Grillage frame made of reinforcement. Reinforcement of the pile foundation grillage. What is a grillage
In order for the building to serve for a long time, it is necessary to reinforce the grillage of the pile foundation in accordance with regulatory requirements. House construction transfers the force of gravity to a concrete strip, which serves to evenly distribute the load on the piles, and through them to stable layers of soil. The need to strengthen a concrete monolithic grillage with reinforcement arises from the fact that concrete responds well to compression forces, but weakly resists tensile and bending loads. In the absence of a reinforcing frame, the structure may become deformed.
Why do you need a grillage?
The house carries an uneven load, with some parts weighing much more than others. It depends on the arrangement of furniture and other items.
The grillage is a structure that connects the supports into a single system. Serves to evenly distribute the load of the building and transfer it through piles (pillars) to the ground. Protects the building from uneven shrinkage.
It is made in the form of a monolithic concrete strip, which must be reinforced with a reinforcing frame, and can be made of wood, reinforced concrete, and steel products that are laid on poles and interconnected into a single whole.
The grillage can be located at a distance above ground level, lie along the upper edge of the soil, or be buried in the soil. For the base of a hanging structure, horizontal beams or a concrete strip can be used. For the recessed option, installation of a monolithic concrete structure is most often used.
Reinforcement technology
Reinforcement of the concrete strip is performed with two rows of metal rods laid along the structure. To obtain sufficient strength, the upper and lower rows of reinforcement are fastened together with vertical and horizontal jumpers.
The rods laid along the grillage must have increased strength. They are made from hot-rolled corrugated profiles of class A3, with a diameter of 13-16 mm. Sometimes fiberglass reinforcement is used; it is good because it is not subject to corrosion.
The following are used as jumpers between the longitudinal rows:
- Rectangular fittings, curved in the form of clamps, made of smooth class A rods with a cross-section of 8-10 mm. Such jumpers are more reliable and have a long service life due to fewer welded joints. In terms of labor intensity, this type of reinforcement is more complex and time-consuming.
- Separate steel rods are welded to the top and bottom rows. The rods must be made of the same material as the longitudinal strapping. Welded seams do not have sufficient strength and are susceptible to corrosion. Doing this kind of work is easier and faster than in the first case.
In longitudinal rows, rods are laid at a distance of 100 mm from each other; there should be at least 3-4 rows of rods in each belt. Transverse lintels of longitudinal reinforcement are installed in increments of 200-300 mm. Vertical rods are fixed at a distance of at least 400 mm from each other.
At the bottom of the grillage, space is left to pour the concrete solution. To do this, lift the steel reinforcement bars above the formwork, placing mushroom-shaped plastic stands under them.
There must be a layer of concrete at least 50 mm thick between the outer contours of the metal frame. If its thickness is less, the rods will be subject to corrosion, and the structure itself will be unable to evenly redistribute the load-bearing load.
Calculation of the base with a grillage
In order to correctly perform all calculations, it is necessary to take into account the characteristics of the soil, the proximity of groundwater and the load from furnished housing construction, taking into account the maximum force of gravity. It is better to show all calculations and diagrams to specialists so that they can check their correctness.
Based on the data obtained, the required number of piles and their burial depth are calculated. The support should be buried 20 cm below the freezing point of the soil. Piles or pillars are located at each corner, at the intersection of load-bearing walls with lintels, under the heaviest structures of the house (under columns, fireplace). The remaining supports are installed at a certain distance from each other.
After installing the piles, a grillage is installed; if it is arranged in the form of a monolithic concrete strip, it must be reinforced with reinforcement.
Calculation of the amount of reinforcement
As an example, a monolithic concrete grillage 8 m long, 6 m wide, 400 x 400 mm thick is presented. For reinforcement you will need two longitudinal belts with 3 rods each. You will need metal rods with a cross section of 14 mm, class A3. The distance between them should be 100 mm, taking into account the fact that a layer of concrete occupies 50 mm on each side.
To install jumpers you will need rods with a cross section of 11 mm, class A1. Install them at a distance of 200 mm from each other.
