below) and the angle of the roof.
Appendix 5 to SNiP 2.01.07-85. To enlarge, click on the image
S = Sg * µ
S- snow load value;
Sg- value of snow cover weight per 1 m² horizontal surface(determined depending on the area on the map according to the table below);
µ - load coefficient on the roof surface depending on its angle of inclination.
- If the angle of inclination is less than 25°, then µ = 1;
- If the angle of inclination is more than 25°, but less than 60°, then µ=0.7
- If the angle of inclination is more than 60°, then the load is not calculated.
Calculation of snow load on a roof in the Moscow region
As an example, let's take a cottage in Troitsk with gable roof th, the angle of inclination of which is 35°.
- This is a snow area |||. In this case, Sg = 180 kgf/m².
- Since the tilt angle is in the range from 25° to 60°, then µ=0.7
- Substitute the obtained values into the formula S = Sg * µ
- S = 180 * 0.7 = 126 kgf/m²
Please note that this value is approximate. In the case of complex roofs with many valleys and slopes located at different angles, the calculation is more difficult to make. Load in different parts will be unevenly distributed. This can cause leaks and even structural collapse. To avoid this take into account all the nuances when calculating and constructing, from the rafter system to the installation of the security system.
It is recommended to calculate rafters as accurately as possible, based on the characteristics of the construction site, external load on rafter system, dimensions and configuration of the structure, characteristics of the material for constructing the roof.
Types of loads on rafters
Construction of a pitched roof requires the creation durable frame– load-bearing roof structure. At the design stage, it is necessary to calculate the rafters in order to determine the length and cross-section of the elements that take on the main loads (constant and variable).
Constant loads include the weight of the roofing pie, which consists of outer covering, sheathing, waterproofing layer, heat insulator, vapor barrier and inner lining of the attic or attic room. This type of load also includes the weight of equipment or other objects that are planned to be placed on the roof or fixed to the rafters from the inside.
Variable loads mean the impact of wind and precipitation, as well as the weight of the person repairing or cleaning the roof. This category also includes special loads, including seismic ones - their presence places increased demands on the reliability of the roof.
Calculation of the weight of the roofing pie
Before approaching the calculation of the cross-section of a rafter leg, single-pitch, gable or hip roof, it is important to determine the weight of the roofing cake. This requires a calculation, the formula of which is extremely simple: the weight of one square meter of each layer is summed up roofing system, and the resulting result is multiplied by 1.1 - a correction factor that allows increasing the reliability of the structure by 10%.
Thus, the standard roof weight calculation looks like this:: (weight of 1 m 2 lathing + weight of 1 m 2 roofing+ weight of 1 m 2 of waterproofing + weight of 1 m 2 of insulation) × 1.1 = weight of the roofing pie taking into account the correction factor. When using most popular roofing materials (except for the heaviest), this load on the rafters does not exceed 50 kg/m2.
When developing a project for a pitched or gable roof, it is enough to focus on the weight of the roofing pie equal to 55 kg/m2. This approach will allow you to build a roof frame with a margin of strength and subsequently change the type of roofing without recalculating the rafter system.
Snow and wind loads
For many regions of Russia, the issue of snow loads on rafters is relevant - the rafter leg is required to withstand the weight of accumulated snow without deforming. The smaller the angle of inclination of the roof (usually this applies to a lean-to structure), the higher the snow load. Construction is almost flat pitched roof requires the use of rafters large section and the minimum step of their installation. In this case, you should regularly clean the pitched roof, the angle of inclination of which does not exceed 25°.
The formula S = Sg × µ allows you to calculate the snow load (S). In this case:
- Sg – reference value of the weight of snow cover on 1 square meter of horizontal surface (selected from the table in SNiP “Rafter systems” depending on the region of construction);
- µ – correction factor, the value of which is determined by the angle of inclination of the roof.
The coefficient µ is equal to:
- 1.0 – slope angle up to 25°;
- 0.7 – slope angle from 25 to 60°.
For roofs with slopes whose slope angle exceeds 60°, snow loads are not taken into account in the calculations.
To calculate the wind load (W), the formula W = Wo × k is used, where:
- Wo – reference value of wind load characteristic of a particular region (selected from the table);
- k is a correction factor, the value of which depends on the height of the structure and the type of terrain.
A – open area (field, steppe, coast);
B – urban development, forest.
