Rafter dimensions and material requirements. The maximum length of the rafters. How the parameters of the rafters are calculated Features of the installation of rafter legs

The house is the final stage of construction. This is a responsible event, on which both the durability of the operation of the building and the safety of the people living here depend. In this case, one cannot do without a correct calculation.

A mandatory element of the entire system is the rafter leg. It will have permanent and temporary loads. Therefore, before producing on your own, you need to familiarize yourself with all the details of this process. There are certain rules and regulations that govern the arrangement of the truss system.

general characteristics

The truss is the main supporting structure. It bears the main load. To ensure the reliability of the system, each rafter leg is rigidly attached. This allows the roof to withstand gusts of wind, snow drifts and other influences.

The material for such structural elements is most often wood. It is easier to correct, and the cost of work is significantly reduced. Special attention should be paid to special antiseptic and fire-fighting impregnations.

The angle of inclination of the truss elements is equal to the slope of the roof slopes. The bottom rests on the Mauerlat. This allows you to evenly distribute the load. The top of these elements rests on a beam under the ridge or intermediate fittings.

Rafter legs are sometimes also called a diagonal or oblique leg, as well as simply a rafter.

Load

The rafter leg is subjected to various loads. These are permanent and temporary varieties. The first group includes the total weight that the truss system has, as well as other roofing materials (vapor barrier, waterproofing, roofing material, slate). It also includes elements of interior decoration of a room or attic.

Live loads can arise due to a variety of reasons. They can only be calculated approximately. In this case, the maximum possible amount of precipitation that can be observed in this climatic zone is taken into account. Wind gusts can also affect the roof. If in this area they are long and have great strength, this feature should be taken into account in the calculations.

It is necessary to take into account the weight of people who will carry out the construction or maintenance of the roof in the future, as well as the total size of their tool. Take into account the load that additional equipment will provide (chimney, antenna, aerator, ventilation, etc.). If additional structures are installed on the roof, their weight is also taken into account.

Calculation rules

To perform the calculation of the rafter leg, after assessing the load, determine the type of material, as well as the type of the roofing system itself. It can be suspended or layered. In the first case, the rafters have two points, but which are the extreme supports. This creates a bursting force horizontally. work in compression and bending. To reduce this force, they are connected by stretching.

The layered type of construction is applicable in buildings with an average load-bearing wall or intermediate supports, the ends of which are adjacent to the outer walls. At the same time, the rafter leg performs the function only for bending. At the same time, the total material costs are reduced. But such a system is suitable only for those buildings in which the supports are separated from each other no further than 6.5 m.

When performing the calculation, the angle of inclination of the roof is also taken into account. At this stage, all additional elements of the system are also considered.

Length

The length of the rafter leg is also calculated in accordance with all existing operating conditions of the roof. Initially, you should determine the size of the building itself. This takes into account all the ledges, verandas, attic, which contains the construction plan.

Next, the shape and angle of the roof slope are thought out. Further, based on the data obtained, choose the length of the rafter leg. Usually this indicator does not exceed 6 m. This is due to the standardization of sawn timber that goes on sale. To purchase longer varieties of products, you will have to pay a decent amount of money.

If necessary, these structural elements are increased. The need for long rafter legs sometimes arises when arranging hip, half-hip or diagonal rafters.

cross section

After calculating the length, it is required to determine the cross section of the rafter leg. Its type is influenced by several factors. All operating conditions specific to the roof will matter in this process. The level of loads, the slope of the roof, the type of roof must be taken into account when choosing a section. An important factor is the total area of \u200b\u200bthe building, its configuration. The more factors that affect the operation of the building, the wider the cross section of the rafters should be chosen.

Also, do not forget to take into account the peculiarities of the climatic zone. It is important to immediately plan what material is used during the construction of the rafters. Most often it is wood. It must be of good quality. for rafters should be 20-22%.

To date, there are many special programs that allow you to calculate the cross section of the rafter leg. To avoid mistakes, you should use them. But it is better to entrust this work to a professional. The most popular section size is 50 by 150 mm.