Calculation formula:
- Calculate the length of the rods in the upper longitudinal row. Determine the length of the entire grillage. To do this, add up the length of all four of its sides: (8*2) + (6*2) = 16 +12 = 28 m. Since three rods are used in a row, the resulting number is multiplied by three: 28 m * by 3 pcs. = 84 m. Since two rows need to be laid, the resulting value is multiplied by two: 84 * 2 = 168 m of reinforcement will be needed to install two longitudinal rows.
- Calculate the jumpers for both contours of the grillage. They are placed at a distance of 200 mm from each other. The length of the jumpers will be 300 mm. The quantity is calculated using the formula: (30/0.2) *2 = 300 pcs. Calculate the length of the metal rods: 300 * 0.3 = 90 m.
In a grillage in which the thickness is the same on all sides, the same number of vertical jumpers will be needed as transverse ones.
You will need 168 m of metal rods of class A3 and 180 m of rods of class A2.
Stronger connections are obtained if the reinforcement is fastened together with wire rather than welding. Each connection requires about 40 cm of wire. Its quantity is determined by the formula: (30/0.2) * 4 = 600 pcs. 0.4 m each = 240 m.
Installation of a monolithic grillage
After installing the piles, the installation of the grillage begins. Its device includes:
- installation of formwork;
- laying reinforcement according to calculated indicators;
- filling the form with concrete mortar;
- dismantling of formwork;
- waterproofing works.
The design of the formwork depends on how the grillage is located above ground level.
Installation of formwork
The strength and appearance of the grillage will depend on the correct installation of the formwork. The removable form is most often assembled from boards, sometimes plywood is used.
It is imperative to control the vertical level of installation of the side walls. Angles should be set at 90 degrees unless other parameters are specified in the project. The walls are reinforced with supports so that the concrete solution does not destroy the formwork.
If the grillage is located above ground level, you need to calculate the load from the reinforcement and concrete mortar on the lower wall of the form. If the bottom falls out, the work will need to start over.
After installing the formwork, pour a layer of sand 150 mm thick into it. Wet it and compact it well. Waterproofing material is laid.
Reinforcement
Reinforcement of hanging grillages is performed with metal rods. Fiberglass reinforcement, as experience shows, is good in cases where it rests on the soil. The type of reinforcement and type of beams for the grillage are determined at the design stage of the house.
After cutting the supports to the required size, the reinforcement will protrude from them. It will be used as a connecting element between the grillage and the support.
Before performing reinforcement, draw a drawing of the location of metal rods. All work is performed based on this scheme. If the reinforcement is installed incorrectly, the structure may not withstand the load and become deformed.
Metal rods, connected together by 3-4 pieces of wire, are lowered into the formwork. The reinforcement should not touch the edges of the wooden form for concrete, so that later it does not turn out that its edges protrude from the concrete base.
To ensure ventilation of the space under the floor of the house, vents are left in the structure by inserting pipes with a diameter of 100 mm into the formwork.
After installing the metal frame, remove all construction debris from the formwork using an industrial vacuum cleaner with high power.
Before pouring the foundation, you need to clean the future grillage from water and dirt. When weather conditions do not allow pumping out water near the foundation, a small hole is dug below its level with a bevel from the base of the house into which the water will flow.
Pouring concrete
The geometry and reliability of the fastening of the formwork and reinforced frame are checked so that the structure does not fall apart during filling with concrete.
Prepare cement mortar. It should be uniform, without lumps. The solution is mixed at the construction site using a mixer or ordered from the factory in a concrete mixer.
After the concrete has hardened, the formwork is dismantled and the sand is removed from under the grillage. You can disassemble the form no earlier than the concrete base is completely dry.
Waterproofing
A hanging grillage can be isolated from moisture by coating the structure with bitumen mastics.
When installing a buried concrete strip and before pouring concrete, roofing felt is placed on the bottom of the formwork, and after dismantling the formwork, the entire grillage is covered with roll insulation.
Rules for grillage reinforcement
By adhering to the following rules, you can avoid many mistakes when building a grillage:
- the reinforcement frame and formwork are installed strictly according to the level;
- the top part of the piles is cut off so that all the heads are in a horizontal plane;
- when installing a metal frame, the jumpers are installed at a distance of 200-400 mm from each other;
- corner elements are connected by bent L- and U-shaped elements;
- the cross-section of the support must be at least 300 mm, the number of rods in the longitudinal belt is 3 or more, the allowance for reinforcement under the grillage must be 50 cm or more;
- welded joints are less durable than wire joints.
