Do you want to calculate the truss system quickly, without studying theory and with credible results? Take advantage online calculator on the site!
Can you imagine a man without bones? In the same way, a pitched roof without a truss system looks more like a structure from a fairy tale about three little pigs, which can easily be swept away by the elements of nature. A strong and reliable rafter system is the key to the durability of the roof structure. In order to qualitatively design a rafter system, it is necessary to take into account and predict the main factors affecting the strength of the structure.
Take into account all roof bends, correction factors for uneven distribution of snow on the surface, snow drift, slope slope, all aerodynamic coefficients, forces on roof structural elements, and so on - calculate all this as close to the real situation as possible, and also take into account all loads and skillfully assemble their combinations is not an easy task.
If you want to understand thoroughly - a list of useful literature is given at the end of the article. Of course, the strength of materials course for a complete understanding of the principles and the impeccable calculation of the truss system cannot be fit into one article, so we will give the main points for the simplified versioncalculation.
Load classification
Loads on the truss system are classified into:
1) Main:
- permanent loads: the weight of the truss structures themselves and the roof,
- continuous loads- snow and temperature loads with a reduced design value (used if it is necessary to take into account the influence of the duration of the loads, when checking for endurance),
- variable short-term influence- snow and temperature effects according to the full design value.
2) Additional- wind pressure, weight of builders, ice loads.
3) Force majeure- explosions, seismic activity, fire, accidents.
To carry out the calculation of the truss system, it is customary to calculate the maximum loads, so that, based on the calculated values, to determine the parameters of the elements of the truss system that can withstand these loads.
The calculation of the truss system of pitched roofs is made for two limit states:
a) The limit at which structural failure occurs. The maximum possible load on the strength of the rafter structure should be less than the maximum allowable.
b) Limit state at which deflections and deformation occur. The resulting deflection of the system under load should be less than the maximum possible.
For a simpler calculation, only the first method is used.
Calculation of snow loads on the roof
For counting snow load use the following formula: Ms = Q x Ks x Kc
Q- the weight of the snow cover covering 1 m2 of a flat horizontal roof surface. It depends on the territory and is determined from the map in figure No. X for the second limit state - calculation for deflection (when the house is located at the junction of two zones, a snow load with a large value is selected).
For strength calculation according to the first type, the load value is selected according to the area of residence on the map (the first digit in the indicated fraction is the numerator), or is taken from table No. 1:
The first value in the table is measured in kPa, in parentheses is the desired converted value in kg/m2.
Ks- correction factor for the angle of inclination of the roof.
- For roofs with steep slopes with an angle of more than 60 degrees, snow loads are not taken into account, Ks=0 (snow does not accumulate on steeply pitched roofs).
- For roofs with an angle of 25 to 60, the coefficient is taken as 0.7.
- For the rest it is equal to 1.
Roof pitch can be determined online roof calculator the corresponding type.
Kc- coefficient of wind drift of snow from roofs. Under the condition of a sloping roof with a slope angle of 7-12 degrees in areas on the map with a wind speed of 4 m/s, Kc = 0.85 is assumed. The map shows zoning by wind speed.
Drift factor Kc not taken into account in areas with January temperatures warmer than -5 degrees, since an ice crust forms on the roof, and snow does not blow off. The coefficient is also not taken into account in the event that the building is closed from the wind by a higher neighboring building.
Snow falls unevenly. Often, a so-called snow bag is formed on the leeward side, especially at the joints, kinks (valley). Therefore, if you want a solid roof, keep the pitch of the rafters to a minimum in this place, and also carefully follow the recommendations of the roofing material manufacturers - snow can break off the overhang if it is of the wrong size.
We remind you that the calculation above is presented to your attention in a simplified form. For a more reliable calculation, we recommend multiplying the result by the load safety factor (for snow load = 1.4).
Calculation of wind loads on the truss system
We figured out the snow pressure, now let's move on to calculating the wind effect.
Regardless of the angle of the slope, the wind has a strong effect on the roof: it tries to drop a steep-pitched roof, and lift a flatter roof from the leeward side.
To calculate the wind load, its horizontal direction is taken into account, while it blows bidirectionally: on the facade and on the roof slope. In the first case, the flow is divided into several - part goes down to the foundation, part of the flow tangentially from below vertically presses on the roof overhang, trying to raise it.
In the second case, acting on the slopes of the roof, the wind presses perpendicular to the slope, pressing it; a tangential vortex is also formed on the windward side, bending around the ridge and turning into lift already on the leeward side, due to the difference in wind pressure on both sides.
To calculate the average wind load use the formula
Mv = Wo x Kv x Kc x safety factor,
where Wo- wind pressure load determined from the map
kv- wind pressure correction factor, depending on the height of the building and the terrain.
Kc- aerodynamic coefficient, depends on the geometry of the roof structure and wind direction. Values negative for leeward, positive for windward
For a shed roof, take the coefficient from the table for Ce1.
To simplify the calculation, the value of C is easier to take as a maximum equal to 0.8.
Calculation of own weight, roofing pie
To calculate the permanent load you need to calculate the weight of the roof (roofing cake - see figure X below) per 1 m2, the resulting weight must be multiplied by a correction factor of 1.1 - the rafter system must withstand such a load throughout the entire service life.
The weight of the roof is made up of:
- the volume of timber (m3) used as battens multiplied by the density of the tree (500 kg/m3)
- truss system weight
- weight of 1m2 of roofing material
- weight 1m2 weight of insulation
- weight of 1m2 finishing material
- weight of 1m2 of waterproofing.