Laying step

The rafter leg, the dimensions of which were calculated in accordance with the building regulations, requires proper installation. To do this, you need to calculate the laying step of these structural elements.

In this case, an important role is played by the load that the factors listed above have on the roof. By correctly choosing the parameters of the rafters using special tables, you can calculate the laying step. With a standard section size (50 by 150 mm) and a length of 3 m, the distance between the rafters will be 1 m.

If the load level is high, it is better to reduce this indicator. This is especially true for slate roofs. In the case of using rafters with a standard section, but 4-4.5 m long, the step between the structural elements is reduced to 60 cm.

The slope of the roof is also important in the calculations. For example, with a slope of 45 degrees, the pitch of the rafters can reach 120-140 cm. But in the event of a large amount of precipitation in this area, this figure is reduced to 60-80 cm.

Installation Basics

The rafter leg assembly consists of several elements. These include crossbar, racks and struts. Their participation in a system of small width is especially important. If this is not done, over time, the truss structure may sag. Therefore, in such cases, equip a special lattice.

The legs of the rafters are known to rest on the Mauerlat. It is placed across the entire width of the building or placed only under the bottom of the structural elements. If the truss system is wooden, the Mauerlat is made of logs or timber. This is the top of the log. But in brick buildings, the mauerlat is set flush with the inner surface of the walls. A layer of waterproofing is laid between them.

Plain roof installation

The rafter leg and Mauerlat are very important elements of the entire structure. The quality of the roof depends on the correctness of their installation. They can be attached in two ways. Installation is rigid or sliding. The correct variety is chosen in accordance with the type of truss system (layered, hanging).

The rigid type of fastening ensures the immobility of the structure. At the same time, cuts are made on the rafters. Structural elements are fixed with metal brackets, corners, wire or long nails.

A sliding joint is also called a hinge joint. If it is necessary to ensure the freedom of settling of wooden elements on the frame, this type of installation is preferable. They make a gash on the rafter leg and fasten it with a Mauerlat with two nails. They are punched in diagonally. Another nail is driven in from above.

Hip roof installation

When arranging a hip roof, the rafter leg often turns out to be longer than 6 m. Here, an extension is required. Two rafter boards pair. In this case, they need to be strengthened. This procedure involves the arrangement of struts. These are vertical racks, which are installed in the design no more than 2 pieces.

These reinforcements are supported by a wood lining. It is mounted on the ceiling or on the puff. Diagonal truss elements are always longer than ordinary ones. They are subjected to a load that is 1.5 times greater than normal. Therefore, the amplification procedure in this case is simply irreplaceable.

Having become acquainted with such an element as a rafter leg, each novice builder will be able to delve deeper into the process of building a roof. Subject to all building codes and regulations, high quality construction can be achieved.

The acquisition of the right material is one of the most important aspects of any construction. When building a truss system, you need to understand that it has a great responsibility, therefore, to create such a structure, you should use only reliable, high-quality raw materials. You can buy ready-made roof trusses, or you can make the necessary details from logs or other material on your own. The dimensions of the rafters, the methods of their installation, fasteners and other nuances should be chosen individually for each building and truss system.

There are three standard options for purchasing rafter boards, it is worth considering the pros and cons of each of them:

cubic meters of lumber. Buying wood in cubic meters is a completely acceptable way, for this you just need to order the required number of cubic meters of timber or boards from a specialized company. Further, they will still need to be cut and processed, only after that you can begin construction. In this case, it is worth considering waste when processing wood, so it’s better to take somewhere 10 percent more than calculated according to the construction plan.
Custom cut boards. The next option is to purchase ready-made rafters after cutting. Now many companies are engaged in cutting wood to order according to the given parameters, the builders will only have to assemble the truss system on the roof. There will be practically no waste here, and the process of constructing the structure will go much faster.
Finished roof trusses. This option is easier than the first two. Such structures are easier to mount on the walls right on the roof, which will reduce the assembly time of the entire structure to one or several days. This farm needs only to be installed and connected in the ridge. The amount of waste is minimal, but the price of this option is much higher.