You cannot skimp on the quality and quantity of metal rods.
You can learn more about how to reinforce a pile-grillage foundation from specialized books or videos:
Reinforcement of a monolithic concrete grillage is a mandatory technological process. If all standards and reinforcement technology are observed, the building will last more than half a century.
A pile-type foundation is used as a reliable foundation for the construction of buildings. A foundation on supporting elements is necessary when constructing objects on problematic soils. A pile foundation is the optimal solution in many situations, including if the structure is being built on permafrost or weak soil with nearby aquifers, as well as if there are significant elevation differences at the construction site. Reinforcing the grillage of a pile foundation allows you to ensure the strength of the foundation and form a reliable base for the building being constructed.
The grillage is a critical, horizontally located part of the load-bearing frame, connecting the supporting columns into a single contour. It ensures the verticality of the pillars and prevents them from moving. Ensuring the strength characteristics of the supporting structure is achieved by strengthening it with steel reinforcement. To strengthen the support contour, a drawing is required, and calculations of the expected forces acting on the foundation during operation of the structure are required.
Pile foundation is a universal foundation for the construction of brick, wood, aerated concrete and foam concrete low-rise buildings
Let's look at how a grillage foundation is strengthened. Let us dwell on the features of the main stages of work, the professional implementation of which will ensure the reliability of the foundation being built.
What is a grillage?
For those who do not know construction terminology, we inform you that the grillage is a critical part of the pile foundation, connecting the pile heads into a single power circuit.
There are different types of grillages used in pile foundations:
- tape type, representing a monolithic concrete tape. It is located along the perimeter of the supports, successively located under the load-bearing main walls;
- slab structure, in the form of a monolithic slab, the dimensions of which correspond to the contour of the base of the building and cover all supports.
The grillage is a ribbon structure that connects free-standing piles to each other
Depending on the characteristics of the grillage foundation, it can be manufactured in the following designs:
- Whole version. Manufacturing is carried out by pouring concrete mortar into pre-prepared formwork. The formation of a monolithic base occurs after the concrete mixture has hardened.
- Composite form. The base is a prefabricated surface made of industrially produced reinforced concrete products, connected during installation with supporting columns, as well as with each other.
Regardless of the design features, the grillage forms a supporting surface intended for the construction of the walls of the building. The tying of columns located in the ground ensures high rigidity of the spatial system and resistance to the effects of existing forces.
Reinforcement of a pile-grillage base allows you to strengthen the monolithic base with steel rods, which contribute to the integrity of the structure and increase durability.
Design features
To form a strip foundation located on piles, grillages are made at different heights relative to the zero level. Depending on the location of the contour relative to the ground level, the following types are distinguished:
- high, the lower mark of which exceeds the ground level by 10 cm or more. Constructed for light buildings located on any type of soil. On problem soils, its device is especially important. The structure needs serious strengthening with reinforcement, which is due to the presence of cavities under the concrete monolith located above the ground surface;
It is in the case of installing a monolithic pile grillage, which is used in the construction of houses made of heavy materials, that it is necessary to reinforce the strapping
- ground version, made on a gravel-sand backfill without penetration into the soil. Its peculiarity is the absence of free space between the concrete monolith and the ground. Installation is carried out on non-problematic soils. When the soil is exposed to frost heaving, cracks may form and the hardened concrete mass may separate from the supporting columns;
- shallow type, formed by burying the lower part into the soil on a previously prepared sand and gravel backfill. The design of such a foundation resembles a strip foundation, the base of which rests on piles. The formation of a buried foundation is associated with significant costs and is used for the construction of massive buildings located on soils characterized by low bearing capacity.
Pile-type foundations are formed mainly for light buildings. That is why the construction of a grillage foundation is quite common, the base of which is a hanging ribbon of concrete reinforced with steel reinforcement. With a base height of up to 40 cm, its width depends on the type and size of the material used to build the walls, and is 30-40 cm.
On the feasibility of reinforcement
The need to strengthen the foundation of a building with steel reinforcement is related to the characteristics of concrete. The material has increased resistance to compressive forces, but is susceptible to bending moments and tension, which cause disruption of the integrity and deformation of the base.