All these parameters can be easily obtained by specifying these data with the seller, or look at the label on the main characteristics: m3, m2, density, thickness, - perform simple arithmetic operations.
Example: for insulation with a density of 35 kg / m3, packed in a roll 10 cm or 0.1 m thick, 10 m long and 1.2 m wide, weight 1 m2 will be equal to (0.1 x 1.2 x 10) x 35 / (0.1 x 1.2) = 3.5 kg / m2. The weight of other materials can be calculated according to the same principle, just do not forget to convert centimeters to meters.
More often the roof load per 1 m2 does not exceed 50 kg, therefore, in the calculations, this value multiplied by 1.1 is taken into account, i.e. use 55 kg/m2, which itself is taken as a reserve.
More information can be found in the table below:
10 - 15 kg/m² |
|
Ceramic tiles | 35 - 50kg/m² |
Cement-sand tiles | 40 - 50 kg/m² |
bituminous tiles | 8 - 12 kg/m² |
metal tile | |
Decking | |
Rough deck weight | 18 - 20 kg/m² |
Lathing weight | 8 - 12 kg/m² |
Rafter system weight | 15 - 20 kg/m² |
We collect loads
According to the simplified version, it is now necessary to add up all the loads found above by simple summation, we will get the final load in kilograms per 1 m2 of roof.
Calculation of the truss system
After collecting the main loads, you can already determine the main parameters of the rafters.
falls on each rafter leg separately, we translate kg / m2 into kg / m.We count according to the formula: N = rafter pitch x Q, where
N - uniform load on the rafter leg, kg / m
rafter pitch - distance between rafters, m
Q - total roof load calculated above, kg/m²
It is clear from the formula that by changing the distance between the rafters, it is possible to regulate the uniform load on each rafter leg. Typically, the pitch of the rafters is in the range from 0.6 to 1.2 m. For a roof with insulation, when choosing a pitch, it is reasonable to focus on the parameters of the insulation sheet.
In general, when determining the installation step of the rafters, it is better to proceed from economic considerations: calculate all the options for the location of the rafters and choose the cheapest and optimal in terms of the quantitative consumption of materials for the rafter structure.
- Calculation of the section and thickness of the rafter leg
In the construction of private houses and cottages, when choosing the section and thickness of the rafters, they are guided by the table below (the section of the rafters is indicated in mm). The table contains average values for the territory of Russia, as well as the dimensions of building materials on the market. In the general case, this table is sufficient to determine which section of wood should be purchased.
However, we should not forget that the dimensions of the rafter leg depend on the design of the truss system, the quality of the material used, the constant and variable loads exerted on the roof.
In practice, when building a private residential building, boards with a section of 50x150 mm (thickness x width) are most often used for rafters.
Self-calculation of the section of the rafters
As mentioned above, the rafters are calculated according to the maximum load and deflection. In the first case, the maximum bending moment is taken into account, in the second, the section of the rafter leg is checked for deflection stability in the longest section of the span. The formulas are quite complex, so we have chosen for you simplified version.
The section thickness (or height) is calculated by the formula:
a) If the roof angle< 30°, стропила рассматриваются как изгибаемые
H ≥ 8.6 x Lm x √(N / (B x Rb))
b) If the roof pitch is > 30°, the rafters are flex-compressed
H ≥ 9.5 x Lm x √(N / (B x Rb))
Designations:
H cm- rafter height
Lm, m- working section of the longest rafter leg
N,
kg/m- distributed load on the rafter leg
B cm- rafter width
Rizg, kg/cm²- resistance of wood to bending
For pine and spruce Rizg depending on the type of wood is equal to:
It is important to check whether the deflection exceeds the permitted value.
The deflection of the rafters should be less L/200- the length of the maximum span to be checked between the supports in centimeters divided by 200.
This condition is true if the following inequality is satisfied:
3,125 xNx(lm)³ / (BxH³) ≤ 1
N (kg / m) - distributed load per linear meter of the rafter leg
Lm (m) - working section of the rafter leg of maximum length
B (cm) - section width
H (cm) - section height
If the value is greater than one, it is necessary to increase the parameters of the rafter B or H.
Sources used:
- SNiP 2.01.07-85 Loads and impacts with the latest amendments 2008
- SNiP II-26-76 "Roofs"
- SNiP II-25-80 "Wooden structures"
- SNiP 3.04.01-87 "Insulating and finishing coatings"
- A.A. Saveliev "Rafter systems" 2000
- K-G.Götz, Dieter Hoor, Karl Möhler, Julius Natterer "Atlas of Timber Structures"
The topic of snow in September is not very relevant even for us - the inhabitants of Siberia. However… the “sleigh” should already be ready, despite the fact that we still continue to ride the “carts”. Moments come to mind when, after a heavy snowfall in the winter and before the snow melts in the spring...
The owners of various buildings - from bathhouses, sheds and greenhouses to huge swimming pools, stadiums, workshops, warehouses - are puzzled by two questions arising from one another: “Will the roof withstand the mass of snow accumulated on it or not? Throw this snow off the roof or not?”
Snow load on the roof is a serious issue and does not tolerate an amateurish approach. I will try to briefly and clearly state the information about the snow and assist in solving the above issues.
How much does snow weigh?
Everyone who has had to remove snow with a shovel is well aware that snow can be both 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.
