Before you start building a roof, it is of course desirable that it be designed for strength. Immediately after the publication of the last article ““, I began to receive questions in the mail regarding the choice of the cross-section of rafters and floor beams.
Yes, understanding this issue in the vastness of our beloved Internet is indeed quite difficult. There is a lot of information on this topic, but as always it is so scattered and sometimes even contradictory that an inexperienced person, who in his life may not even have encountered such a subject as “Sopromat” (lucky someone), can easily get confused in these wilds.
I, in turn, will now try to create a step-by-step algorithm that will help you independently calculate the rafter system of your future roof and finally get rid of constant doubts - what if it won’t hold up, or what if it will fall apart. I will say right away that I will not delve into the terms and various formulas. Well, why? There are so many useful and interesting things in the world that you can fill your head with. We just need to build a roof and forget about it.
The entire calculation will be described using the example of a gable roof, which I wrote about in
So, Step #1:
We define snow load on the roof. To do this, we need a map of snow loads in the Russian Federation. To enlarge the picture, click on it with the mouse. Below I will give a link where you can download it to your computer.
Using this map, we determine the number of the snow region in which we are building a house and from the table below we select the snow load corresponding to this region (S, kg/m²):
If your city is located on the border of regions, choose higher value loads. There is no need to adjust the resulting figure depending on the angle of inclination of the slopes of our roof. The program we will use will do this itself.
Let's say in our example we are building a house in the Moscow region. Moscow is located in the 3rd snow region. The load for it is 180 kg/m².
Step #2:
Determine the wind load on the roof. To do this, we need a map of wind loads in the Russian Federation. It can also be downloaded from the link below.
Using this map, we also select the corresponding region number and determine the wind load value for it (the values are shown in the lower left corner):
Here, column A is the open coasts of seas, lakes and reservoirs, deserts, steppes, forest-steppes and tundra; Column B - urban areas, forests and other areas evenly covered with obstacles. It should be taken into account that in some cases the type of terrain may differ in different directions(for example, the house is located on the outskirts of a populated area). Then select the values from column “A”.
Let's return to our example again. Moscow is located in I-th wind region. The height of our house is 6.5 meters. Let's assume that it is being built in locality. Thus, we accept the value of the correction factor k=0.65. Those. wind load in in this case will be equal to: 32x0.65=21 kg/m².
Step #3:
You need to download a calculation program made in the form of an Excel table to your computer. We will continue to work in it. Here is the download link: ". Also here are maps of snow and wind loads in the Russian Federation.
So, download and unpack the archive. Open the file “Calculation” rafter system", and we get to the first window - “Loads”:
Here we need to change some values in the filled cells blue. All calculations are done automatically. Let's continue with our example:
In the “Initial data” plate we change the angle of inclination to 36° (whatever angle you have, write that, well, I think this is clear to everyone);
We change the pitch of the rafters to the one we chose. In our case it is 0.6 meters;
Load roof (self-weight load roofing material) — we select this value from the table:
For our example, we choose metal tiles with a weight of 5 kg/m².
Snow. region - here we enter the sum of the values of snow and wind loads that we received earlier, i.e. 180+21=201 kg/m²;
Insulation (mans.) - we leave this value unchanged if we lay insulation between the rafters. If we do cold attic without insulation - change the value to 0;
We write in the “Lathing” sign required dimensions sheathings. In our case, for metal tiles, we will change the sheathing pitch by 0.35 m and the width by 10 cm. We leave the height unchanged.
All other loads (from the own weight of the rafters and sheathing) are taken into account by the program automatically. Now let's see what we got:
We see the inscription “The load-bearing capacity of the sheathing is ensured!” We don’t touch anything else in this window; we don’t even need to understand what the numbers are in other cells. If, for example, we choose a different rafter pitch (more), it may turn out that the load-bearing capacity of the sheathing will not be ensured. Then it will be necessary to select other dimensions of the sheathing, for example, increase its width, etc. In general, I think you will figure it out.