For self-manufacturing of rafters, it is enough to know their necessary parameters, such as the angle of inclination, distance, section, fastening method, and so on. In order to calculate the desired size of the rafters, carpentry tools are used. Modern special building calculators carry out these calculations extremely quickly.

lumber requirements

When calculating a truss structure involving the production of rafter legs from a board, in addition to all sizes, it is also extremely important to take into account the quality of the lumber used. First you need to decide what kind of wood is best suited for rafters. Here the answer is unequivocal - it is hardwood and coniferous lumber that meets the requirements of GOST 2695-83 and GOST 8486-86. Based on these standards, a board for the manufacture of rafters may have:

Non-through cracks not exceeding half the board in length;
No more than three knots per linear meter of material, while the maximum allowable knot diameter is 30 mm;
Humidity is not more than 18% (from the moisture meter).

According to the requirements of SNiP, when buying lumber for the production of rafters and other components of the roof frame, it is imperative to check the documents informing about the quality of the purchased products. They must indicate:

The name of the product and its standard number;
Name of the manufacturer with basic data about him;
Type of wood, size of lumber, degree of humidity;
Date of release of the consignment;
The number of units in the package.

Wood is a natural material, so it is quite susceptible to biological degradation. To reduce the risk of damage to the structure assembled from this lumber, it must be prepared before installation. Preparation involves carrying out various constructive and protective measures.

Protective ones include:

Impregnation of wood with special flame retardants to prevent the likelihood of fire;
Treatment of wood with antiseptic compounds to prevent premature decay;
Treatment of rafters and other wooden elements with pest control agents.

Construction activities include:

Formation of waterproofing under the roof and vapor barrier from the side of the room;
Installation of waterproofing pads at the junction of wooden structures to brick ones;
Roofing pie ventilation equipment.

The rafters made from the board, subject to all prescribed roof construction technologies, will last a long time without any repair.

Rafter dimensions

The choice of section and length of truss elements is extremely important. All rafter legs must be created the same length and thickness, which depend on the pitch of the rafters and the expected load on them. The materials for the rafters are logs, timber or boards. The disadvantages of log rafters include their considerable weight and the need to make complex cuts for fasteners, which significantly weakens the bearing capacity of the structure. The beam is more suitable for the manufacture of rafters, but it costs more than boards.

Important! For rafters, the best option is a beam that has already aged for some time: it does not change shape after the construction of the roof, which ensures the stability of the structure. A beam with a width of 10–15 cm is used, while its thickness should be about 5 cm.

Rafter legs from the board are the best choice, this material is very popular and is actively used to create side rafter legs. It is also convenient to make side rafters of an elongated type and strong sloping rafter legs from the board.

The thickness of the board significantly affects its characteristics. An ordinary board suitable for the manufacture of rafters has a thickness of 40-60 mm.

Important! It is worth noting that when building a roof on outbuildings, it is better to take a board with a thickness of 40 mm, this will help reduce costs. When erecting the roof of a residential building, this parameter should not be lower than 50 mm.

The width of the rafter board is selected depending on the length of the opening to be blocked - the longer the length of the rafter leg, the wider the board is needed for its production. When the length of the rafters does not exceed 6 meters, for their manufacture, you can take a board 150 mm wide - that is, the minimum size of the rafter section for the roof of a residential building is 50 × 150 mm. If the rafter leg is more than 6 meters, then the width of the board must be at least 180 mm. The elongated leg is made from cohesive boards, the width of which is 150 mm.

Another important parameter is the cross section of the rafters. It depends on the loads on the roof: both from external weather and from the severity of the truss system itself. Also, the calculation of the section is affected by the angle of inclination of the slope, the width of the building and the length of the overlapped opening. For calculations, special formulas and tables are used; there are many corresponding computer programs. Having determined the value of the section of the rafter legs, it is worth paying attention to the installation features of this design.

Features of mounting rafter legs

After choosing the optimal material for the rafter system, adjusting the size of the rafters, going through all the stages of their processing, it is also necessary to correctly install the rafter legs on the Mauerlat. The strength and reliability of the entire structure largely depends on this connection. There are two mounting options - sliding and rigid. Each of them is applicable to a certain type of rafters, and its choice depends on many factors.