In late autumn and early winter, the specific gravity of snow lying on horizontal and slightly inclined surfaces is usually 160±40 kg/m3.
At the moments of prolonged thaws, the specific gravity of snow begins to increase significantly (snow "sets" as in spring), sometimes reaching values of 700 kg/m3. That is why in warmer regions the density of snow is always greater than in cold northern regions.
By the middle of winter, the snow is compacted under the influence of the sun, wind and from the pressure of the upper layers of snowdrifts on the lower layers. The specific gravity becomes 280±70 kg/m3.
By the end of winter, under the action of a 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 spring, before abundant melting, the specific gravity of "wet" snow can be 750 ± 100 kg/m3, approaching the density of ice - 917 kg/m3.
Snow, which was raked into heaps, thrown from place to place, increases its specific gravity by 2 times.
The most probable average density of "dry" compacted snow is in the range of 200...400 kg/m3.
For receiving information about the release of new articles and for the ability to download working files of programs I ask you to subscribe to announcements in the window located at the end of the article or in the window at the top of the page.
Enter your email address, click on the "Receive article announcements" button, confirm your subscription in a letter that will immediately come to you at the specified mail !
Remove snow from roofs or not?
It is necessary to understand a simple thing - the mass of snow lying on the roof, in the absence of snowfall, remains unchanged regardless of density !!! That is, the fact that the snow “became heavier” did not increase the load on the roof !!!
The danger lies in the fact that a layer of loose snow can absorb, like a sponge, precipitation in the form of rain. That's when the total mass of water in its various forms, located on the roof, will increase dramatically - especially in the absence of a runoff, 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 is the pressure of the distributed load - how many kilograms per square meter - the roof can really hold before the start of unacceptable deformations of the structure.
For an objective answer to this question, it is necessary to examine the roof, draw up a new one or confirm the design design scheme, perform a new calculation or take the results of the old design one. Next, it is necessary to experimentally determine the density of snow - for this, a sample is cut out, weighed and its volume is calculated, and then the specific gravity.
If, for example, the roof, according to calculations, must withstand a specific pressure of 200 kg / m2, the snow density determined empirically 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 with a depth of more than 0.2 ... 0.3 m and a high probability of rain with subsequent cooling, it is necessary to take measures to dump the snow.
Regulatory and design snow load.
in 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 builders-designers and delve into options for geometric types of coatings, slope angles, snow drift coefficients and other difficulties. But we will compose a general algorithm and write a program that implements it. We will learn how to determine the normative and design snow pressure on the horizontal projection of the coverage for objects in any area of Russia that interests us.Let's remember a few "axioms". If on a simple shed or gable roof, the slope of the coating over 60˚ , then it is considered that there can be no snow on such a roof (μ =0) . He's all "rolling down". If the slope angle 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 by linear interpolation. For example, at an angle equal to 45˚ only 50% of the fallen snow will lie on the roof (μ=0.5).
Designers calculate by limit states, which are divided into two groups. The transition beyond the limit states of the first group is the destruction and loss of the object. The transition beyond the limit states of the second group is the excess of deflections of the permissible limits and, as a result, the need to repair the object, possibly a capital one. In the first case, the calculated snow load is used in the calculation, which is equal to the standard load increased by 40%. In the second case, the calculated snow load is the normative 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 very powerful free alternative - OOo Calc from the Open Office package.
Before starting work, search the Internet and download SP 20.13330.2011 with all applications.
Some important material from SP 20.13330.2011 are in the file, which site subscribers can be downloaded from the link at the very end of this article.
We turn on the computer and start calculating the snow load on the coatings in Excel.
In the cells with a light turquoise fill, we write the initial data selected by SP 20.13330.2011. In cells with a light yellow fill, we count the results. In the cells with a pale green fill, we place the initial data that is little subject to change.
In comments to all cells of a column C put formulas and links to paragraphs 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 the weight of the snow cover”, we determine the number of the snow area for the area where the building is 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 of
You can read more about how the INDEX function works in conjunction with a combo box.
2. We read the mass of snow cover per 1 m2 of the horizontal surface of the earth Sg in kg/m2 for the selected area
3. We accept, in accordance with paragraph 10.5-10.9 of SP 20.13330.2011, the value of the coefficient that takes into account the drift of snow from building coverings by wind Ce
in cell D4: 1,0
Ce— write 1.0.
4. We assign, in accordance with paragraph 10.10 of SP 20.13330.2011, the value of the thermal coefficient Ct
in cell D5: 1,0
If you do not understand how to assign Ct— write 1.0.
5. We assign, in accordance with clause 10.4 according to Appendix D of SP 20.13330.2011, the value of the transition coefficient from the weight of the snow cover of the earth to the snow load on the cover μ
in cell D6: 1,0
We recall the "axioms" from the previous section of the article. Do not remember and do not understand anything - write 1.0.
6. We read the standard value of the 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. We write down, in accordance with paragraph 10.12 of SP 20.13330.2011, 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 the "simple" buildings of the third snow region withμ =1 design snow load is 180 kg/m2. This corresponds to a snow cover height of 0.90…0.45 m at a snow density of 200…400 kg/m3, respectively. Conclusions to draw each of us!
I ask those who RESPECT the work of the author to download the file AFTER SUBSCRIBE to the announcements of articles.
REST can be downloaded just... - there are no passwords!