The relationship between the cross section and the length of the rafters
Calculating the length of the rafters is quite simple, if you take into account that almost the entire roof is a system of triangles (it doesn’t matter whether we are talking about a single-pitch, gable or complex roof). Knowing the length of the walls of the building, the angle of inclination of the slope or the height of the ridge, using the Pythagorean theorem the length of the rafter from the edge of the wall to the ridge is calculated. To the obtained value you need to add the amount of the eaves overhang (if the rafters protrude beyond the edge of the wall). In some cases eaves overhang is formed by installing fillies - boards for building up the rafter legs. The length of the fillies is added to the length of the rafters when calculating the roof area - this will allow you to determine the exact amount of materials for installing the roofing pie.
To determine which board or timber section is suitable for the construction of a particular pitched, gable or hip roof, you can use the table of standards, which shows the correspondence between such parameters as the thickness of the lumber, the length of the rafter leg and the installation pitch of the rafters.
The cross-sectional parameters of the rafters vary from 40×150 mm to 100×250 mm. The length of the rafter leg depends on the angle of inclination of the slope and the length of the span between opposite walls. As the angle of inclination of the slope increases, the length of the rafter increases, which requires the use of lumber larger section to ensure the required structural strength. At the same time, the snow load on the roof is reduced, and the rafter installation step can be made less frequent. At the same time, a decrease in the pitch of the rafters leads to an increase in the total load on the rafter leg.
When performing calculations, it is necessary to take into account all factors in order to achieve the required strength of the roof frame, including taking into account the characteristics of wood (density, degree of humidity, quality) during construction wooden structures, thickness of metal elements - during the construction of metal roof frames.
The supporting structure of the roof must have a high degree of rigidity - it is necessary to prevent deflection of the rafters under loads. Deflection occurs if errors were made when calculating the cross-section of the roof elements and the installation pitch of the rafters. If the deflection of the rafters was detected after the roof was installed, additional elements (struts) can be used to stiffen the structure. If the length of the rafter leg of a single-pitched, gable or hip roof exceeds 4.5 meters, without installing struts, a deflection can form regardless of the cross-section of the wooden rafter legs. This should be taken into account when calculating the length of the rafters.
The basic principles of calculation are based on the fact that the choice of beam thickness depends on the total load on the roof. An increase in the thickness of the rafters leads to an increase in the strength of the roof, eliminates deflection, but at the same time the total weight of the rafter system increases significantly, that is, the loads on building structures and foundation. Rafters on residential buildings set in increments of 60 - 100 cm, the specific value depends on:
- design load;
- rafter sections;
- characteristics roofing material;
- slope angle;
- width of the insulating material.
The calculation of the number of rafter legs is directly related to the step of their installation. Initially, a suitable installation step is selected, then the length of the wall should be divided by this value, add one to the result and round the number. By dividing the length of the wall by the result obtained, you can get the required gap between the rafters.
When determining the number of rafters on one slope, it is important to remember that the distance between the axes of the rafter legs is taken into account.
Metal rafter structures
In private housing construction, the use of metal rafter systems is less common, since a metal frame needs to be installed by welding - this leads to an increase in the complexity and volume of work. You can order the construction to be manufactured in production, but its installation will require the use of special equipment. Designing a metal roof frame requires accurate calculation and compliance with the dimensions of all elements, since there is no possibility to adjust the part directly during installation.
There are no complaints about the strength of metal rafter systems: the use of metal profiles makes it possible to eliminate rafter deflection even when covering large spans without installing additional elements for strength and rigidity. Metal rafters can span spans of more than 10 meters without forming a deflection under design loads.
When calculating a metal rafter system, you should take into account the weight of the material, the loads on building structures and the foundation. The strength parameters of metal rafters and their high resistance to deflection loads make it possible to significantly reduce the number of these elements compared to a wooden structure.
Calculation metal frame roofs should be maintained based on reference values for the strength of elements (channels, corners, beams, etc.) depending on their shape and thickness. The size of the spans and the angle of inclination of the slopes should be taken into account.
The supporting structure for a metal rafter system (mauerlat) must be metal beam securely attached to the top edge of the wall.
Calculation of rafters: length, load, cross-section and number of rafters per roof
Calculation of the length and cross-section of rafters and rafter legs on the roof. Calculation of load on wooden rafters according to the formula. Calculation of the angle, pitch and thickness of rafters.
How to calculate the loads on a truss structure
City dwellers often want to live in their own home. If you decide to build this house, when preparing its technical design, do not forget to first calculate the rafters, which determines the parameters of all load-bearing structures. Thanks to preliminary calculation you will avoid design errors and after construction you will be able to live peacefully in your home without worrying about its integrity.