Note that foundations that use two types of piles are subject to reinforcement - driven and bored.
Reinforcement of a pile-grillage structure allows you to prevent the likelihood of destruction, increase the stability and service life of the building being constructed. The reinforcement frame located inside the concrete mass absorbs tensile loads and bending forces, ensuring the stability of the constructed foundation.
Regardless of the design of the piles used, located inside the soil, the supporting columns are also reinforced with reinforcement. The steel rods located in the piles are connected into a common structure with the reinforcing frame of the supporting surface.
Reinforcement of the pile foundation grillage allows:
- Prevent destruction of the monolithic massif as a result of soil reaction.
- Significantly increase the strength of the base, which absorbs the load from the mass of the structure.
- Prevent shrinkage of the structure caused by low strength characteristics of the base.
Strengthening the grillage foundation allows you to avoid negative phenomena.
Specifics of reinforcement
Reinforcement of the pile foundation grillage is carried out by a pre-fabricated volumetric frame, consisting of two tiers of rods, combined into a single structure using steel lintels.
Reinforcement of the strip grillage is carried out using a spatial reinforcement frame consisting of two longitudinal reinforcement belts
For the longitudinal chords of the frame, corrugated rods are used, produced by the hot-rolled method, which corresponds to class A3 reinforcement. Depending on the load perceived by the base, the diameter of the rods is 12-16 mm.
The combination of connecting elements located in the vertical and horizontal planes into a common power circuit can be carried out:
- separate steel corrugated rods of rectilinear shape, the diameter of which corresponds to the range of longitudinal reinforcement;
- steel clamps of rectangular configuration, made of smooth rods of class A2 with a cross-section of up to 10 mm. Despite the increased complexity of manufacturing and installation, rectangular jumpers ensure the reliability and durability of the reinforcement structure.
When strengthening a strip foundation placed on piles, adhere to the following recommendations:
- Use at least 4 rods located in the upper and lower tier of the strip contour frame, ensuring an interval between elements of 10-15 cm.
- Maintain a distance of 15-25 cm when installing jumpers in the longitudinal belt.
- Ensure the pitch of the vertically located frame jumpers is 30-40 cm.
Amplification of the grillage begins after completing all the previous stages of arranging the pile foundation - installation of piles, their trimming and arrangement of formwork
The need for a protective layer between the metal frame structure and the concrete surface of the grillage is determined by the following factors:
- the need for proper distribution of the current forces by the metal structure of the frame;
- the susceptibility of reinforcing bars to corrosion processes arising from moisture penetrating into the concrete.
Maintaining a fixed distance from the reinforcement to the formwork while providing a protective layer is achieved by using stands made of plastic.
Calculation method
To determine the need for reinforcement that allows for the reinforcement of the pile foundation grillage, it is necessary to first develop a drawing. The documentation contains the following information:
- Design dimensions.
- Number of reinforcement belts.
- Reinforcement profile.
- Diameter of the rods used.
- Distance between jumpers.
Scheme of correct reinforcement of grillage corners and junctions
Knowing the overall dimensions of the grillage foundation, it is easy to calculate the length of the steel reinforcement bars in the upper and lower chords, as well as the dimensions of the lintels.
Having summed up the obtained values, we obtain the total length of each standard size of the reinforcement used. Knowing the footage and weight of one linear meter of a certain rod, it is not difficult to determine the need for reinforcing bars, expressed in kilograms.
If you do not plan to connect the elements using electric welding, you will need knitting wire. Having a drawing that shows information about the number of connection points, you can calculate the required amount of binding wire. Considering that about 30 centimeters are needed to securely fix two perpendicularly located rods, the total need for wire is determined by multiplying the number of connections by the length of the material.
Carrying out the calculations is not difficult. The main thing is to first develop a reinforcement drawing.
Grillage reinforcement technology
If the reinforced piles have been installed and the formwork has been installed, then you can begin to form the reinforcement frame. Please note that the frame is fixed to the reinforcement protruding from the concrete piles to the height of the formwork. Fastening steel rods can be done by welding, as well as using binding wire.