Waiting for your comments, dear readers!!! Professionals - builders, I ask "not to hit hard." The article is written not for specialists, but for a wide audience.
If you've ever shoveled snow, you know how hard it can be. And what can we say about the roof, on which, during the first month of winter, such a hat is assembled that can break through even a fairly solid structure! And the topic of competent roof arrangement is especially relevant for residents of the northern regions of Russia, where there are already snowdrifts in September. That is why, when building a house, everyone asks the question: will the roof withstand the entire mass of snow, dump it every 2 weeks, or not.
For this purpose, such a concept as the normative snow load and its combination with the wind load was developed. There are really a lot of subtleties and nuances, and if you want to figure it out, we will be happy to help!
So, the calculation of the snow load on the roof is done taking into account two limiting states of the roof - for destruction and deflection. In simple terms, this is precisely the ability of the entire structure to resist external influences - until such time as it receives local damage or unacceptable deformation. Those. until the roof is caved in or damaged enough to require repair.
Roof load capacity limit
As we have already said, there are only two limiting states. In the first case, we are talking about the moment when the rafter structure has exhausted its bearing capacity, including its strength, stability and endurance. When this limit is exceeded, the roof begins to collapse.
This limit is defined as follows: σ ≤ r or τ ≤ r. Thanks to this formula, professional roofers calculate what load for the structure will still be the maximum allowable, and which will exceed it. In other words, this is the calculated load.
For this calculation, you need data such as snow weight, slope angle, wind load, and roof self-weight. It also matters what kind of rafter system, crate and even thermal insulation was used.
But the standard load is calculated based on data such as the height of the building and the angle of inclination of the slopes. And your task is to calculate both the calculated load and the standard load, and convert them into a linear one. There is a special document for SP 20. 13330. 2011 in paragraphs 4.2.10.12; 11.1.12.
Roof limit for truss deflection
The second limit state indicates excessive deformations, static or dynamic loads on the roof. At this moment, unacceptable deflections occur in the structure, so much so that the compositions are revealed. As a result, it turns out that the rafter system is, as it were, intact, not destroyed, but still it needs repair, without which it will not be able to function further.
This load limit is calculated using the formula f ≤ f. It means that the death of the rafters under load should not exceed a certain limit state. And for the floor beam there is a formula - 1/200 , which means that the deflection should not be more than 1 in 200 of the measured length of the beam.
And it is correct to calculate the snow load immediately for both limit states. Those. your task when calculating the amount of snow and its effect on the roof is to prevent deflection more than possible.
Here is a valuable video lesson for the "patient" on this topic:
Standard snow load in your area
When they talk about calculating the snow load on the roof, they talk about how many kilograms of snow can fall on each square meter of the roof, while it can actually hold such a weight before the structure begins to deform. In simple terms, what head of snow can be allowed to lie on the roof every winter without fear that it will break the roof or loosen the entire truss system.
Such a calculation is done at the design stage of the house. To do this, first of all, you need to study all the data on special tables and maps of SP 20.3330.2011 "Loads and Impacts". Based on this, find out whether your planned design will be reliable.
For example, if, according to calculations, it should calmly withstand a layer of snow of 200 kilograms per square meter, then you will need to carefully monitor that the snow cap on the roof is not higher than one height. But, if the snow on the roof already exceeds 20-30 cm and you know that it will rain soon, then it is better to remove it.
So, to find out the standard snow load in the area where you are building a house, refer to this map:
Also, the same factor is not used for buildings that are well protected from the wind by other buildings or tall forest. Your calculation equation will look like this:
- for the first limit state where strength is calculated, apply the formula qr. Сн = q×µ,
- for the second limit state, where the possible deflection of the roof is calculated, use the following formula qn. Сн = 0.7q×µ.
In this case, as you have already noticed, for the second group of limit states, the weight of snow should be taken into account with a factor of 0.7, i.e. the formula itself will look like this: 0.7q.
Specific gravity: such light and heavy snow
And now let's move on to practice. If you live in Russia, and not on the southern continent without winter, then you know what snow really is: incredibly light and incredibly heavy. For example, the same fluffy snowball in frosty and dry weather at a temperature of -10 ° C will have a density of about 10 kg per cubic meter. But the snow at the end of autumn and at the beginning of winter, which lay for a long time on horizontal and inclined surfaces and “packed”, already has a much larger mass - from 60 kilograms per cubic meter. By the way, it is not difficult to find out the density of snow - in winter it is enough to cut a sample of one cubic meter of snow with a large shovel and weigh it.
If we are talking about loose snow, which, in theory, is light and does not cause problems, then know that there is some danger lurking here. Loose snow, like no other, quickly absorbs all the precipitation in the form of rain and becomes already wet snow. And its presence on the roof, where there is no well-organized flow, is fraught with big problems.
Further, in the spring, during a long thaw, the proportion of snow also increases significantly. In dry compacted snow, the average density is in the range from 200 to 400 kg per cubic meter. Also, do not miss such an important moment when the snow remained lying on the roof for a long time and there was no new snowfall, and you did not remove it. Then, regardless of its density, it will have the same mass, although visually the “cap” itself has become half as large. In a particularly humid climate in spring, the specific gravity of snow reaches 700 kg per cubic meter!
Snow bag and air temperature
"Snowbag" refers to snow on a roof that exceeds the average thickness standards for a particular area. Or more simply: if above 50 cm per eye.