The roof rafter system is the most important and important element roof structure, which ensures its stability and strength.
Based on what factors should the calculation be made?
In order for the calculation of the rafter system to be carried out correctly, it is necessary to determine the intensity of the loads on the roof. Such loads are divided into several types:
Construction of the rafter system. In order for the frame to be strong, the wooden rafter legs rest firmly on the outer walls through the mauerlat (longitudinal beam).
- Permanent in nature. This is a load that will constantly affect the rafter system; it includes the own weight of the roof, sheathing, waterproofing and vapor barrier, insulation and other elements that form a constant value with a stable fixed weight.
- Variables. These are loads determined by climatic factors: wind and its intensity, the amount of snow and other precipitation. They only affect the rafters occasionally.
- Special. In this type of load, extreme manifestations are taken into account climatic factors or their increased intensity. This type of load must be taken into account in areas where seismic activity, hurricanes or storm winds are likely.
Taking all these factors into account at the same time, especially if you are doing it for the first time, is quite difficult. After all, it is necessary not only to take into account the loads, but also the weight and strength that the rafter beam has, the method of fastening the boards to each other, and other quantities. Many people think that this work can be made easier by a rafter calculation program, but this is not entirely true. Such programs operate with already calculated data on the loads that the rafter system will have to withstand. Therefore, after making your own calculations, you will feel everything design features the roof you will be constructing.
Calculation of permanent loads
Schemes of standard snow loads. If the roof slope is more than 60 degrees, the snow load is not taken into account in the calculation of the rafter system.
Before determining what the length of the rafters will be, you need to understand what to focus on. Therefore, it is right to start with something simple, that is, by determining the weight of the roof structure itself. To do this, you must calculate what the weight of one square meter will be. m of each layer. First you need to study technical specifications material that should be, usually the required value is indicated there. After all the data has been received, add all the values together and increase the result by 10%, thereby setting the safety margin of the rafter system. It is better to select materials so that per square meter. m of roof area did not account for more than 50 kg of weight.
Snow load calculation
To undertake further calculations of rafters, you should move on to calculating variable loads, and specifically snow loads, since many areas experience the long-term influence of snowy winters. And the weight of the snow acting on the roof should not break the beam used as a rafter leg.
This type of load is calculated using the formula: snow weight per 1 sq.m × correction factor = total snow load. The first value is an average value and varies depending on the regional location of the house. The correction factor must be taken from SNiP 2.01.07-85. This result should also be increased by 10%, thereby creating a safety margin.
Wind load calculation
Wind load diagram. They depend on the area where the house is located.
This indicator is very important for inclined structures, which are roof slopes. At small angles of inclination there is a danger of roof destruction, and at large angles the wind pressure is very high over the entire surface of the slope, so the height of the roof must be thought out as carefully as possible. The calculation formula looks like this: region indicator × coefficient = wind load. To determine the region's indicator, there is a table of values; the coefficient varies depending on the height of the house and the area around it (forest, steppe, high-rise buildings). You can find out the exact values of these two quantities in the same SNiP, as they should be suitable for your project.
Calculation principle
Calculation of loads on rafter systems. Calculation of the truss structure and arrangement of elements is carried out by developing plans and roofing diagrams.
Having set out to correctly calculate the length of the rafter leg, realize that almost the entire roof is a system of triangles, regardless of the configuration of the trusses. Therefore, determining the length of the boards required for the structure will not be special labor. What section of beam to choose or the number of legs is another matter. A guide to the correctness of these calculations can be a table of standards, where you can see the correspondence between the length, cross-section and pitch of the legs.
For example, the cross-section of rafters for a pitched roof can vary from 40*150 mm to 100*250 mm. The shorter the installation step, the greater the length of the rafter leg, which means that the total load on it increases, and as a result, the cross-section of the rafters should be larger. Everything matters in these calculations: what kind of wood you use for the timber, how the wood was dried, where the structure is located, what loads it will be subjected to. Don't neglect any factors. Detailed example rafter calculations can be found in SNiPs for building design.
What algorithm of actions to follow
Table of weights of roofing materials. The value of loads on rafter systems can vary significantly depending on the selected roofing covering.
Calculation of rafters: loads that need to be taken into account
Calculation of rafters is the basis for a properly designed roof. Thanks to preliminary calculations you will avoid design errors
Calculator for calculating the load on rafters to determine the optimal cross-section
For the manufacture of rafter legs, high-quality lumber of a certain cross-section is used. Its strength characteristics must be guaranteed to be sufficient so that the roof structure can withstand all the loads placed on it.