Rigid fastening eliminates the possibility of any turns and bends of the rafters. For him, cuts are created and the rafter legs are fixed on the Mauerlat with the help of various fasteners.

A sliding joint, also often referred to as a swivel joint, has a couple of degrees of freedom. It is usually used in the construction of a roof over a wooden house, as it allows the roof to settle over time on the frame, giving some shrinkage in the first few years. In this case, the connection of the ridge with the rafters is not so rigid. The rafter leg is fixed on the Mauerlat by washing down and strengthened with nails on both sides.

When erecting a hip roof, the rafter is more than 6 meters in size. Because of this, the rafters increase in length. To strengthen the rafter legs, struts are made for them from vertical racks. More than two racks are rarely installed at the same time.

Many people have such questions: how to make rafters with your own hands, how to properly install rafters, what is the best material for them to choose, and so on. Now you can find a lot of information about this in various sources and do the work yourself. If there are any questions and it is not possible to cope on your own, then it is better to turn to the services of professionals, whose reliability there is no doubt.

In order to calculate the roof truss system, a person who is not familiar with all the nuances of complex design calculations in accordance with SNIP and other standards can use our roof construction calculators.

As initial parameters, it is necessary to enter the data of some elements of the truss system:

specify the step of the rafters (the distance between them - the step regulates the load on the rafter system),
rafter dimensions - the so-called section = thickness x width of a board or beam

Here it is worth saying that the board is a more affordable option for installing a roofing system, since it can withstand loads, and, importantly, it costs several times more budget.

In the two tables below we have collected commonly used in construction sizes of rafters and battens broken down by type of roofing. The minimum angle of inclination of the roof is also given as optimal, depending on its type, in some places the angle is indicated as minimal, but all according to SNIP.

The main most commonly used parameters of the elements of the truss system are the pitch and cross section of the rafters, the angle of inclination of the roof, depending on the type of roofing material:

roof type	Optimum roof slope, degrees	rafter step,	rafter section,
Decking	(optimum - 20-30)		board 5 x 15
			board 5 x 20
Cement-sand tiles		≤ 75; ≤ 90; ≤ 110	board 5 x 15
ceramic tiles			board 5 x 15; 6x18
soft roof (roll; bituminous tiles)			board 5 x 15
metal tile			board 5 x 15; 5 x 20 (for insulation)
			board 5 x 15; 5x15
asbestos-cement sheets of ordinary profile
asbestos-cement sheets of a unified profile

To calculate the rafters of a gable roof in automatic mode, the rafter calculator on our website will help you.

The following table contains data on crate, counter-crate and according to the roofing material:

roof type	Shelter. material Length x width x thickness, mm	Roof slope, degrees	Lathing pitch, cm	Lathing section, cm	Counter-batten, cm (pitch = rafter pitch)	An overlap of blood. sheets, cm
Decking:		Min 12 (optimum - 20-30)	according to the angle of inclination	board 3 x 10	the width of the beam is slightly less than the rafters with a thickness of 2.5 - 4	horizon. overlap:roof angle less than 15° - 20 cm; 15-30° - 15 -20; from 30° - 10 -15
NS-20	thickness 0.55	30; 45	40; 60
NS-20	0,75	30; 45	50; 70
NS-35	0,55	30; 45	100; 100
NS-35	0,75	30; 45	120; 130
S-44	0,55	30; 45	90; 150
S-44	0,75	30; 45	110; 140
Cement-sand tiles and ceramic tiles	from manufacturer and type	22 - 30	31,2 - 33,5	timber from the pitch of the rafters:3x5; 4x5; 4x6 or 5x5	from 3 x 5	8,5 - 10,8
Cement-sand tiles and ceramic tiles	from manufacturer and type	30 - 90	32,1 - 34,5	board 5 x 15; 6x18	from 3 x 5	7,5 - 10,8
soft roof (roll; bituminous tiles)	from the manufacturer	from 7	1. rolled - on a continuous crate 3 - 5 mm gap;2. soft tiles - 30 cm step of the boards of the sheathing under the OSB	1.solid 2. lathing from a board 2.5 x 10-15 + OSB 9mm	from 3 x 5	for rolled - 15-30; for soft tiles - from 15
Metal tiles	opt. 4500 x 1160 - 1190 x 0.5 profile height 1.8 - 2.5 cm wave pitch 35-40 cm	from 20	80 - 100 (from wave)	board 5 x 20; timber 4 x 6	from 3 x 5	depending on brand 6 - 9
Slate	3600 x 1500 x 8-10 3000 x 1500 x 8-10 2500 x 1200 x 6-8-10	14 - 60; opt. 25-45	the sheet should rest on 2 beams of the crate		from 3 x 5	from 12 to 30
asbestos-cement sheets are common. profile			50 - 54	board 5-6 x 10; timber from 5 x 5		should cover the wave
asbestos-cement sheets unified. profile			60 - 75	board 5-6 x 10; timber from 7.5 x 7.5		should cover the wave
bitumen corrugated sheet (euroslate)- For example ondulin	2000 x 950 x 3 wave height 36	5 - 10	5	solid (gap up to 5 cm)	from 3 x 5	3; lateral - 2 waves
		10 - 15	45			2; lateral - 1 wave
		from 15	60	board 5 x 20; timber 4 x 5; 5x5		1.7; lateral - 1 wave

In order to independently determine the dimension of the entire rafter system, it is necessary to calculate the main influence of wind, snow masses, as well as the weight of roofing materials and structural load-bearing roof elements in the aggregate.

Again, we remind you that the calculation is given for review in a much simplified format, since for an accurate calculation it is necessary to take into account the vertical and horizontal loads on the rafter legs, additionally calculate the resistance of the rafters to bending, compression and tension, check the structures for their ability to resist chipping and crushing.

If you do not have a complex architectural structure, you can easily build a roof yourself, based on the optimal dimensions of a beam or board, on standardized roof design parameters.

The figure and table below show standard sections of elements roof structure:

Cross-sections of wooden floor beams depending on the span and beam installation step, using the example of a case with a full load of 400 kg / m2

Span (m)
Installation step (m)

We repeat once again that in a simplified format, everyone is able to calculate the ability of a roofing system to withstand loads.

O online roof calculators will help you calculate the amount of timber, roofing and under-roofing materials for the construction of the roof and truss system, as well as the parameters of the roof, battens and rafter legs.

Thus, you can roughly estimate how much building materials you need to purchase, how and in what quantity the crate and rafters will be located.

The completion of the construction of any house is the construction of its roof, which requires a truss system.

This system includes such components as rafter legs, struts, mauerlat, racks, sprigs, puffs, lathing, trussed and other elements that give the roof structure strength and rigidity.

From this article you will learn exactly about the rafter legs.

Characteristics of rafter legs

In various structures, rafter legs may have the names of slanting or ordinary rafters.

Their size is selected based on the variable and constant loads that will necessarily affect the roof.

If this is not done, then the rafters may turn out to be fragile, which will lead to the destruction of the roof structure.

The roof of the house must necessarily include load-bearing elements, the main of which are the rafters.

Their task is to take on the entire load from other parts of the roof, as well as additional weight in the form of precipitation.

Wooden rafters are most often made from softwood.

The blanks are treated with special compounds that provide the wood with fire resistance and protect it from various biological factors and phenomena.

The loads that the rafters take on can be divided into permanent, temporary and special loads.

Permanent loads are the weight of the entire roof of the building, live loads are the weight of repair equipment, workers, snow, wind, etc.

A special load on the rafters is a seismic effect.

The designs of the rafter legs can be different, but the layered and hanging ones are especially distinguished.

The foundation is the walls of the house.

Their middle part must rest against specially constructed central supports.

Such rafters are installed where there is a main wall, which is a carrier or on installed intermediate supports - pillars.

This is done when a single roof structure is mounted, consisting of several spans.

If there are no intermediate supports, hanging rafters are attached, and where they are, layered ones are installed.