Usually, snow bags accumulate on the non-windy side of the roof and in places where dormer windows and other roof elements are located. Just in such places they put double and reinforced rafter legs, or even make a continuous crate. In addition, here, according to all the rules, there should be a special roofing substrate to avoid leaks.
Therefore, in the warmer regions of Russia, the snow density is always greater than in the cold ones. Indeed, in such areas in winter, the snow is compacted under the action of the sun, the upper layers of the snowdrift put pressure on the lower ones. Also keep in mind that the snow that is thrown from place to place increases its specific gravity at least twice. Thanks to all this, the average specific gravity is usually equal in the middle of winter 280 + - 70 kg per cubic meter.
And in spring, during the period of abundant melting, sleet can weigh almost a ton! Can you imagine having several tons of snow on your roof at the same time? That is why the fact that during the construction of the roof several workers hang on the truss system at once and this allegedly indicates its strength should not be taken into account. After all, a couple of people definitely do not weigh several tons at once.
Please note that the calculation of the standard load also takes into account the average air temperature in January. Which one do you have, look already on the map SP 20.13330.2011:
If it turns out that your average temperature in January is less than 5 degrees Celsius, then the snow load reduction factor of 0.85 does not apply. Indeed, because of such a temperature, the snow in winter will constantly thaw from below, forming ice and lingering on the roof.
And, finally, the greater the angle of the slope, the less snow always remains on it, because it gradually slides under its own weight. And on those roofs whose slope angle is greater than or equal to 60 degrees, there is no snow at all. Therefore, in this case, the coefficient µ must be equal to zero. At the same time for a slope with an angle 40° µ is 0.66, 15° 0.33 and for 45° degrees 0.5.
Wind and snow distribution on two slopes
In those regions where the average wind speed during all three winter months exceeds 4 m / s, on gentle roofs and with a slope of 7 to 12 degrees, the snow is partially blown away and here its standard amount should be slightly reduced by multiplying by 0,85 . In other cases, it should be equal to one, or it can be omitted, which is quite logical.
In this case, your formula will now look like this:
- strength calculation Q r.cn = q × µ × c;
- deflection calculation Q n.sn = 0.7q × µ × s.
The accumulation of snow on the roof is also directly dependent on the wind. What matters is the shape of the roof, how it is located relative to the prevailing winds, and what the angle of inclination of its slopes (not in terms of how easily the snow moves out, but in terms of whether it is easily blown away by the wind).
Because of all this snow on the roof, there can be either less than on a flat surface of the earth, or more. Plus, on both slopes of the same roof, there can be completely different heights of the snow cap.
Let us explain the last statement in more detail. For example, such a frequent phenomenon as a snowstorm constantly transfers snowflakes to the leeward side. And this is prevented by the ridge of the roof, which, delaying the wind, reduces the speed of movement of snow flows and snowflakes settle more on one slope than on another.
It turns out that on one side of the roof there may be less snow than normal, and on the other - much more. And this also needs to be taken into account, because it turns out that in this case, almost twice as much snow is collected on one of the slopes than on the ground!
To calculate such a snow load, the following formula is used: for gable roofs with an angle of inclination of 20 degrees, but less than 30, the percentage of snow accumulation will be 75% on the windward side and 125% on the leeward side. This percentage is calculated from the amount of snow cover that lies on flat land. The value of all these coefficients is indicated in the regulatory document SNiR 2.01.07-85.
And, if you have determined that the wind in your region will create a noticeable difference in snow cover on different slopes, then you will need to arrange twin rafters on the leeward side:
If you don’t have data on the wind rose of the area at all, or they are not accurate, then give preference to the maximum load to make sure - as if both slopes of your roof are on the leeward side and there will always be more snow on them than on the ground .
So what happens then to the snow bag downwind? He gradually slips and presses already on the overhang of the roof, trying to break it. That is why, according to the rules, the roof overhang must be equally strengthened, depending on its roofing.
By the way, if your roof also has a height difference, it will be useful for you to watch this video tutorial:
The formula for the actual snow load on the roof
The next important point. Often the snow load is calculated with such a simple and understandable end result as the nth number of kilograms per square meter of roofing. But the truss system itself is much more complicated, and it is not entirely correct to assess the pressure only on its continuous coating.
The fact is that each element of the roof truss system takes on a certain load, which was originally designed only for him alone, and not for the entire roof at once. Therefore, it is necessary to convert the units of measurement kg / m 2 into the unit of kg / m, i.e. kilograms to meters.
This means to measure the linear pressure on the rafters, or crate, overhangs and girders. And all these are linear structures, the loads act along the longitudinal axis of each:
If we take a separate rafter, the load that will be located directly above it acts on it. And in order to change the area of \u200b\u200bthe total load on the roof, you need to change the width of the installation step of the rafters.
Bottom line: taking into account the totality of all loads
And finally, let's summarize and note the most common mistake when calculating snow loads on a roof. This is an omission of the fact that all loads act in combination. The roof itself has weight, a person standing on it, insulation and much more!
Therefore, all loads that affect the roof must be summed and multiplied by a factor of 1.1. That's when you get some real value. Why at 1.1? To account for additional unexpected factors, you do not want the truss system to work at its limit, do you? Repairs are usually complex and expensive.
Depending on the value obtained, you now need to calculate the installation step of the rafters. It will also be necessary to take into account the length of the wall of the building and the convenience of placing a whole number of stable legs on it at the same distance: for example, 90 cm, 1.5 meters, 1.2 meters.