Calculator for calculating the load on rafters to determine them optimal cross section
To determine this parameter, you will have to carry out some calculations. A calculator for calculating the load on rafters to determine the optimal cross-section of lumber for their manufacture can provide all possible assistance.
The necessary explanations for the calculations will be given below.
Algorithm for calculating the cross-section of rafter legs
The work will be built in two stages. First, using the calculator, the distributed load per 1 linear meter rafter leg. Then, according to the attached table, it will be possible to select optimal size timber for making rafters.
Step one - calculating the distributed load on the rafter legs
The calculation calculator will ask for the following values:
- Slope angle. This value is directly related to the levels of external loads on the roof - snow and wind.
With the steepness of the slope and, accordingly, with ridge height(ridge unit) the special calculator to which the link leads will help you figure it out.
- Type of planned roofing. Naturally, various coatings have their own mass, which determines the static load on the rafter system. The calculator already takes into account not only weight characteristics various coatings, but also lathing and roof insulation materials.
- It is necessary to indicate the zone of your region according to the level of possible snow load. It is easy to determine from the diagram map below:
Schematic map for determining your zone by snow load level
- The zone is determined in a similar way by the level of wind pressure - for this there is its own schematic map.
Schematic map for determining the zone according to the degree of wind impact on the roof
- It is necessary to take into account the specific location of the building on the ground. To do this, you need to evaluate its “surroundings” and choose one of the three proposed zones, “A”, “B” or “C”.
There is a nuance to this. All natural or artificial wind barriers can be taken into account only if they are located at a distance from the house not exceeding 30×N, Where N– this is the height of the building along the ridge. For example, for a building 7 meters high, a circle with a radius of 210 meters is obtained. If the obstacles are further away, it will be considered open terrain.
- Finally, you will need to enter the height of the house in meters (at the ridge).
- The last window of the calculator is the step of installing rafter legs. The more often they are installed, the less will be the distributed load falling on each of them, but at the same time, their number naturally increases. You can “play” with the step value to track the dynamics of changes in the distributed load - this will give you the opportunity to select optimal value to further determine the cross-section of the rafters.
Step two - determining the cross-section of the rafter leg
So, there is a value of the distributed load per linear meter of the rafter leg. Surely it was calculated in advance and rafter length(if not, it is recommended to go to the appropriate calculator). With this data you can already enter the table to determine the cross-section of the beam.
There is one more nuance. If the rafters are too long, then to increase their rigidity, additional reinforcing elements of the system are often provided - racks (headstocks) or struts. They allow you to reduce the “free span” distance, that is, between adjacent support points. It is this value that will be needed to enter the table.
The illustration with arrows shows an example of determining the cross-section of a rafter for a distributed load of 75 kg/linear meter and with a distance between support points of 5 meters. On the left side of the table, you can take any of the proposed values, which seems more convenient: boards or beams with minimum sections: 40 × 200; 50×190; 60×180; 70×170; 80×160; 90×150; 100x140. In addition, you can use a log with a diameter of 140 mm.
Rafters are the main load-bearing elements of the roof structure
The durability and reliability of the entire system depend on their quality and correct calculation. roofing structure generally.
Calculator for calculating the load on rafters to determine the optimal cross-section - with the necessary explanations
A calculator for calculating the load on the rafters to determine the optimal cross-section is an assistant when designing your own roof. With detailed explanations.
Calculator for calculating the load on rafters to determine the optimal cross-section for design
When installing rafters, lumber of suitable sizes is used to withstand the applied roof loads. The cross-section of elements must be determined taking into account all factors influencing performance characteristics designs. By using our calculator you can make the calculation process much easier.
The thickness and width of the rafters corresponds to the expected load
Introduction to the calculation algorithm
All work can be divided into two main stages. In the first of them, using the presented program, the load per linear meter is calculated. Next, using a special table, the acceptable cross-section of the beam used as a rafter leg is determined.
Stage No. 1: obtaining the result in the form of a distributed load
The calculator fields require you to enter certain parameters.
Auxiliary map for determining the load created by snow cover
- The slope angle is indicated first of all in order to understand what kind of load will be exerted by external factors in the form of snow and wind. Optimal slope must be selected taking into account the roofing coating used and other characteristics.