Calculation

In order to draw up a technical project of your house, a mandatory one is required.

There are several methods for calculating such structures.

It is ideal to entrust the calculation process to competent specialists.

As experience shows, the cost of their services during the construction of the roof quickly pays off.

If you yourself have such knowledge, and also connect the Internet to this process, where there are special programs and calculators, then you can calculate the rafter legs without any problems, that is, their length and size.

The main size of the cross section of the rafters in the construction of houses is 150 x 150 mm.

It is successfully used for the construction of roofs of any shape.

Length, that is, the distance between them, is usually taken equal to one meter.

Given the weight of the roofing material on the roof, you need to know that the currently popular shingles are the heaviest.

This means that the rafters must have a margin of safety in order to hold it for a long time.

In this case, the legs play an important role.

Rafters that are supported by only two supports are known as rafters without struts.

They are mainly used for single-pitched roofs with a span of 4.5 meters or for double-pitched structures with a span of 9 meters.

The section of the beam from which the rafters are made is affected by the length of the rafter leg, its step, as well as the calculation of the loads on it.

Below are the points that directly affect the choice of section:

temporary and permanent loads on the rafters
roofing material
roof pitch angle
roof view
the size of the house, the complexity of construction and the shape of its corners
climatic and natural features of the area where the house is being built
the quality and reliability of the material from which the house is being built.

Fastening rafters to Mauerlat

Mauerlat is the cornerstone of the building's roof.

It evenly distributes the considerable weight of the roof over all structures.

Mauerlat is connected to the rafter legs and takes the load from them.

This element is solid and is laid around the entire perimeter of the roof.

It can also be pieces of 1 meter and fit directly under the rafters.

Only timber, boards and logs go under the Mauerlat, the section of which is 100 x 100, 100 x 150, or 150 x 150.

If a log is used, then its side is cut off until there is a snug fit against the wall.

Fixing rafters to the Mauerlat is a very important point in the construction of the roof.

The durability of the roof when exposed to winds, snow, frost and heat depends on the method by which the rafter leg was attached.

As you know, wet wood expands, and if it is exposed to high temperatures, it will shrink.

That is why it is impossible for all connections to be rigid, since there will certainly be displacements and breaks.

To prevent this, you should know the rules for fastening rafters to the Mauerlat.

There are two ways to connect the rafter legs and the Mauerlat - hard and sliding.

A rigid connection eliminates the effects of torsion, bends, shifts and turns between parts of the structure.

This result is achieved when fastening with corners using hemmed support bars, as well as by sawing the saddle on the rafter, followed by connection with staples, nails and wire.

The second method of attaching the rafter legs to the Mauerlat is more common.

It involves driving nails from the sides at an angle so that they cross inside the Mauerlat.

After that, another nail is hammered vertically, which gives a fairly rigid attachment point.

Usually, insurance is used for both types of fastening: the rafters are connected to the wall with wire rod and anchors.

If the angle of the roof slope is the same in all places, then the same type of rafters are used, made according to a single template.

Some working methods

To increase the bearing qualities of the rafters, it is important to strengthen the truss system.

To do this, the installation of unloading beams, struts and double-sided overlays is carried out.

Based on the results of practice, truss structures selected taking into account strength characteristics sometimes do not fall under the deflection calculations according to SNiP “Loads and other influences”.

Therefore, the cross-sectional area must be made larger.

It is not difficult to reinforce the rafters: you can apply help - an additional beam.

This element must be attached to the bottom of the rafters in the span between them and the Mauerlat.

The reinforcement of the rafter leg is fixed with metal plates with teeth or bolted clamps.

If the rafter has already been reinforced with help, then you can increase its length and take it beyond the edge of the support to the strut.

Two goals are achieved here: we obtain a satisfying deflection index and reinforce the reference node.