Quite often, the decisive criterion for choosing a rafter pitch is economic, although the chosen roofing also dictates its own conditions. But remember that when arranging the roof, everything is calculated so that the rafters can easily withstand the pressure placed on them. And for this, consider several options for installing rafters and determine for each of these options the cross section of the boards and the material consumption.
The correctly chosen step is considered to be the one where the material consumption is the smallest, while the final properties remain the same. And keep in mind that, in addition to rafters, battens and girders, there are always additional load-bearing elements such as racks in the roof structure.
When designing a roof, it is necessary to take into account the loads acting on it - snow and wind. To determine the indicators of these values, you can contact a special construction organization, where engineers will help you with the calculations. But if you want to do everything yourself and do not doubt your abilities, then here you will find the necessary formulas with a detailed description of the quantities that will be needed in the calculation. So, for starters, let's figure out what these loads are and why they must be taken into account.
The Russian climate is very diverse. It is important to understand that the roof of a house under construction will be affected by temperature changes, wind pressure, precipitation and other physical and mechanical factors. Moreover, the degree of their influence will directly depend on the area of construction. All this will put pressure not only on the roof railing - the roof, but also on the supporting structures, such as rafters and battens. It must be understood that the house is a single structure. By chain reaction, the load from the roof is transferred to the walls, and from them to the foundation. Therefore, it is important to calculate everything to the smallest detail.
The snow cover that forms on the roof of the house in winter puts a certain pressure on it. The further north the area, the more snow. It seems that the threat of breakdowns is higher, but it is worth being more careful when designing a house in an area where there is a periodic change in temperature that can cause snow to melt and then freeze. The average weight of snow is 100 kg/m3, but in the wet state it can reach 300 kg/m3. In such cases, the snow mass can cause deformation of the truss system, hydro and thermal insulation, which will lead to roof leaks. Such weather conditions will also affect the rapid and uneven snow cover from the roof, which can be dangerous for humans.
The greater the slope of the roof, the less snow deposits will linger on it. But if your roof has a complex shape, then at the junction of the roof, where internal corners are formed, snow can accumulate, which will contribute to the formation of an uneven load. It is best to install snow retainers in areas where rainfall is high enough so that snow that collects near the edge of the eaves cannot damage the gutter system. Snow removal can be done independently, but this process cannot be called one hundred percent safe.
In order to ensure the safe melting of snow and prevent the formation of icicles, a cable heating system is used. It can be controlled automatically or manually. Depends on your desire and choice. The heating elements of such a system are located along the entire edge of the roof in front of the gutter.
For Russia, the value of snow load will depend on the area of construction. A special map will help determine how much snow cover will be in your area.
Snow load calculation technology: S=Sg*m, where Sg is the calculated value of the weight of snow cover per 1 m2 of the horizontal surface of the earth, taken from the table, and m is the coefficient of transition from the weight of the snow cover of the earth to the snow load on the cover.
The calculated value of the weight of the snow cover Sg is taken depending on the snow region of the Russian Federation.
Determining the snow load of the terrain
| snow area | I | II | III | IV | V | VI | VII | VIII |
|---|---|---|---|---|---|---|---|---|
| Snow cover weight Sg (kgf/m2) | 80 | 120 | 180 | 240 | 320 | 400 | 480 | 560 |
The coefficient m depends on the angle of inclination of the roof slope, at the angles of inclination of the roof slope:
less than 25 degrees m is taken equal to 1
from 25 to 60 degrees, the value of m is taken equal to 0.7 (approximately, each slope has its own value)
more than 60 degrees, the value of m, in the calculation of the total snow load, is not taken into account.
The wind exerts lateral pressure on the walls of the house and the roof. The air flow, colliding with an obstacle, is distributed, going down to the foundation and up into the eaves of the roof. If you do not calculate the wind pressure, then the roofing can simply be torn off by a hurricane wind. Such destruction can not always be corrected by some kind of cosmetic repair, often this leads to the need to replace the roof. An important indicator in calculating the impact of wind is the aerodynamic coefficient. It depends on the angle of the roof slope. The steeper the slope, the greater the load, and the wind will try to “tip” the roof. If the angle of your roof is small, then the wind will act on the roof like a lifting force, trying to rip it off and carry it away. In order to prevent this from happening, you need to properly observe the design of the roof. The stability of the rafter system depends on the provision of spatial rigidity, which consists of the correct combination of braces, struts and diagonal braces in it, as well as their rigid fastening to each other. In addition, the wind can carry objects that, when colliding with the roof, will leave mechanical damage. To prevent this from happening, you need to carefully choose the roofing and properly organize the crate for its installation.
Wind pressure, as well as the weight of the snow cover, will depend on the construction area. You can determine the zoning on the map below.
Wind Load Calculation Technology
The coefficient k, which takes into account the change in wind pressure with height z, is determined from the table below, depending on the type of terrain. The following terrain types are accepted:
A - open coasts of the seas, lakes and reservoirs, deserts, steppes, forest-steppes, tundra;
B - urban areas, forests and other areas evenly covered with obstacles more than 10 m high;
C - urban areas with buildings over 25 m high.