- It is necessary to indicate the type of roofing material, because the weight of coatings can vary significantly. In this way, it is possible to find out the static load that will be exerted on the rafter legs. The presented program already contains weight indicators different materials, and not only roofing.
- In a special field, you should also select a region zone corresponding to a certain snow load. To determine it, a special map is used.
- In the same way, the pressure exerted by the wind is recognized and entered. To do this, use the appropriate card.
- The location of the building must also be taken into account. You are asked to evaluate and mark one of the options. The building may be located in an open area, in wooded areas or in a dense urban area. When choosing an item, you need to take into account the most acceptable option. All artificial and natural wind barriers must be considered if they are within a certain distance. To determine in which zone the building is located, you should multiply 30 meters by its height (from the ground to the ridge). The result obtained will be the radius for drawing the circle. If the main obstacles are outside the circle, then the building is in an open area.
- The height of the building in meters must be indicated in a special field of the initial data. It is necessary to reflect the distance to the highest point, which is usually the ridge.
- The final point is the step of installing the rafters. With frequent installations, the distributed load decreases. If necessary, you can change the distance between them to look at the value of the force transmitted to each linear meter of the element.
A special map is presented to determine the load created by wind
Stage No. 2: determining the cross-section of the beams used for the rafter system
When the distributed load acting on each meter of the beam has been obtained, you can use the table to find out the appropriate dimensions for each specific case. The length of the rafter leg must also be determined. Having such data, you can refer to the table to help you select the section.
One more point needs to be taken into account. If the beams are relatively long, then special elements such as racks or struts are used to improve the strength properties. They make it possible to reduce the flight distance directly between reference points.
It is proposed to use a table to determine the cross-section of the rafters
If the load distributed between the rafters is 75 kg per meter of length, and the step between the support points is 5 meters, then after studying the table you can understand that certain sections are suitable for the work.
A little about choosing lumber
If you plan to build a residential building, then you can use it for rafters. pine wood. For baths where hot air rises, you can purchase lumber from larch or other moisture-resistant species. There should not be any cracks or excessively large knots on the surface of the beams.
The moisture content of the lumber used should be within 18-22 percent, otherwise deformation changes in the system are possible, which will certainly affect the durability of the structure. In addition, poorly dried beams quickly rot. Raw elements create installation difficulties. It is much more difficult to lift them to a height than dry ones, since a significant proportion of the weight is water.
Calculator for calculating the load on rafters to determine the optimal cross-section with explanations
The cross-section of beams must be determined taking into account all factors affecting the operational characteristics of the structure. By using our calculator you can make the calculation process much easier.
When designing and constructing hangars, it is necessary to take into account the snow loads that the load-bearing structure. This is necessary so that during the operation of the hangar, due to overpressure snow cover, the roof of the building did not collapse. IN different regions In Russia, the weight of snow cover per square meter can vary significantly. When making calculations, you can use snow load maps, from which it is easy to determine the region number and correctly calculate the load.
Entire territory Russian Federation is divided into 8 regions, with varying snow loads. In the first, the weight of the cover will be minimal, respectively, the heaviest load falls on areas with indexes 8. Here the weight of snow (wet and sticky) can reach 560 kg/m2.
| snow area | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 80 | 120 | 180 | 240 | 320 | 400 | 480 | 560 |
In addition to the snow load, it is also necessary to take into account the wind load on the structure. Wind load is the pressure of wind on a structure over a long period of time. Depends on the shape of the object. When moving, air currents encounter the walls and roof of the structure. The strength of these flows must be taken into account and included when designing a building. There are 8 wind regions, with different pressure levels in each.
| wind region | Ia | I | II | III | IV | V | VI | VII |
| 17 | 23 | 30 | 38 | 48 | 60 | 73 | 85 |
The MOSTENT company has long been engaged in the design and construction of prefabricated structures, thanks to professional and competent calculations, our hangars are successfully operated under any snow and wind loads.