The algorithm for performing the work is as follows:
- fix horizontally located longitudinal rods at a distance of 5 cm from the bottom of the formwork;
- place and secure the perpendicularly located rods of the lower chord;
- install rectangular clamps or vertically located rods for fastening the bars of the upper tier;
- secure the longitudinal rods of the upper chord;
- Perform angular reinforcement of the grillage by installing curved rods in the corners of the structure.
Reliably strengthen the corners with reinforcement, since in these places the frame absorbs significant forces.
Conclusion
Reinforcing the pile foundation grillage with steel reinforcement allows you to form a reliable base that ensures the stability of the structure being built. The work is not difficult to carry out independently, having previously developed a drawing according to which the pile foundation grillage is reinforced.
Reinforcement of pile foundation grillage: drawing, technology, calculation
How to reinforce a pile-grillage foundation? Correct calculation method. Features of grillage strengthening technology. Drawings and diagrams of grillage reinforcement.
This article was inspired by a question from Vladimir B. sent by email. In the initial data there was such a foundation; it was necessary to properly reinforce the grillage.
I will say right away that in the presence of a bending moment, the grillage needs to be designed on two piles - then the moment is divided into a couple of forces, and the piles experience only compressive and pull-out loads; they can simply be calculated and designed. The operation of the pile under the influence of torque is doubtful; it is better to avoid this option. But the question still interested me, and I decided to write an article to develop design thinking (with the assumption that our pile has already been calculated, designed, and can withstand all loads with a margin). I will be glad to discuss in the comments.
So, in the initial data we have a monolithic grillage 800x600 mm, 500 mm high, resting on a single reinforced concrete pile with a section of 300x300 mm. A metal column rests on top of the grillage. Vertical load from the column N = 18 t, moment along axis 2 Mx = 4.5 tm, shear force along axis 2 Qy = 1 t. The base of the column and the distance between the bolts are shown in the figure below.
Let's look at what design scheme should be adopted for the grillage. We have a rigid support on the pile. The load from the column is transmitted exactly along the axis of the pile, without piles. But we have a bending moment that is transmitted through the foundation bolts, the position of which extends beyond the boundaries of the pile. In fact, if we depict the design diagram for the grillage, we will see the following.
The vertical force N = 18 t is distributed along the length of the column base plate of 0.62 m and turns into a uniformly distributed load q = 18/0.62 = 29 t/m.
The bending moment Мх = 4.5 t∙m is decomposed into a pair of forces P, located at a distance of 0.5 m, and one acting upward, the other acting downward. Each force P = 4.5/0.5 = 9 tons.
Considering that the estimated length of the console is equal to the overhang of the console, we obtain for each console the following design diagram with pinching in the middle:
We will have three loads:
1) uniformly distributed q r.v. = 0.5∙0.6∙2.5 = 0.75 t/m – load from the own weight of a grillage with a cross-section of 0.5x0.6 m (2.5 t/m 3 – own weight of concrete);
2) uniformly distributed q – load from the column (from vertical force N);
3) vertical force P (down and up) – load from the column (from the bending moment Mx).
The length of each console is equal to the length of the grillage overhang. The binding of the vertical force P and the distributed load q is according to the actual bindings of the plate and bolts.
Now we need to find the maximum bending moment M and the maximum shear force Q in the cantilever.
To calculate the reinforcement, we will need the standard and design values of M and Q, highlighting permanent and temporary loads. The load from your own weight is constant. Loads N and Mx include permanent and temporary parts; to clarify the task, you should contact a metal structure designer, but for the sake of example, we will simply assume that the short-term part of the load is 30%.
For the convenience of calculating the console, we summarize the load on it in a table:
|
Normative value |
Load safety factor |
Estimated value |
|
|
Initial data |
|||
|
0.75∙1.1 = 0.83 t/m |
29∙0.7 = 20.3 t/m |
20.3∙1.1 = 22.3 t/m |
|
|
Load q (short-term part, 30%) |
29∙0.3 = 8.7 t/m |
±0.7∙9 = ±6.3 t |
±6.3∙1.1 = ±6.9 t |
|
Load ±P (short-term part, 30%) |
±0.3∙9 = ±2.7 t |
±2.7∙1.2 = ±3.2 t |
|
As a result of the calculation, we obtain the following diagrams M and Q:
From the moment diagram we see that with a load like ours in the example (when the moment tries to rotate the column counterclockwise, and it presses through the bolts on the left side of the grillage, while the right one tries to lift up), the maximum moment is on the left side grillage, and in the diagram it rises up above the zero line - which means it requires installing the upper working reinforcement along the grillage so that it absorbs the tension from bending. The moment on the diagram above means that the upper part of the section is stretched. In the same way, on the right side of the grillage we see that the moment diagram first goes down (lower working reinforcement is required), and then at the place where the bolt is installed it comes up - tension appears there, requiring upper reinforcement. Thus, we need to install both upper and lower working reinforcement in the grillage. We will calculate the upper one based on the value of M1 (it is larger than M3), and the lower one based on the value of M2.
