Contents
One of the popular foundations for small houses and outbuildings is columnar. It is attractive for its low cost, ease of execution and the fact that even a person who is not particularly experienced in construction can build it with his own hands. A good columnar foundation is also the fact that it can be designed for most buildings and conditions.
Device and types
The columnar foundation consists of a number of supporting pillars that transfer the load from the building to the ground. Poles can have a rectangular or round section, can be made of:
- monolithic reinforced concrete;
- bricks (solid ceramic, well-fired);
- concrete (from heavy concrete grade not lower than B15) and reinforced concrete blocks;
- buta and butobeton.
The most reliable are made of monolithic reinforced concrete. They can be used on soils prone to heaving with high groundwater levels. All the rest consist of elements fastened with concrete mortar and are inferior in strength to a monolith. They are recommended for use on normal soils.
Pillars must be placed in the corners of the building, at the junction of walls and piers. If at the same time the distance between the supports exceeds 3 m, put additional ones. The installation step is on average 1,5-2,5 m. The heavier the building will be, the smaller the step, but more often than after 1 m it makes no sense to put: the foundation will be too expensive.
Types of grillage
In order for the load from the building to be transferred to the pillars evenly, they are connected with a transverse beam – a grillage or a shallow tape. This eliminates one of the main disadvantages of columnar bases – the possible uneven shrinkage of the supports.
In the construction of wooden houses or carcasses, a large-section wooden beam is often used as a grillage, sometimes metal is used – a large-section profiled pipe or T-beams / I-beams. To connect the beams and strapping during the construction of pillars, they lay studs, special fasteners, or release reinforcement. The form of mortgages is selected based on the planned type of grillage.
For heavy houses, a concrete beam is made. It can be composite – from ready-made reinforced concrete beams, or it can be monolithic. Although the components are easier to implement (buy, install, tie), but the monolithic one comes out cheaper, and even more reliable. Therefore, a monolithic grillage is used more often. Under houses made of bricks and other heavy materials, it is more suitable.
In the manufacture of a columnar foundation with a monolithic grillage, the reinforcement of the pillars is done with an outlet of at least 70 cm. These outlets are then connected to the reinforcement of the grillage frame.
Outbuildings and their foundation
If there are lighter extensions to the house – a porch, a veranda, a track – the foundations are made separate and incoherent. This means that they make their own contour for the house, and their own for the porch. And they should not have points of contact with each other. Since grillages (beams passing along the top of the supports) are located side by side, a damper layer is laid between them. For example, two-layer roofing material, mineral wool cardboard and other similar materials.
This is necessary since the weight of these elements is very different, respectively, the magnitude of the load on the foundation differs. If you make it coherent, a large difference will cause a distortion, and possibly the destruction of the building.
How to close the gaps between the pillars
When installing a columnar foundation, the building turns out to be raised above the ground and a draft walks under the subfloor. This is good for maintaining the normal moisture content of the wood, but bad for heating: the floor is too cold. One more thing: all living creatures like to settle under the house. And homemade and not very … For these reasons, they try to close the space between the pillars. You can do this in two ways:
- pick-up device – stone or brickwork between the pillars;
- by attaching pieces of sheet material or finishing materials.
It is easier, of course, to attach sheet material. It is then cut off in such a way that it does not rest on the ground. The remaining gap is useful for ventilation and to compensate for heaving.
A pick-up is done if they plan to make an insulated blind area around the house. At the same time, it is installed so that it does not support the grillage or the lower trim – they are made on the same level with the outer edge of the pillars (they should be at least 10 cm wider than the wall).
It is advisable to prepare a base for the pick-up: dig a trench with a depth of at least 20-30 cm, pour and compact a layer of sand, crushed stone on it, which is also rammed. Then a decorative wall is placed on this compacted base.