To strengthen or restore damaged structures, you can use the following methods:

Wooden lining. They are used if a single rafter has been damaged. The lining made of wood is fastened with bolts or nails, which leads to reinforcement of the roof section.
Bar prostheses. This method is used when massive damage to the rafter legs has occurred. With the help of temporary supports, damaged rafters are fixed. The coating is disassembled and the rotten part is cut out. After that, an element from a new beam is inserted into a free space and rests on the Mauerlat.
Overlays based on a beam. The method is suitable for replacing a rotten area at the end of the rafters or part of the Mauerlat. Temporary supports are installed and rotten elements are cut out. Crutches are hammered into the masonry, where a meter beam is laid. After that, a piece of laying of the same length is placed on the wall or ceiling. Two struts, previously fixed with nails, are attached to the new beam.

Filly in truss structures

A filly is a piece of board, with the help of which the rafter leg is lengthened.

It is used for roof overhangs.

A filly is needed when the length of the board from which the rafter leg is made is less than what is required to build a canopy.

Boards for making a filly are usually smaller in width than the boards from which the rafters are made.

This allows you to reduce the effort to remove the line of the eaves.

When the filly gets rotted or damaged, it's much easier to replace than the entire rafter without taking the roof apart.

Today, when building technologies are constantly being improved, new methods are emerging for reliable fastening of rafters to the Mauerlat, fastening fillies to rafters, etc.

To keep abreast of such novelties, study the news in the field of construction and do not be afraid to introduce innovative developments.

Video about the truss system.

Collectionloads

Previously, to determine the loads, we set the cross section of the rafter leg 75x225 mm. The constant load on the rafter leg is calculated in Table. 3.2.

Table 3.2 Estimated constant load on the rafter leg, kPa

	Exploitation		limiting
Elements and loads		γ fm	meaning
	meaning		loads
	loads

Rafter leg 0.0750.2255/0.95
	g str. e \u003d 0.372		g c tr. m = 0.403

Estimated maximum load on the rafter leg (combination of constant plus snow)

The geometric scheme of the rafters

Schemes for calculating the rafter leg are shown in fig. 3.2. With the width of the corridor in the axes = 3.4 m distance between the longitudinal axes of the outer and inner walls.

The distance between the axes of the Mauerlat and the lying, taking into account the binding to the axis (
\u003d 0.2 m) m. We set the brace at an angle β = 45° (slope 2 = 1). The slope of the rafters is equal to the slope of the roof i 1 \u003d i \u003d 1/3 \u003d 0.333.

To determine the dimensions necessary for the calculation, you can draw a geometric diagram of the rafters on a scale and measure the distances with a ruler. If the mauerlat and the bed are on the same level, then the spans of the rafter leg can be determined by the formulas

Node heights h 1 =i 1 l 1 \u003d 0.333 * 4.35 \u003d 1.45 m; h 2: = i 1 l\u003d 0.333 * 5.8 \u003d 1.933 m. Height mark: we take the crossbar 0.35 m below the intersection point of the axes of the rafter leg and the rack h = h 2 - 0.35 (m) = 1.933 -0.35 = 1.583 m.

Forces in the rafter leg on the crossbar

The rafter leg works like a three-span continuous beam. Support settlements can change the support moments in continuous beams. If we assume that the bending moment on it has become equal to zero due to the subsidence of the support, then it is possible to conditionally cut the hinge into the place of the zero moment (above the support). To calculate the rafter leg with a certain margin of safety, we consider that the subsidence of the strut lowered the support bending moment above it to zero. Then the design scheme of the rafter leg will correspond to Fig. 3.2, c.

Bending moment in rafter leg

To determine the thrust in the crossbar (puff), we consider that the supports have sagged in such a way that the reference moment over the strut is equal to M 1 and above the racks - zero. We conditionally cut the hinges into places of zero moments and consider the middle part of the rafters as a three-hinged arch with a span l cp = 3.4 m. The thrust in such an arch is

Vertical component of the strut reaction

Using the diagram in Fig. 3.2.d, determine the force in the strut

Rice. 3.2. Schemes for calculating rafters

a-cross section of the attic; b - diagram for determining the estimated length of the rafter leg; c - design scheme of the rafter leg; d - scheme for determining the thrust in the crossbar; l - also for a scheme with one longitudinal wall; 1 - Mauerlat; 2 - bed; 3 - run; 4 - rafter leg; 5 - rack; 6 - brace; 7 - bolt (tightening); 8 - spacer; 9, 10 - persistent bars; 11 - filly; 12 - overlay.