Step #4:
Sling.1"and go to the window for calculating rafters with two support points. Here, all the input data we previously entered is already entered by the program automatically (this will be the case in all other windows).
In our example from the article “ Gable roof do-it-yourself houses” rafters have three points of support. But let’s imagine that there are no intermediate posts and let’s do the calculation:
On the rafter diagram we change the length of its horizontal projection (the cell is filled in blue). In our example, it is 4.4 meters.
In the “Calculation of rafters” plate, change the value of the rafter thickness B (specified) to what we have chosen. We set 5 cm. This value must be greater than that indicated in the cell Tue (stable);
Now in the line " We accept N"We need to enter the selected rafter width in centimeters. It must be greater than the values indicated in the lines " Ntr.,(strong)" And " Ntr., (deflection)". If this condition is met, all the inscriptions at the bottom under the rafter diagram will look like “Condition met.” In the line " N, (by variety)" indicates the value that the program itself offers us to choose. We can take this number, or we can take another. We usually choose sections available in the store.
So, what we got is shown in the figure:
In our example, to meet all the strength conditions, it is necessary to choose rafters with a section of 5x20 cm. But the roof diagram I showed in the last article has rafters with three support points. Therefore, to calculate it, we move on to the next step.
Step #5:
Click on the tab " Sling.2" or " Sling. 3″. This opens a window for calculating rafters with 3 support points. We select the tab we need depending on the location of the middle support (rack). If it is located to the right of the middle of the rafter, i.e. L/L1<2, то пользуемся вкладкой "Strop.2". If the post is located to the left of the middle of the rafter, i.e. L/L1>2, then use the tab "Sling.3". If the stand is exactly in the middle, you can use any tab, the results will be the same.
On the rafter diagram, we transfer the dimensions in cells filled with blue (except for Ru);
Using the same principle as described above, we select the cross-sectional dimensions of the rafters. For our example, I took the dimensions 5x15 cm. Although 5x10 cm was also possible. I’m just used to working with such boards, and there will be a larger margin of safety.
Now it is important: from the drawing obtained during the calculation, we will need to write down the value of the vertical load acting on the post (in our example (see figure above) it is equal to 343.40 kg) and the bending moment acting on the post (Mop. = 78.57 kghm). We will need these numbers later when calculating the racks and floor beams.
Next, if you go to the “ Arch“, a window will open for calculating the rafter system, which is a ridge arch (two rafters and a tie). I won’t consider it; it’s not suitable for our roof. We have too large a span between the supports and small angle slope of the slopes. There you will get rafters with a cross section of about 10x25 cm, which is of course unacceptable for us. For smaller spans such a scheme can be used. I am sure that those who understand what I wrote above will understand this calculation themselves. If you still have questions, write in the comments. And we move on to the next step.
Step #6:
Go to the “Rack” tab. Well, everything is simple here.
We enter the previously determined values of the vertical load on the post and the bending moment in the figure in the cells “N=” and “M=”, respectively. We recorded them in kilograms, we enter them in tons, and the values are automatically rounded;
Also in the figure we change the height of the rack (in our example it is 167 cm) and set the dimensions of the section we have chosen. I chose a 5x15 cm board. At the bottom in the center we see the inscription “Central secured!” and “Off-center.” secured." So everything is fine. The safety factors "Kz" are very large, so you can safely reduce the cross-section of the racks. But we will leave it as it is. The calculation result in the figure:
Step #7:
Go to the tab "Beam". Floor beams are subject to both distributed and concentrated loads. We need to take both into account. In our example, beams of the same section span spans of different widths. Of course, we make calculations for a wider span:
— in the “Distributed load” plate we indicate the pitch and span of the beams (from the example we take 0.6 m and 4 m, respectively);
— we take the values Load (normal) = 350 kg/m² and Load (calc.) = 450 kg/m². The values of these loads in accordance with SNiP are averaged and taken with a good margin of safety. They include the load from the dead weight of the floors and the operational load (furniture, people, etc.);
- in the line " B, given» enter the section width of the beams we have chosen (in our example it is 10 cm);
In the lines " N, strength" And " N, deflection» the minimum possible cross-sectional heights of the beams will be indicated at which it will not break and its deflection will be acceptable. We are interested in the larger of these numbers. We take the height of the beam section based on it. In our example, a beam with a cross section of 10x20 cm is suitable:
So, if we didn’t have racks resting on the floor beams, the calculation would have ended there. But in our example there are racks. They create a concentrated load, so we continue to fill out the “” and “ Distributed + concentrated«:
In both plates we enter the dimensions of our spans (here I think everything is clear);
In the “” plate, we change the values of Load (normal) and Load (calculated) to the figure that we received above when calculating rafters with three points of support - this is the vertical load on the rack (in our example, 343.40 kg);
In both plates we enter the accepted width of the beam section (10 cm);
The height of the beam section is determined by the sign “ Distributed+concentrated." . Again we focus on a larger value. For our roof we take 20 cm (see figure above).