Support-columnar foundation
On normal soils, small light buildings with an area of u30buXNUMXbnot more than XNUMX square meters – such as a barn, gazebos and other similar buildings – can be made on a lightweight foundation. It is called columnar. Usually this is one or more concrete blocks laid on a sand and gravel cushion.
The planning of the pillars is the same: at the corners, at the junction of the walls and intermediate, if necessary. With this type, it is important to compact the pillow well, thereby increasing the bearing capacity of the soil. And one more thing: do not use this type of foundation on loose or clayey water-saturated soils. Even under a shower or shed. It will be completely unreliable.
What is the difference between pile and column foundation
If you look at a columnar foundation with round pillars and a pile foundation, at first glance there is no difference. But she is. These are the soils on which they are placed and the depth to which they are buried. Pile foundations are placed on soils with weak bearing capacity and their task is to pass through unstable soils and transfer the load from the house to denser layers. Columnar are placed on soils with normal bearing capacity and their depth is determined by the type of soil, climatic zone and groundwater level. Structurally, they are similar, but the tasks are different, as well as a different scope.
What is soil heaving and how does it affect the choice of foundation
The most difficult for construction are clay soils. They do not drain water well, and it accumulates in small cavities. When frozen, it significantly increases the volume of the soil. Increasing in volume, the soil presses on everything that is in it, including the foundation. This phenomenon is called heaving (the soil swells), and the soils, in which they are clearly expressed, are heaving.
Heaving forces are applied both from below, trying to push the column, and from the sides, trying to move the support to the side. It is for this reason that in clayey – heaving – soils it is better to make the pillars monolithic with reinforcement.
Also, so that the pillars do not push up, the lower part is made extended. This area is called the heel and in size it is usually twice the size of the pillar itself, and its height is equal to 1/3 of the section of the pillar. For example, under a 40 cm post, make a heel 80 * 80 cm, 13 cm high or so.
The seams between blocks / bricks / stones in prefabricated pillars are far from always able to withstand lateral loads, therefore, to reduce the effect of heaving forces on prefabricated columnar foundations, they are made with beveled walls. This increases their reliability in heaving soils. On normal soils, the pillars are stacked with even walls.
Application area
As already mentioned, a columnar foundation is also called a columnar foundation or on pillars) behaves normally on soils with normal bearing capacity. On loose sands, peat bogs and other unstable grounds, they are not used. Its main features are that it is impossible to organize a basement and a blown gap between the ground level and the subfloor. They also need to be borne in mind when choosing.
They are suitable for the following cases:
- under houses made of light materials – wooden, frame, panel;
- under heavy walls if you need to dig a foundation deeper than 1,8-2 meters and tape becomes very expensive;
- on heavily heaving soils – less forces act on the poles than on the tape.
One of the columnar foundations – bottom view
At the same time, there are a number of situations (except for soft soils) when it is not recommended to use foundations on pillars:
- with a large height difference on the building site – more than 2 m;
- with complex geology with possible displacement of layers.
In other cases, they can be used.
Depth
The main criterion for choosing the parameters of the foundation is to ensure its integrity. Therefore, when planning, they always lay a margin of safety of the order of 20-40%. It compensates (to some extent) for unforeseen changes in conditions (groundwater levels, unforeseen low winter temperatures) or loads. This is especially pronounced when planning foundations: they take the depth of the foundation with a margin and the bearing capacity is usually underestimated. It is understandable: adding something to an already finished foundation is either a very expensive or unrealistic undertaking.
The greatest margin of safety is laid on heaving soils: it is impossible to calculate their manifestation, therefore they try to make it as reliable as possible. To do this, for private houses, foundations are buried below the depth of soil freezing. You will find out the average data for your region, and add about 15-25 cm to this figure. This is how you get the depth of the foundation. For example, in the region, the soil freezes by 1,5 m, which means the depth of the foundation is 1,65-1,75 m.