| city | wind region | snow area |
|---|---|---|
| 3 | 2 | |
| 2 | 5 | |
| Angarsk | 3 | 2 |
| Arzamas | 2 | 4 |
| Artem | 4 | 3 |
| Arkhangelsk | 2 | 4 |
| Astrakhan | 3 | 1 |
| Achinsk | 3 | 4 |
| Balakovo | 3 | 3 |
| Balashikha | 1 | 3 |
| Barnaul | 3 | 4 |
| Bataysk | 3 | 2 |
| Belgorod | 2 | 3 |
| Biysk | 1 | 4 |
| Blagoveshchensk | 3 | 1 |
| Bratsk | 2 | 3 |
| Bryansk | 1 | 3 |
| Velikie Luki | 1 | 3 |
| Veliky Novgorod | 1 | 3 |
| Vladivostok | 4 | 2 |
| Vladimir | 1 | 3 |
| Vladikavkaz | 2 | |
| Volgograd | 3 | 2 |
| Volzhsky Volgogr. Region | 3 | 2 |
| Volzhsky Samarsk. Region | 3 | 4 |
| Volgodonsk | 3 | 2 |
| Vologda | 1 | 4 |
| Voronezh | 2 | 3 |
| Grozny | 4 | 2 |
| Derbent | 5 | 2 |
| Dzerzhinsk | 1 | 4 |
| Dimitrovgrad | 2 | 4 |
| Ekaterinburg | 2 | 3 |
| Dace | 2 | 3 |
| Railway | 2 | 3 |
| Zhukovsky | 1 | 3 |
| Zlatoust | 2 | 4 |
| Ivanovo | 1 | 4 |
| Izhevsk | 1 | 5 |
| Yoshkar-Ola | 1 | 4 |
| Irkutsk | 3 | 2 |
| Kazan | 2 | 4 |
| Kaliningrad | 2 | 2 |
| Kamensk-Uralsky | 1 | 3 |
| Kaluga | 1 | 3 |
| Kamyshin | 2 | 3 |
| Kemerovo | 3 | 4 |
| Kirov | 1 | 5 |
| Kiselevsk | 2 | 4 |
| Kovrov | 1 | 4 |
| Kolomna | 1 | 3 |
| Komsomolsk-on-Amur | 3 | 4 |
| Kopeysk | 2 | 3 |
| Kopeysk | 1 | 4 |
| Krasnogorsk | 1 | 3 |
| Krasnodar | 6 | 2 |
| Krasnoyarsk | 3 | 3 |
| Mound | 2 | 3 |
| Kursk | 2 | 3 |
| Kyzyl | 1 | 2 |
| Leninsk-Kuznetsky | 3 | 4 |
| Lipetsk | 2 | 3 |
| Lyubertsy | 1 | 3 |
| Magadan | 5 | 5 |
| Magnitogorsk | 3 | 4 |
| Maykop | 2 | |
| Makhachkala | 5 | 2 |
| Miass | 2 | 3 |
| Moscow | 1 | 3 |
| Murmansk | 4 | 5 |
| Moore | 1 | 3 |
| Mytishchi | 1 | 3 |
| Naberezhnye Chelny | 2 | 5 |
| Nakhodka | 5 | 2 |
| Nevinnomyssk | 5 | 2 |
| Neftekamsk | 2 | 5 |
| Nefteyugansk | 2 | 4 |
| Nizhnevartovsk | 2 | 5 |
| Nizhnekamsk | 2 | 5 |
| Nizhny Novgorod | 1 | 4 |
| Nizhny Tagil | 2 | 4 |
| Novokuznetsk | 3 | 4 |
| Novokuibyshevsk | 3 | 4 |
| Novomoskovsk | 1 | 3 |
| Novorossiysk | 5 | 2 |
| Novosibirsk | 3 | 4 |
| Novocheboksarsk | 2 | 4 |
| Novocherkassk | 3 | 2 |
| Novoshakhtinsk | 3 | 2 |
| Novy Urengoy | 2 | 5 |
| Noginsk | 1 | 3 |
| Norilsk | 3 | 5 |
| Noyabrsk | 2 | 5 |
| Obnisk | 1 | 3 |
| Odintsovo | 1 | 4 |
| Omsk | 2 | 3 |
| Eagle | 2 | 3 |
| Orenburg | 3 | 4 |
| Orekhovo-Zuevo | 1 | 3 |
| Orsk | 2 | 4 |
| Penza | 2 | 3 |
| Pervouralsk | 2 | 4 |
| Permian | 2 | 5 |
| Petrozavodsk | 5 | 2 |
| Petropavlovsk-Kamchatsky | 7 | 7 |
| Podolsk | 1 | 3 |
| Prokopyevsk | 2 | 4 |
| Pskov | 1 | 3 |
| Rostov-on-Don | 3 | 2 |
| Rubtsovsk | 3 | 3 |
| Rybinsk | 1 | 4 |
| Ryazan | 1 | 3 |
| Salavat | 3 | 5 |
| Samara | 3 | 4 |
| Saint Petersburg | 2 | 3 |
| Saransk | 2 | 3 |
| Saratov | 3 | 3 |
| Severodvinsk | 2 | 4 |
| Serpukhov | 1 | 3 |
| Smolensk | 1 | 3 |
| Sochi | 4 | 2 |
| Stavropol | 5 | 2 |
| Stary Oskol | 2 | 3 |
| Sterlitamak | 3 | 5 |
| Surgut | 2 | 4 |
| Syzran | 3 | 3 |
| Syktyvkar | 1 | 5 |
| Taganrog | 3 | 2 |
| Tambov | 2 | 3 |
| Tver | 1 | 4 |
| Tobolsk | 2 | 4 |
| Tolyatti | 3 | 4 |
| Tomsk | 3 | 4 |
| Tula | 1 | 2 |
| Tyumen | 2 | 3 |
| Ulan-Ude | 3 | 1 |
| Ulyanovsk | 2 | 4 |
| Ussuriysk | 3 | 2 |
| Ufa | 2 | 5 |
| Ukhta | 2 | 5 |
| Khabarovsk | 3 | 2 |
| Khasavyurt | 5 | 2 |
| Khimki | 1 | 3 |
| Cheboksary | 2 | 4 |
| Chelyabinsk | 2 | 3 |
| Chita | 2 | 1 |
| Cherepovets | 1 | 4 |
| Mines | 3 | 2 |
| Shchelkovo | 1 | 3 |
| Elektrostal | 1 | 3 |
| Engels | 3 | 3 |
| Elista | 3 | 2 |
| Yuzhno-Sakhalinsk | 4 | 4 |
| Yaroslavl | 1 | 4 |