This completes the calculation of the rafter system.
I almost forgot to say: the calculation program we use is applicable for rafter systems made of pine (except Weymouth), spruce, European and Japanese larch. All wood used is 2nd grade. If you use other wood, some changes will need to be made to the program. Since other types of wood are rarely used in our country, I will not describe now what needs to be changed.
Few people will have to explain the important role the roof of a house plays. The roof should not only protect Vacation home from unfavorable natural phenomena, but also play one of the determining roles in the complex aesthetic perception of the building. Shape, color, proportions - all this will influence the exterior of the house. And how well the rafter system (the skeleton of the future roof) was designed will depend on its strength, practicality and cost.
Many professional architects prefer to design houses in the AutoCAD program, but when they get to designing a rafter system, their life turns into a constant nightmare, especially if the client constantly makes some changes and clarifications.
Many builders low-rise buildings prefer to communicate with their clients without intermediaries, but when it comes to what the roof of the future cottage will look like, the customer begins to get nervous, because... he is unable to imagine his house completely, and the construction specialist throws up his hands, because... he has neither the time nor the desire to spend a lot of effort to understand professional software for designing and visualizing houses (such as: AutoCAD, Archikad, 3D Max, K3-Cottage, etc.). But there is a way out - this is the Arkon program. This product has already proven itself well Russian market. The program is especially good for preliminary design country houses and in particular it has functions for fast and efficient design of roofs and rafter systems. The Arkon program is successfully used by architects, builders, and even private users to design small houses.
How does the Arkon program work? The program allows the user to choose desired type roofs, and then make the necessary adjustments to the design, i.e. There is no need to draw a rafter system from scratch! In particular, the following roof types are presented: free form, pitched roof, gable roof, hip roof, half hip roof, mansard roof with pediment, mansard-hipped roof, spherical roof, gable roof with fastening rail, flat roof. Various combinations of roof types are also possible. In addition to the standard types, you yourself can design the roof that suits you.
The ArCon program allows you to independently select the roof input mode. You can use the function Roof installation and in the appropriate dialog box, set the basic parameters of the future roof and save them as standard.
Or use the window Roof editor and enter all the necessary parameters.
Using the Roof Editor you can modify standard form roofs. To do this, individual values are entered for individual roof slopes. For example, you can install gables, height of gutters, change roof slopes, etc.
By clicking the button View, you will immediately see the results of your changes, which, you see, is very convenient, especially when designing complex roofs.
By clicking the button Info, you will get a window Roof information. It shows all lengths and areas wooden structure actual floor separate from the main roof and dormer windows. Based on these values, costs and estimates are prepared. All output values are calculated automatically based on the designed 3D roof model, i.e. the possibility of errors in calculations is eliminated.
Beautiful and reliable.
What is the basis of any roof?
How strong and reliable the roof will be will depend on how correctly the parameters of the rafter system elements are calculated.
Therefore, even at the stage of drawing up the building design, a separate calculation of the rafter system is performed.
It is impossible to perform the calculation correctly if you do not take into account the intensity of the various loads that will affect the roof of the house at different periods.