Calculation of the rafter leg according to the strength of normalsections

Required moment of resistance run

App. M accept the width of the rafter leg b = 5 cm and find the required section height

App. M accept a board with a section of 5x20 cm.

There is no need to check the deflections of the rafter leg, since it is located in a room with limited access to people.

Calculation of the joint of boardsrafter leg.

Since the length of the rafter leg is more than 6.5 m, it is necessary to make it from two boards with an overlap joint. We place the center of the joint at the place of support on the brace. Then the bending moment at the joint during the subsidence of the strut M 1 = 378.4 kN * cm.

The joint is calculated similarly to the joint of the runs. We accept the length of the overlap l nahl =1.5 m= 150cm, nails diameter d= 4mm=0.4cm and long l guards = 100 mm.

Distance between axes of nail connections

150 -3 * 15 * 0.4 \u003d 132 cm.

The force taken by the nail joint

Q \u003d M op / Z \u003d 378.4 / 132 \u003d 3.29 kN.

Estimated nail pinching length, taking into account the normalized limiting gap between the boards δ W = 2 mm with a board thickness δ D = 5.0 cm and a nail tip length l, 5d

a p = l gv -δ d -δ w -l,5d \u003d 100-50-2-1.5 * 4 \u003d 47.4 mm \u003d 4; 74 cm

In the calculation of the dowel (nail) connection:

is the thickness of the thinner element a= a p =4,74 cm;

- the thickness of the thicker element c = δ d = 5.0 cm.

Finding a relation a/c = 4,74/5,0 = 0,948

App. T, we find the coefficient k n \u003d 0.36 kN / cm 2.

We find the bearing capacity of one seam of one nail from the conditions:

– wrinkling in a thicker element

\u003d 0.35 * 5 * 0.4 * 1 * 1 / 0.95 \u003d 0.737 kN

– crumpling in a thinner element

\u003d 0.36 * 4.74 * 0.4 * 1 * 1 / 0.95 \u003d 0.718 kN

- nail bending

= (2,5* 0,4 2 + 0,01* 4,74 2)
/0.95=0.674 kN

- but not more than kN

Of the four values, choose the smallest T = 0.658 kN.

Find the required number of nails P guards ≥ Q/ T =2,867/0,674=4,254.

Accept P guards = 5.

We check the possibility of installing five nails in one row. The distance between the nails across the wood fibers S 2 \u003d 4d \u003d 4 * 0.4 \u003d 1.6 cm. The distance from the extreme nail to the longitudinal edge of the board S 3 \u003d 4d \u003d 4 * 0.4 \u003d 1.6 cm.

According to the height of the rafter leg h = 20 cm should fit

4S 2 + 2Sz \u003d 4 * 1.6 + 2 * 1.6 \u003d 9.6 cm<20 см. Устанавливаем гвозди в один ряд.

Calculation of the junction of the crossbar with the rafter leg

According to the assortment (app. M), we take a crossbar from two boards with a section bxh = 5x15 cm each. The force at the joint is relatively large (N = 12, kN) and may require the installation of a large number of nails in the conditions of the construction site. To reduce the complexity of the installation of the coating, we design a bolted connection of the crossbar with the rafter leg. We accept bolts with a diameter of d = 12 mm = 1.2 cm.

In the rafter leg, dowels (bolts) crush the wood at an angle to the fibers α = 18.7 0 . App. Ш we find the coefficient corresponding to the angle α =18.7 0 coefficient k α =0.95.

In the calculation of the dowel joint, the thickness of the middle element is equal to the width of the rafter leg c \u003d 5 cm, the thickness of the extreme element is equal to the width of the crossbar board a = 5 cm.

We determine the bearing capacity of one seam of one dowel from the conditions:

– crumpling in the middle element
= 0.5*5* 1.2*0.95* 1 *1/0.95 = 3.00 kN