A structure is considered to be located in a locality of this type if this locality remains on the windward side of the structure at a distance of 30h - with a height of the structure h up to 60 m and 2 km. - at a higher altitude.
| Height z, m | k coefficient for terrain types | ||
|---|---|---|---|
| ≤ 5 | 0,75 | 0,50 | 0,40 |
| 10 | 1,00 | 0,65 | 0,40 |
| 20 | 1,25 | 0,85 | 0,55 |
| 40 | 1,50 | 1,10 | 0,80 |
| 60 | 1,70 | 1,30 | 1,00 |
| 80 | 1,85 | 1,45 | 1,15 |
| 100 | 2,00 | 1,60 | 1,25 |
| 150 | 2,25 | 1,90 | 1,55 |
| 200 | 2,45 | 2,10 | 1,80 |
| 250 | 2,65 | 2,30 | 2,00 |
| 300 | 2,75 | 2,50 | 2,20 |
| 350 | 2,75 | 2,75 | 2,35 |
| ≥ 480 | 2,75 | 2,75 | 2,75 |
Note: when determining the wind load, terrain types can be different for different calculated wind directions.
Wind and snow loads when designing canopies
Particular attention should be paid to the calculation of those who are thinking about designing a canopy - for example, for a gazebo or a car park. Typically, in such cases, an economical design is used that does not have sufficient rigidity. Therefore, the pressure of snow cannot be ignored. It is recommended to clean the snow on time, preventing the formation of a snow cover more than 30 cm thick. For a canopy made of wood, it will be more reliable to make a solid crate and reinforced rafters. If you have chosen a metal structure, then it must have an appropriate profile thickness. In any case, it is better to use the results of the calculation to select materials of the required stiffness.
Examples of calculating snow and wind loads for Moscow and the Moscow region
Example #1: Snow Load Calculation
Initial data:
region: Moscow
roof slope: 35 degrees
Find the total design value of the snow load S:
the total design value of the snow load is determined by the formula: S=Sg*m
according to the map of snow cover zones of the territory of the Russian Federation, we determine the number of the snow area for Moscow: in our case, this is III, which corresponds according to the table to the weight of the snow cover Sg = 180 (kgf / m2);
coefficient of conversion from the weight of the snow cover of the ground to the snow load on the cover for a roof angle of 35 degrees m=0.7
we get: S \u003d Sg * m \u003d 180 * 0.7 \u003d 126 (kgf / m2)
Example #2: Wind Load Calculation
Initial data:
region: Moscow
roof slope: 35 degrees
building height: 20 meters
terrain type: urban areas
Find the full design value of the wind load W:
The calculated value of the average component of the wind load at a height z above the earth's surface is determined by the formula: W=Wo*k ,
According to the map of wind pressure zones on the territory of the Russian Federation, we determine region I for Moscow
The normative value of the wind load corresponding to the I region, we take Wo = 23 (kgf / m2)
The coefficient k, which takes into account the change in wind pressure along the height z, is determined from Table. 6k=0.85
We get: W \u003d Wo * k \u003d 23 * 0.85 \u003d 19.55 (kgf / m2)
The weight of snow in winter creates a significant load on the roof truss system, and through it - on the foundation of the building. The calculation of the snow load on the roof is necessary both for determining the parameters of the roof structure and for designing the foundation, where the total weight of the house is important. This article discusses methods for determining the weight of snow cover on the roof of a house, determines what threat it poses to people and dwelling structures. The information will be useful to all people living in regions with snowy and long winters, planning to build a private house.
Types of roof loads
The main loads affecting the roof are:
Snow weight.
.They have a different degree and nature of the impact on the roof and the truss system as a whole. The snow load is more static, all changes occur relatively slowly and smoothly. The only exception can be the avalanche-like descent of large snowdrifts, which is characteristic of modern types of metal roofing. In addition, snow lies for several months, there are no loads in summer.
For the wind, the season does not matter, it is able to rise both in winter and in summer. The wind is dangerous because of its unpredictability, it is impossible to foresee and somehow prepare. Most often, strong winds do not last long, but the consequences are very deplorable. At the same time, strong gusts that create noticeable pressure on the structure of the house are relatively rare.
In most cases, the wind load is minimal and does not have a constant value. The episodic nature and unevenness of wind manifestations create significant difficulties in determining the real load on the structure of the house, therefore, it is customary to take into account the maximum tabular values for a given region.
Dependence of loads on the angle of inclination of the roof
Snow and wind loads are inversely related to the angle of the roof. The wind is directed parallel to the surface of the earth, any vertical objects are a hindrance to it. Snow falls on the plane and presses on it in the direction from top to bottom. Therefore, the steeper the angle of inclination of the roof slopes, the greater the wind loads and, conversely, the weaker the pressure of snowdrifts. Therefore, to reduce wind loads, it is necessary to reduce the angle of inclination, and to reduce snow loads, increase it.
Such a discrepancy requires the designer to have accurate knowledge of the amount of snow cover and the strength of the winds prevailing in the region, the possibility and frequency of squally gusts. Otherwise, you can get an excessively steep roof, forming a strong sail, or too flat, not allowing snow to roll down an inclined plane.
The roof must be designed to allow snow to roll down an inclined plane. Source pxhere.com
Why are snow loads dangerous?
High snow loads are dangerous in several ways:
Creation excessive pressure on the truss system, causing deflection, sagging coatings or destruction bearing roof elements.
Appearance additional load on house walls, and through them to foundation.
Big weight of snow dangerous with a sudden descent of snowdrifts from the roof, as those below may suffer people, cars or otherwise property.