Factors influencing the roof are usually classified into:
And it is difficult for a beginner to do it, since many factors that affect the roof must be taken into account.
Indeed, in addition to the above factors, it is also necessary to take into account the weight of all elements of the rafter system and fastening elements.
Therefore, special calculation programs come to the aid of calculators.
To find out the load on the rafters of our house, we must first calculate the weight of the roofing pie.
This calculation is easy to do if you know total area roofing and materials that are used to create this very pie.
First, count the weight of one square meter pirogue.
The mass of each layer is summed up and multiplied by a correction factor.
This coefficient is equal to 1.1.
Here is a typical example of calculating the weight of a roofing pie.
Let's say you decide to use ondulin as a roofing material.
And that's true!
After all, ondulin is reliable and inexpensive material. It is for these reasons that it is so popular among developers.
So:
Let's sum up the data obtained: 3+5+10+15= 33 kg.
Now the result must be multiplied by 1.1.
Our correction factor.
The final figure is 34.1 kg.
This is the weight of one square meter of roofing cake.
The total roof area is, for example, 100 square meters. meters.
This means that she will weigh 341 kg.
This is very little.
This is one of the advantages of ondulin.
The moment is very important.
Because in many areas in our winter a fairly decent amount of snow falls.
And this is a very large weight, which must be taken into account!
To calculate the snow load, a snow load map is used.
Determine your region and calculate the snow load using the formula
In this formula:
— S is the desired snow load;
— Sg is the mass of snow cover.
The weight of snow per 1 square meter is taken into account. meter.
This indicator is different in each region.
It all depends on the location of the house.
A map is used to determine the mass.
— µ is the correction factor.
The indicator of this coefficient depends on the angle of inclination of the roof.
If the angle of inclination of the slopes is less than 25 degrees, then the coefficient is equal to 1.
At an inclination angle of 25 - 60 degrees, the coefficient is 0.7.
If the angle of inclination is greater than 60 degrees, then the coefficient is not taken into account.
For example, a house was built in the Moscow region.
The slopes have an inclination angle of 30 degrees.
The map shows us that the house is located in the 3rd district.
Mass of snow per 1 sq. meter is 180 kg.
We carry out the calculation, not forgetting about the correction factor:
180 x 0.7 = 126 kilograms per 1 sq. meter of roof.
To calculate wind loads, a special map broken down by zones is also used.
Use this formula:
Wo is a standard indicator determined from the table.
Each region has its own wind tables.
And the k indicator is a correction factor that depends on the height of the house and the type of terrain.
Length calculation rafter leg refers to the simplest geometric calculations.
Because you only need two dimensions: width and height, and the Pythagorean theorem.
To make the calculation more clear, look at the figure below.
We know two distances:
- a is the height from the bottom to the top point of the inside of the rafters.
First leg;
- b is the quantity equal to half roof width.
Second leg.
- c is the hypotenuse of the triangle.
c²=(2 x 2)+(3 x 3).
Total c²=4+9=13.
Now we need to get the square root of 13.
You can, of course, take Bradis tables, but it’s more convenient to use a calculator.
We get 3.6 meters.
To this number you now need to add the extension length d to get the required rafter length.
The cross-section of the boards that we will use for the manufacture of rafters and other elements of the rafter system depends on how long the rafters are, at what pitch they will be installed and on the magnitude of the snow and wind loads that exist in a particular region.
For simple designs use a table standard sizes and board sections.
If the design is very complex, then it is better to use special programs.
The distance between their bases is called.
Experts believe that minimum distance should be 60 cm.
And the optimal distance is 1 meter.
We calculate the distance between the rafters:
For example, the length of the roof slope is 12 meters.
We first select a rafter pitch of 0.8 meters.
12/0.8 = 15 meters.
We add the unit 15+1=16 rafters.
If the result was a fractional number, we would round it up.
Now 12 meters should be divided by 16.
As a result, 1216 = 0.75 meters.
Here optimal distance between the rafters on the same slope.