| Yakutsk | 2 | 2 |
The topic of snow in September is not very relevant even for us - residents of Siberia. However... the “sleigh” should already be ready, despite the fact that we still continue to ride on “carts”. Moments come to mind when after heavy snowfall in winter and before the snow melts in spring...
Owners various buildings- from bathhouses, sheds and greenhouses to huge swimming pools, stadiums, workshops, warehouses - they are puzzled by two questions arising from each other: “Will the roof withstand or not withstand the mass of snow accumulated on it? Should I throw this snow off the roof or not?”
Snow load on a roof is a serious issue and cannot tolerate an amateurish approach. I will try to present information about snow as briefly and clearly as possible and provide assistance in resolving the issues raised above.
How much does snow weigh?
Anyone who has had to shovel snow knows that snow can be very light and incredibly heavy.
Fluffy light snow that fell in relatively frosty weather with an air temperature of about -10˚C has a density of about 100 kg/m3.
Late autumn and early winter specific gravity snow lying on horizontal and slightly inclined surfaces is usually 160±40 kg/m3.
During prolonged thaws, the specific gravity of snow begins to increase significantly (the snow “sits” like in spring), sometimes reaching values of 700 kg/m3. This is why in warmer areas the snow density is always greater than in cold northern areas.
By mid-winter, the snow becomes compacted under the influence of the sun, wind and the pressure of the upper layers of snowdrifts on the lower layers. The specific gravity becomes equal to 280±70 kg/m3.
By the end of winter, under the influence of more intense sun and February winds, the density of snow crust can become equal to 400±100 kg/m3, sometimes reaching 600 kg/m3.
In the spring, before heavy melting, the specific gravity of “wet” snow can be 750±100 kg/m3, approaching the density of ice - 917 kg/m3.
Snow that has been raked into heaps and thrown from place to place doubles its specific gravity.
The most probable average density of “dry” compacted snow is in the range of 200...400 kg/m3.
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To remove snow from roofs or not?
Need to understand simple thing– the mass of snow lying on the roof, in the absence of snowfalls, remains unchanged regardless of density!!! That is, the fact that the snow “became heavier” did not increase the load on the roof!!!
The danger is that a layer of loose snow can absorb precipitation in the form of rain like a sponge. That's when total mass water in its various forms located on the roof will increase sharply - especially in the absence of drainage, and this is very dangerous.
To correctly answer the question about removing snow from the roof, you need to know what load it is designed for and built. You need to know what the distributed load pressure is - how many kilograms per square meter - roof can really hold before unacceptable deformations of the structure begin.
To answer this question objectively, it is necessary to examine the roof, draw up a new or confirm the design calculation diagram, perform a new calculation or take the results of the old design one. Next, you should experimentally determine the density of the snow - for this, a sample is cut out, weighed and its volume is calculated, and then the specific gravity.