As you understand, if you make prefabricated poles – from brick, rubble, blocks – you will have to dig a decent-sized hole for each. The bottom must be 20 cm below the laying depth in order to backfill. Also, the pit should be decently wider – it will be necessary to work in it by folding the supports. This is another reason why monolithic pillars are made: holes are drilled under them, gravel or sand is poured onto the bottom and formwork is inserted (most often pipes of the desired diameter). Inside which a reinforcing element is installed – a frame made of reinforcement or a metal pipe.
This rule works for heavy buildings. But under them, columnar foundations are rarely made. They are placed mainly under light buildings: wooden or frame houses or outbuildings. If a basement is not planned in such a building, it is more expedient to make the foundation shallow (laying depth from 0,5 to 0,7 freezing levels) or not buried (from 0,3 to 0,5 freezing depths).
With this choice, the heaving forces will act on the pillars, but since their area is smaller than that of the tape, the effect will not be so strong. These impacts are successfully compensated by the construction of a frame or wooden house. Moreover, additional measures are being taken to reduce the impact of heaving forces:
- at the bottom of the pit under the post, a gravel cushion is made, which takes on part of the load;
- make the side surfaces smooth and additionally lubricate them with grease, bituminous mastic, epoxy, etc.
- make an insulated blind area around the house.
As a result, in most cases, the owners do not even notice that the foundation is “swollen”. Some supports rose slightly, the grillage and piping compensated for them. After thawing the soil, everything returned to its place.
In soils that drain water well, the columnar foundation is made shallow or not buried. Here it is only important to choose the right area of the columns in order to evenly transfer the load.
Pole sizes
There are minimum dimensions for column foundation supports. They depend on the materials:
- Monolithic reinforced concrete 30 cm.
- Burnt brick pillars – 38 cm;
- Booth, concrete blocks and rubble concrete – 40 cm;
- Stonework – 60 cm.
But at the same time, the size of the pillar should be 10 cm larger than the width of the wall. For normal operation of the foundation, it is necessary that the wall falls in the middle of the support and protrudes at least 5 cm on the sides.
Construction Stages
As usual, everything begins with cleaning the site and marking. Sod is removed from the entire site, the fertile layer is removed. It is not considered bearing and is taken out. At the same time, alignment is done – they cut off the bumps, fall asleep and tamp the pits and depressions. Horizontal level is controlled using a two-three-meter board with a building level installed on it.
Layout
Along the perimeter of the building, a cast-off is installed – pillars or pillars with nailed planks – benches. They are placed at a distance of about a meter from the perimeter of the future home. With the help of cords stretched between them, they indicate the perimeter and width of future pillars.
When setting, make sure that the angles are strictly at 90 °, measure the diagonals of the rectangles. They must be equal. At the intersection of the stretched cords, a perpendicular is lowered down (using a plumb line), marking the dimensions of the pillars on the ground.
This way you can make them all the same, as well as monitor the height when setting up the formwork for monolithic pillars or when laying prefabricated ones.
Digging holes for poles
Under prefabricated foundations, pits are dug manually or with the help of an excavator. If the depth is required up to 1 m and the soil is not loose, you can dig them with straight walls. If the depth is greater or there are signs of shedding, the slopes are made inclined.
When they reached the design depth of the foundation, they dig another 20-30 cm lower. The bottom is leveled, its dimensions should be 10-20 cm larger than the planned dimensions of the heel or pillar.
Easier with round poles. Under them, wells are drilled with a manual or automated drill. If you plan to expand at the bottom – the heel, you can either make the hole wider – according to the size of the heel, or use a drill with a folding blade. It is called a TISE drill. The additional blade opens after the set depth has been reached.
Sand and gravel pad device
A layer of crushed stone of the order of 10-15 cm is poured at the bottom and it is well rammed. Coarse-grained sand is poured onto the compacted gravel. It is spilled and rammed. The total layer of the sand and gravel pad should be such that it reaches the specified depth of the foundation.
In the case of round posts, the pillow is also made, tamping is done if possible. They take a long pole, which compacts the bedding.