The table mentioned earlier can also be used.
For wooden beams the optimal span is from 2.5 to 4 meters.
The optimal cross-section is rectangular.
Height to width ratio 1.4:1.
The beam must extend into the wall at least 12 cm.
Ideally, the beams are attached to anchors that are pre-installed in the wall.
Waterproofing of beams is carried out “in a circle”.
When calculating the cross-section of beams, the load from its own weight (usually 200 kg/sq. meter) and operational live load are taken into account.
Its value is equal to the constant load - 200 kg/sq. meter.
Knowing the span and the installation pitch of the beams, their cross-section is calculated from the table:
| Span (m)/ Installation pitch (m) | 2.0 | 2.5 | 3.0 | 4.0 | 4.5 | 5.0 | 6.0 |
| 0.6 | 75x100 | 75x150 | 75x200 | 100x200 | 100x200 | 125x200 | 150x225 |
| 1 | 75x150 | 100x150 | 100x175 | 125x200 | 150x200 | 150x200 | 175x250 |
If a more accurate calculation is required, then use Romanov’s calculator.
A pitched roof is the simplest roofing option.
But this option is not suitable for every building.
And rafter calculations are required in any case.
Calculations for a pitched roof begin with determining the angle of inclination.
And it depends, first of all, on what material you plan to use for the roof.
For example, for corrugated sheets minimum angle equals 8 degrees.
And the optimal temperature is 20 degrees.
If online calculators perform simple calculations, then special software can calculate everything you need.
And there are quite a lot of such programs!
The most famous of them are 3D Max and AutoCAD.
Such programs have only two drawbacks:
There are a number of free programs.
Most programs can be downloaded to your computer.
Or use them online.
Video about calculating rafters.
Rafter system. Calculation of rafters and floor beams. Before starting to build a roof, it is of course desirable that its rafter system be designed for strength. Immediately after the publication of the last article, “Gable roof of a house with your own hands,” I began to receive questions in the mail regarding the choice of the cross-section of rafters and floor beams. Yes, understanding this issue in the vastness of our beloved Internet is indeed quite difficult. There is a lot of information on this topic, but as always it is so scattered and sometimes even contradictory that an inexperienced person, who in his life may not even have encountered such a subject as “Sopromat” (lucky someone), can easily get confused in these wilds. I, in turn, will now try to create a step-by-step algorithm that will help you independently calculate the rafter system of your future roof and finally get rid of constant doubts - what if it won’t hold up, or what if it will fall apart. I will say right away that I will not delve into the terms and various formulas. Well, why? There are so many useful and interesting things in the world that you can fill your head with. We just need to build a roof and forget about it. The entire calculation will be described using the example of a gable roof, which I wrote about in a previous article. So, Step No. 1: Determine the snow load on the roof. To do this, we need a map of snow loads in the Russian Federation. To enlarge the picture, click on it with the mouse. Below I will give a link where you can download it to your computer. Using this map, we determine the number of the snow region in which we are building a house and from the table below we select the snow load corresponding to this region (S, kg/m²): If your city is located on the border of the regions, choose a higher load value. There is no need to adjust the resulting figure depending on the angle of inclination of the slopes of our roof. The program we will use will do this itself. Let's say in our example we are building a house in the Moscow region. Moscow is located in the 3rd snow region. The load for it is 180 kg/m². Step #2: Determine the wind load on the roof. To do this, we need a map of wind loads in the Russian Federation. It can also be downloaded from the link below. Using this map, we also select the corresponding region number and determine the wind load value for it (the values are shown in the lower left corner): Next, the resulting figure must be multiplied by the correction factor “k”, which in turn is determined from the table: Here column A is open coasts seas, lakes and reservoirs, deserts, steppes, forest-steppes and tundras; Column B - urban areas, forest areas, etc. terrain evenly covered with obstacles. It should be taken into account that in some cases the type of terrain may differ in different directions (for example, a house is located on the outskirts of a populated area). Then select the values from column “A”. Let's return to our example again. Moscow is located in the 1st wind region. The height of our house is 6.5 meters. Let's assume that it is being built in a populated area. Thus, we accept the