If, for example, according to calculations, the roof must withstand a specific pressure of 200 kg/m2, the snow density determined experimentally is 200 kg/m3, then this means that snowdrifts should not be more than 1 m deep.
If there is a snow cover on the roof more than 0.2...0.3 m deep and there is a high probability of rain followed by cold weather, it is necessary to take measures to dump the snow.
Standard and design snow load.
during the design and construction of facilities? The answer to this question is set out for specialists in SP 20.13330.2011 Loads and impacts. Updated version of SNiP 2.01.07-85*. We will not “take bread” from construction designers and delve into the options for geometric types of coatings, slope angles, snow drift coefficients and other complexities. But let’s create a general algorithm and write a program that implements it. We will learn to determine the standard and calculated snow pressure on the horizontal projection of the surface for objects in any area of Russia that interests us.Let's remember a few “axioms”. If on a simple pitched or gable roof the slope angle of the covering more than 60˚ , then it is considered that There can be no snow on such a roof (μ =0) . He will all “roll off”. If the slope of the coating less than 30˚ , then it is considered that all the snow on such a roof lies in the same layer as on the ground (μ =1) . All other cases are intermediate values determined linear interpolation. For example, at angle equal to 45˚ only 50% of the fallen snow will lie on the roof (μ=0.5).
Designers carry out calculations based on limit states, which are divided into two groups. Go beyond limit states The first group is the destruction and loss of an object. The transition beyond the limit states of the second group is when deflections exceed permissible limits and, as a consequence, the need for repair of the object, possibly a major one. In the first case, the calculation uses a calculated snow load equal to the standard load increased by 40%. In the second case, the calculated snow load is the standard snow load.
Calculation in Excel of snow load according to SP 20.13330.2011.
If you do not have MS Excel on your computer, you can use a freely available, very powerful alternative - the OOo Calc program from the Open Office package.
Before you start, search the Internet and download SP 20.13330.2011 with all applications.
Part important materials from SP 20.13330.2011 are in the file that site subscribers can download from the link located at the very end of this article.
We turn on the computer and start calculating the snow load on the surfaces in Excel.
In cells with a light turquoise fill we will write the initial data selected by SP 20.13330.2011. We calculate the results in cells with a light yellow fill. In cells with a pale green fill we will place the original data, which is little subject to change.
In the notes for all cells in a column C put formulas and links to points SP 20.13330.2011!!!
1. We open Appendix G in SP 20.13330.2011 and using the map “Zoning the territory of the Russian Federation by weight of snow cover” we determine the number of the snow region for the area where the building was built (or will be built). For example, for Moscow, St. Petersburg and Omsk this is the III snow region. Select the corresponding line with entry III in the drop-down list box located on top
You can read in detail about how the INDEX function works together with a combo box.
2. We read the mass of snow cover per 1 m2 of horizontal surface of the earth Sg in kg/m2 for the selected area
3. In accordance with clause 10.5-10.9 of SP 20.13330.2011, we accept the value of the coefficient that takes into account the removal of snow from building surfaces by the wind Ce
in cell D4: 1,0
Ce- write 1.0.
4. We assign, in accordance with clause 10.10 SP 20.13330.2011, the value of the thermal coefficient Ct
in cell D5: 1,0
If you don't understand how to prescribe Ct- write 1.0.
5. We assign, in accordance with clause 10.4 of Appendix G SP 20.13330.2011, the value of the coefficient of transition from the weight of the snow cover of the ground to the snow load on the surface μ
in cell D6: 1,0
Let us recall the “axioms” from the previous section of the article. If you don’t remember and don’t understand anything, write 1.0.
6. Reading normative meaning snow load on the horizontal projection of the coating S0 in kg/m2, calculated
in cell D7: =0.7*D3*D4*D5*D6 =128
S0 =0.7*Ce *Ct *μ * Sg
7. In accordance with clause 10.12 of SP 20.13330.2011, we record the value of the reliability coefficient for snow load γ f
in cell D8: 1,4
8. And finally, we read the calculated value of the snow load on the horizontal projection of the coating S in kg/m2, calculated
in cell D9: =D7*D8 =180
S = γ f * S0
Thus, for “simple” buildings of the third snow region withμ =1 calculated snow load is 180 kg/m2. This corresponds to a snow cover height of 0.90...0.45 m with a snow density of 200...400 kg/m3, respectively. Let each of us draw conclusions!
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I look forward to your comments, dear readers!!! I ask professional builders to “not hit too hard.” The article was written not for specialists, but for a general audience.