From the shear force diagram we can see the need for shear reinforcement. We have very stressed sections Q1-Q3 and Q2-Q5; they will have maximum transverse reinforcement. This is logical, because at points Q3 and Q5 we have concentrated forces from the bolts, and according to the design rules we must supply reliable transverse reinforcement from the support to the point of application of the concentrated load - this was confirmed by the calculation.
Let's take a look at the effort that went into our grillage.
The values of the diagrams are summarized in the table:
As we can see, the maximum moment is M1, the maximum lateral force is Q1.
Having the calculation results in hand, we can calculate the reinforcement and start designing the grillage. The percentage of reinforcement according to the calculation results turned out to be small - only 0.05%, but the minimum percentage of grillage reinforcement is not standardized.
According to our calculations, we obtained an upper working reinforcement with an area of 1.5 cm², a lower working reinforcement with an area of 0.6 cm² (and this is logical, the moment M1 is greater than the moment M2), transverse reinforcement with an area of 0.28 cm² with a step of 200 mm. Now we need to construct the grillage.
The recommended reinforcement spacing in the grillage is 200 mm. It is also recommended to reinforce the grillage with welded mesh (welding must be contact welding, and in no case - manual arc!), but if the mesh is knitted, then along the perimeter of the grillage two rows of intersections of rods must be connected by welding. In addition, to anchor the working rods, a perpendicular rod of half (compared to the working rods) diameter must be welded at a distance of 25 mm from their edge. All these requirements are taken from the manual “Reinforcement of elements of monolithic reinforced concrete buildings”, Tikhonov I.N.
Please note, because the grillage is not a slab, but a beam, we should not neglect the design requirements for the reinforcement of beams (they can be studied in the same manual).
So, we accept the upper and lower mesh of the grillage as welded, the working reinforcement is longitudinal (along the grillage), with a diameter of 12 mm with a pitch of 200 mm, reinforcement class A400C - in total you get 4 reinforcement rods, the reinforcement area is 4.52 cm² (this is significantly more than 1.5 cm² and 0.6 cm², but it is better not to install reinforcement of smaller diameters in the beam grillage). Perpendicular to this reinforcement we will install rods with a diameter of 6 mm (the requirement for half the diameter is met - 12/2 = 6 mm) with a pitch of 200 mm, reinforcement class A400C or A240C.
Now let's deal with transverse reinforcement; we will reinforce the grillage with welded frames, in which we will install transverse reinforcement with the pitch we need. With a step of 200 mm, the cross-sectional area of all rods must be at least 0.28 cm² - it turns out that we only need one rod with a diameter of 6 mm in cross-section. But now let's look at Figure 3.1 of the manual. If the beam width is more than 350 mm, we must install not even two, but three frames with transverse reinforcement. Next, let’s clarify the reinforcement pitch. According to Figure 3.10 of the manual and the explanations below it, at length Lsup we must install transverse reinforcement with a step Sw1, which should not exceed 500 mm or a third of the height of the beam section (500/3 = 160 mm). In our case, Lsup is equal to the distance from the support to the point of application of the concentrated load (i.e. to the foundation bolt). On the rest of the console we can install reinforcement with a step of 3h/4 = 3∙500/4 = 375 mm, but with our length of the console such a step will be too large; from the foundation bolt to the edge of the grillage we only have 150 mm left, so we accept for throughout the beam, the pitch of the transverse rods is 150 mm (which is less than 160 mm, i.e. acceptable). It is better not to use values such as 160 mm, but to stick to sizes that are multiples of 50 mm.
We will take the longitudinal reinforcement of the frames to be the same as the transverse reinforcement - 6 mm in diameter, these will be two rods - at the top and bottom of the frame.