value of the correction factor k=0.65. That is, the wind load in this case will be equal to: 32x0.65=21 kg/m². Step No. 3: You need to download to your computer a calculation program made in the form of an Excel table. We will continue to work in it. Here is the download link: “Calculation of the rafter system.” Also here are maps of snow and wind loads in the Russian Federation. So, download and unpack the archive. We open the file “Calculation of the rafter system”, and we get into the first window - “Loads”: Here we need to change some values in the cells filled in blue. All calculations are done automatically. Let's continue to look at our example: - in the “Initial data” plate we change the angle of inclination to 36° (whatever angle you have, write that, well, I think this is clear to everyone); - change the pitch of the rafters to the one we chose. In our case it is 0.6 meters; - Load roofing (load from the own weight of the roofing material) - we select this value from the table: For our example, we select metal tiles with a weight of 5 kg/m². - Snow. region - here we enter the sum of the values of snow and wind loads that we received earlier, i.e. 180+21=201 kg/m²; - Insulation (mans.) - we leave this value unchanged if we lay insulation between the rafters. If we make a cold attic without insulation, we change the value to 0; - in the “Lathing” sign we enter the required dimensions of the sheathing. In our case, for metal tiles, we will change the pitch of the sheathing by 0.35 m and the width by 10 cm. We leave the height unchanged. All other loads (from the own weight of the rafters and sheathing) are taken into account by the program automatically. Now let’s see what we got: We see the inscription “The load-bearing capacity of the sheathing is ensured!” We don’t touch anything else in this window; we don’t even need to understand what the numbers are in other cells. If, for example, we choose a different rafter pitch (more), it may turn out that the load-bearing capacity of the sheathing will not be ensured. Then it will be necessary to select other dimensions of the sheathing, for example, increase its width, etc. n. In general, I think you will figure it out. Step No. 4: Click at the bottom of the working screen on the “Sling 1” tab and go to the window for calculating rafters with two support points. Here, all the input data we previously entered is already entered by the program automatically (this will be the case in all other windows). In our example from the article “Do-it-yourself gable roof of a house,” the rafters have three support points. But let's imagine that there are no intermediate posts and let's make a calculation: - change the length of its horizontal projection on the rafter diagram (the cell is filled in blue). In our example, it is 4.4 meters. - in the “Calculation of rafters” plate we change the value of rafter thickness B (specified) to the one we selected. We set 5 cm. This value must be greater than that indicated in the Vtr (sustainable) cell; - now in the line “Accept N” we need to enter the selected width of the rafters in centimeters. It must be greater than the values indicated in the lines “Ntr., (strength)” and “Ntr., (deflection)”. If this condition is met, all the inscriptions at the bottom under the rafter diagram will look like “Condition met.” The line “N, (by grade)” indicates the value that the program itself asks us to choose. We can take this number, or we can take another. We usually choose sections available in the store. So, what we got is shown in the figure: In our example, in order to meet all the strength conditions, it is necessary to choose rafters with a section of 5x20 cm. But the roof diagram I showed in the last article has rafters with three support points. Therefore, to calculate it, we move on to the next step. Step No. 5: Click at the bottom of the working screen on the “Sling 2” or “Sling. 3″. This opens a window for calculating rafters with 3 support points. We select the tab we need depending on the location of the middle support (rack). If it is located to the right of the middle of the rafter, i.e. L/L1<2, то пользуемся вкладкой «Строп.2″. Если стойка расположена левее середины стропила, т. е. L/L1> 2, then use the “Sling 3” tab. If the stand is exactly in the middle, you can use any tab, the results will be the same. - on the rafter diagram, we transfer the dimensions in cells filled with blue (except for Ru); - using the same principle as described above, we select the cross-sectional dimensions of the rafters. For our example, I took the dimensions 5x15 cm. Although 5x10 cm was also possible. I’m just used to working with such boards, and there will be a larger margin of safety. Now it is important: from the drawing obtained during the calculation, we will need to write down the value of the vertical load acting on the post (in our example (see figure above) it is equal to 343.40 