In addition, a large amount of snow begins to melt as the temperature rises, forming a layer of ice on the roof surface. It is dense and heavy, well kept on the surface, gradually increasing its thickness. During thaws, this ice rolls down and causes severe damage to all objects on which it falls. It must be remembered that a relatively thin layer of ice of 5 cm on the surface of a slope with an area of 20 m 2 weighs about a ton.
The calculation of the snow load on a flat roof shows the magnitude of the impact of snow on a horizontal plane. The angle of inclination of the slopes is taken into account by special coefficients. It is believed that with a slope of more than 75 ° there is no snow load, although in practice wet snow sticks to vertical planes. There is another danger in this, when the structures of the house are unprepared to receive significant pressure.
On our website you can find from the best construction companies with an impeccable reputation in the market. You can choose or from any modern building material. You can directly communicate with representatives by visiting the exhibition of houses "Low-Rise Country".
Features of the distribution of snow load on the roof surface
The snow load is distributed on the roof surface in different ways, evenly over the entire area, or with a noticeable skew to the leeward side. Sometimes huge hanging layers grow on the slopes, which create a corresponding pressure on the eaves of the roof.
Such distortions can deform or destroy the structures of the rafters, create significant pressure on the foundation. It must be understood that a uniform load from the weight of snow affects the structure of the house in an extremely unfavorable way. There are regions where the thickness of the snow cover exceeds 2 m. In such conditions, it is extremely important to take the correct slope angles so that the snow masses can roll off them without reaching excessive thickness and without creating an unbearable load on the supporting structures.
The amount of snow cover is more than 2 meters - an unbearable load for supporting structures Source ko.decorexpro.com
Determination of snow pressure on the roof according to SNiP
When it becomes necessary to determine what snow load on the roof exists in a given region, a lot of questions immediately arise. First of all, how can you find out the amount of snow cover? Direct measurement with a ruler will not provide useful information - each winter has its own characteristics, there are seasons with little snow, when the level of precipitation is less than usual.
The magnitude of the snow impact can be determined using the SNiP applications. There is a map of the Russian Federation, in which all regions with the same amount of snow cover are outlined and numbered. Consider the current version of this application for today:
To determine the snow pressure on the roof, you need to find the point of interest on the map and find out which snow area it belongs to. Then we use the table:
| Snow regions of the Russian Federation | Load value kg/m² |
|---|---|
| 1 | 80 |
| 2 | 120 |
| 3 | 180 |
| 4 | 240 |
| 5 | 320 |
| 6 | 400 |
| 7 | 480 |
| 8 | 560 |
If the roof area is known, then it will not be difficult to determine the weight of the snow - you just need to divide it by the tabular value for this region. But the resulting value shows the load on the horizontal plane. A correction factor is used to account for the angle of inclination. It was found empirically and has the following meanings:
At tilt angle up to 25° - 1.
At tilt angle from 25 to 60 ° - 0.7.
At tilt angle more than 75° - 0.
The zero value of the correction factor was adopted because it is believed that such a slope provides an independent snow flow from the slopes, and there is no pressure. For such roofs, snow retainers are often used to prevent too massive snow from falling.
The most popular manufacturers and construction companies are gathered at the exhibition and presented on our website. Here you can find contacts, choose and order any service, including , . You can directly communicate with representatives by visiting the exhibition of houses "Low-Rise Country".
Snow load calculation online calculator
There is another way to calculate the weight of snow on the roof. This is the use of an online calculator, a specialized resource that automatically performs calculations based on the user's initial data. The debate about the benefits of online calculators has been going on since the very first day of their appearance. Most users are convinced that, if it is necessary to perform a qualitative calculation of the snow load on the roof, the calculator is useless.
Relying on an unknown algorithm for such a crucial issue is dangerous. Supporters of the use of these resources argue that the criterion for the quality of the work of such resources can be the duplication of the calculation on other calculators. It is difficult to say for sure which of them is right. However, given the relative simplicity of self-calculation, it is much more correct to perform these few arithmetic operations on your own.
Calculation of snow load on the roof in the Moscow region
As an example, consider how the snow load on the roof in the Moscow region is calculated. Initial data:
House from two slopes, total roof area 64 m 2 .
Slope angle is 36°.
On the map of snow regions, we determine which of them the Moscow region belongs to. This is district 3. According to the table, we obtain a specific load value equal to 180 mg / m 2.
64 × 180 = 11520 kg.
The resulting value must be multiplied by the slope factor. In the case under consideration, it is equal to 0.7. Then we get:
11520 × 0.7 = 8064 kg.
Snow weight will be 8t and 64 kg. As you can see, this calculation does not present any complexity, it is required to perform literally 2 steps.
Simple clear arithmetic operations for calculating the magnitude of the snow load Source domik.ua
Video description
In the video lesson, an educational program is conducted on the subject of sopromat. In an accessible form, the material is presented for calculating the structures of the house, taking into account the snow load:
Online roof calculator
To find out the approximate cost of various types of roofing, use the following calculator.:
Finally
It should be reminded once again of the importance and responsibility of such calculations. They will be needed in several situations, they will affect the bearing capacity of the foundation and rafters. The magnitude of the snow load should not be forgotten or neglected - the calculation just under consideration showed that 8 tons of snow lie on the roof of a small house in the relatively little snow of the Moscow Region. If the amount of precipitation in the region is greater, as is the roof area, the impact will be much more intense, which can lead to destruction. There is no point in taking risks, it is better to complete all the necessary calculations on time.