So, we have decided on the grillage reinforcement. We have two meshes with working reinforcement and three frames with transverse reinforcement. The builders will connect them together into a three-dimensional frame using knitting reinforcement.
A pile foundation consists of vertical rods (piles) buried in the ground and serving as support for the remaining elements of the structure. According to the type of construction, piles are:
- poured piles, when concrete is poured into prepared holes in the ground;
- driven piles - finished reinforced concrete pillars are driven into the ground with a special machine;
- screw piles made of metal, shaped like a screw or self-tapping screw, are simply screwed into the ground.
Of all the above, we are interested in the first type of piles—cast piles. Arranged in the following sequence:
- using a manual or mechanized drill, a hole is drilled in the ground, the depth is equal to the length of the future pile plus 15 - 20 cm for the gravel bed;
- coarse sand or small crushed stone (shock-absorbing cushion) is poured into the bottom of the well and compacted;
- prepare a structure of three or four reinforcement rods, which are connected to each other by welding using short sections and knitting wire;
- above the well, formwork of a circular or other cross-section is installed up to the estimated height of the pile above the ground;
- the reinforcement structure is immersed in the well, and the well itself, together with the rising formwork, is filled with concrete;
- after the concrete has cured, the remnants of the reinforcement are cut off, sometimes leaving short sections to which supporting elements are attached by welding.
Strip foundation
Tape is widespread due to its strength, ability to withstand buildings of any weight and resist any processes occurring in the soil. It is a ribbon, like a continuation of the outer walls of the house, directed downwards. The thickness of the strip foundation in private construction is assumed to be 40 - 50 cm; it is recommended to lower the foundation into the ground below the maximum depth of soil freezing. This is done so that heaving processes do not occur under the base.
Reinforcement of the strip foundation is carried out after the construction of the trench and formwork.
The skeleton structure consists of two tiers, each consisting of at least two veins.
Requirements for the location of reinforcement in a concrete mass:
- the distance from the outer side wall to the reinforcement should not be less than 50 - 55 mm;
- the distance from the lower and upper planes of the array to the horizontal reinforcement should not be less than 70 - 75 mm;
- the distance between the upper and lower reinforcing layers is not made less than 300 - 350 mm, but it is better to make this distance maximum;
- distance between lintels: vertical 400 - 700 mm, horizontal 800 - 1400, the distance is chosen to ensure the immobility of the reinforcement during pouring of concrete and compacting it using vibrating machines and other devices.
A few tips to make it easier to assemble the reinforcement cage for a strip foundation:
- some of the vertical reinforcement is made of increased length, so that it is convenient to mount the entire skeleton on them, driven into the bottom of the trench;
- The upper horizontal reinforcement is made with a length equal to the width of the foundation, thanks to which, resting against the walls of the formwork, they fix the position of the entire reinforcement structure in space.
Pile with grillage
A pile foundation with a grillage is a symbiosis of strip and pile. The support is provided by piles buried below the freezing point. The number of piles is determined by their diameter and the bearing capacity of the soil. In the upper part there is a grillage - a tape with a thickness equal to or slightly greater than the diameter of the piles. The reinforcement in piles is installed in exactly the same way as in pile foundations; the grillage reinforcement is no different from the reinforcement of the strip base. The only nuance of a pile foundation with a grillage is the connection between the reinforcement frame of the piles and the frame of the grillage. This is done as follows:
- concrete is initially poured into reinforced wells to the level of the lower plane of the grillage tape;
- then a trench is built under the grillage (if everything is done correctly, the plane of the bottom of the trench will coincide with the top of the concrete poured into the wells);
- the result of the work looks like this: there is a trench, from the bottom of which the ends of the reinforcing bars rise at a certain interval (a continuation of the pile reinforcement);
- further actions completely repeat the algorithm for tying the reinforcement of a strip foundation, only the basis is not the rods specially driven into the bottom of the trench, but the ends of the pile reinforcement.
Tiled
Of all the types of building foundations under consideration, it is the tiled (floating) foundation that most needs a high-quality reinforcement frame. The foundation is a flat product made of reinforced concrete, completely following the contours of the building, with a thickness of only about 30 cm. Needless to say, only proper reinforcement of the slab foundation can guarantee the strength of the structure, its ability to withstand the weight of a building, sometimes several floors.