kg) and the bending moment acting on the post (Mop. = 78.57 kghm). We will need these numbers later when calculating the racks and floor beams. Next, if you go to the “Arch” tab, a window will open for calculating the rafter system, which is a ridge arch (two rafters and a tie). I won’t consider it; it’s not suitable for our roof. We have too large a span between the supports and a small angle of inclination of the slopes. There you will get rafters with a cross section of about 10x25 cm, which is of course unacceptable for us. For smaller spans such a scheme can be used. I am sure that those who understand what I wrote above will understand this calculation themselves. If you still have questions, write in the comments. And we move on to the next step. Step #6: Go to the “Rack” tab. Well, everything is simple here. - we enter the previously determined values of the vertical load on the stand and the bending moment in the figure in the cells “N=” and “M=”, respectively. We recorded them in kilograms, we enter them in tons, and the values are automatically rounded; - also in the figure we change the height of the rack (in our example it is 167 cm) and set the dimensions of the section we have chosen. I chose a 5x15 cm board. At the bottom in the center we see the inscription “Central secured!” and “Off-center.” secured." So everything is fine. The safety factors "Kz" are very large, so you can safely reduce the cross-section of the racks. But we will leave it as it is. The result of the calculation is in the figure: Step No. 7: Go to the “Beam” tab. Floor beams are subject to both distributed and concentrated loads. We need to take both into account. In our example, beams of the same section span spans of different widths. We, of course, make calculations for a wider span: - in the “Distributed Load” plate we indicate the pitch and span of the beams (we take 0.6 m and 4 m from the example, respectively); - accept the values of Load. (norm.)=350 kg/m² and Load (calc.)=450 kg/m². The values of these loads in accordance with SNiP are averaged and taken with a good margin of safety. They include the load from the dead weight of the floors and the operational load (furniture, people, etc.); - in the line “B, given” we enter the section width of the beams we have chosen (in our example it is 10 cm); - in the lines “H, strength” and “H, deflection” the minimum possible heights of the cross-section of the beams will be indicated at which it will not break and its deflection will be acceptable. We are interested in the larger of these numbers. We take the height of the beam section based on it. In our example, a beam with a cross-section of 10x20 cm is suitable: So, if we did not have racks resting on the floor beams, the calculation would be completed at this point. But in our example there are racks. They create a concentrated load, so we continue to fill out the “Concentrated load” and “Distributed + concentrated” plates: - in both plates we enter the dimensions of our spans (here I think everything is clear); - in the “Concentrated load” plate we change the values of Load (normal) and Load (calculated) to the figure that we received above when calculating rafters with three points of support - this is the vertical load on the rack (in our example 343.40 kg) ; - in both plates we enter the accepted width of the beam section (10 cm); - the height of the beam section is determined according to the “Distribution + Focus” plate. Again we focus on a larger value. For our roof we take 20 cm (see figure above). This completes the calculation of the rafter system. I almost forgot to say: the calculation program we use is applicable for rafter systems made of pine (except Weymouth), spruce, European and Japanese larch. All wood used is 2nd grade. If you use other wood, some changes will need to be made to the program. Since other types of wood are rarely used in our country, I will not describe now what needs to be changed. Read more.
This article will focus on the simple Rafters program. It is intended for calculating a two-span wooden beam. The software will provide data on the maximum moment, deflection and bearing capacity. Let's take a closer look at the representative.
"Rafters" does not require installation, you just need to run the file from the archive. All functionality is in one window. You will need to enter the necessary parameters about spans, angles of inclination, height and width in the lines and press the button "Calculation", so that the calculation results are displayed below. Please note that there are three types of wood and two calculation modes, this helps to determine the most accurate parameters.
"Rafters" provides a minimum set of tools that are necessary to calculate the roof. However, it fully copes with its task and provides accurate information about the parameters of a two-span beam. The software is easy to use and does not require special skills.