Heel device
If the pillars are prefabricated – from blocks, bricks, buta, it is easier to make a heel from a finished concrete block. They put on the sand, aligned to the level, a block pillow. It has a trapezoidal section, which is convenient.
For monolithic ones, it makes sense to make a half-shell from a monolith. They put the formwork according to the given dimensions (twice the size of the column in the section and at least 1/3 of the section in height). Lay reinforcement in two tiers. Use a rod 12-14 mm in diameter. Also, reinforcement is installed in the heel, which will then go to the pole. The result is a single reinforced concrete structure.
Pillar construction
If we are talking about a monolithic pillar, then they put the formwork, they install reinforcement inside. Formwork for square pillars is knocked down from boards, for round ones you can use plastic pipes of a suitable diameter. There is an economical formwork option – rolled roofing felt of the required length. It is twisted into a pipe of the required diameter, screwed onto the standard. After making two or three layers, they are fixed with adhesive tape. It turns out a reliable formwork, which even in the part protruding above the ground normally holds the solution.
For reinforcement, three or four rebars of class A III with a diameter of 12-14 mm are usually used (4 for square ones, 3 for round ones). They are interconnected into a single structure by transverse dressings installed every 20-25 cm. They can be made from AI class smooth rebar, with a diameter of 6-8 mm. It is better to knit the frame, rather than weld it – higher strength.
Please note that there must be a distance of at least 50 mm from the edge of the formwork to the reinforcement bar. It is necessary that the metal does not rust. The photo below is an example of incorrect reinforcement: the reinforcement is close to the formwork. It will quickly rust and the pole may break.
If pillars are made of brick, then it must be taken at least grade 100, full-bodied, well-burnt. If the site has a high level of groundwater, brick pillars will not work: they will quickly collapse. The mortar for laying brick pillars is made on Portland cement not lower than M 300, but preferably 400 or 500. There are several mortar options:
- 1 part cement, 3 sand;
- 1 cement, 2 lime paste, 10 sand;
- 1 part cement, 1 part clay adze, 10 parts sand.
Laying is carried out with dressing, carefully checking the verticality. Even small deviations can then lead to the destruction of the foundation, and maybe the building.
When laying a rubble columnar foundation, the stones are chosen flat with even edges. When laying, the vertical load should be transferred to the entire plane of the stone, and not to its individual sections. Stones are also laid with dressing, laying the largest fragments in the corners, filling the gaps with smaller ones.
The thickness of the stones should not be more than 30 cm. They are laid on the mortar, tightly fitting to each other. The gaps are filled with rubble, ramming it well. To increase the strength, such pillars can be reinforced both vertically and horizontally. Horizontal reinforcement is done every 25-40 cm, bars of at least 6 mm in diameter are inserted vertically, a reinforcing mesh made of wire of the same diameter can be used.
Important: the pillars must all be kicked out to the same level. To do this, during construction or pouring, constantly beat off the vertical plane: cutting off the tops is long and difficult.
Grill device
As mentioned earlier, when pouring monolithic pillars, embedded parts are installed at the tops:
- studs for tying with a bar of wooden or metal strapping;
- reinforcing outlets of at least 70 cm in length for connection with the reinforcing cage of a monolithic grillage.
The strapping device is considered when describing the construction of houses from a bar. You can read about the device of a monolithic grillage in the article on pile-grillage foundations. Everything coincides, up to the calculation of the bearing capacity.
Water proofing
To prevent the suction of moisture from the soil between the foundation / grillage and the strapping or the first row of masonry, a waterproofing layer is required. You can use a coating (the most common is bituminous mastic) or rolled or a combination of them.
There is no point in isolating the columns. For concrete pillars, the presence of moisture is not a minus, as well as for rubble. And brick is better to put on dry soils. Here is their surface, it may be worth impregnating it with some kind of deep penetration impregnation, which significantly reduces hygroscopicity. The only problem is that they are expensive.