Slab foundations are a real godsend when constructing buildings and structures on difficult soils. No matter what processes occur, no matter what local zones of heaving or soil subsidence are formed, if the reinforcement of the slab is carried out efficiently, the composition of the concrete mixture is correctly determined and the pouring technology is not violated, the geometric parameters of our parallelepiped (even despite a possible slight tilt) will still remain unchanged . And this guarantees the integrity of the structure, as it eliminates the formation of stress zones in load-bearing building structures. All stress is absorbed and dampened by the monolithic slab.
The process of reinforcing the foundation: the reinforcement is installed after the foundation pit has been dug, sand and drainage (crushed stone) cushions are laid on its bottom, and formwork panels are installed around the perimeter. Elongated vertical reinforcement bars are driven into the bottom of the pit, on which first the lower and then the upper reinforcing tiers are mounted. By the way, there is no need to supply absolutely all nodes with vertical segments. Their number should only ensure the immobility of the tiers in space during the process of pouring and compaction.
After the foundation reinforcement is completed, the structure is filled with concrete to the upper edge of the formwork.
In the field of individual construction, when using a pile foundation, a monolithic reinforced concrete grillage is the most popular option, since even with significant dimensions it can always be made on your own.
The materials used for this can be delivered to the construction site by ordinary freight or even passenger transport without the use of special platforms or cranes.
However, the installation of a monolithic grillage is more complex than a prefabricated grillage, and the main difficulty lies in the correct reinforcement of the pile grillage.
You can often hear about the similarity between the structure of a grillage and a conventional strip foundation, but this statement is only partly true. Indeed, in its appearance and function, the grillage is very similar to the strip base, however, the operating conditions of these structures differ significantly:
- If for a strip foundation the occurrence of a bending moment in the vertical plane is rather an out of the ordinary phenomenon, then for grillages, which are beams laid on piles, this is the norm. The span located between the supports takes the weight of parts of the building and other loads, while being, as it were, suspended in the air, which causes deflection;
- Another difference is that the load on a strip foundation is less predictable. The underlying soil under various sections of the foundation may “float” or swell. This causes multidirectional deflections, in which both the upper and lower parts of the cross section can be stretched. And the stretched zone that occurs when a reinforced concrete element deflects is, as is known, exactly the zone in which the reinforcement should be located. Thus, a conventional strip foundation has to be reinforced equally in both the upper and lower parts.
In the case of a grillage, the impact from the soil is completely eliminated, so the stresses arising in it are quite predictable: in the spans between the piles, the lower part of the cross section is always stretched, and in the zones of support on the piles, the upper part is always stretched.
This determines the reinforcement scheme for the pile foundation grillage. The lower chord of the reinforcement frame in the areas between the piles is made more powerful, and at the points of support on the piles the upper chord is strengthened.
Selection of reinforcement frame materials and determination of its parameters
The diameter of the reinforcement used and the parameters of the frame are selected based on calculations that take into account permanent and temporary loads.
Calculation of the pile foundation grillage reinforcement should be carried out by an experienced civil engineer with a good knowledge of the topic of reinforced concrete structures.
Standard solutions
In practice, in individual construction the following rules are followed:
- in the stretched areas of the grillage, several longitudinal reinforcement bars of class AIII with a diameter of 20 mm or more are laid;
- reinforcement with a diameter of 8–15 mm is placed in the compressed belt. The pitch between the rods of the longitudinal reinforcement, also called working reinforcement, is 80 - 100 mm.
- To absorb transverse tensile forces, as well as to combine longitudinal reinforcement into a single frame, transverse rods are attached to it - smooth AI class reinforcement with a diameter of 6 to 8 mm. The distance between them should not be less than 250 mm, but usually it is taken equal to 3/8 of the height of the grillage section.
If the height of the grillage exceeds 150 mm, vertical bars are installed in the reinforcement frame, the pitch of which corresponds to the pitch of the transverse reinforcement.
Most often, instead of separate longitudinal and transverse rods, clamps are used - parts made of reinforcement in the form of a closed rectangle or an inverted letter “P”.
Reinforcement of abutment zones of grillage tapes
Where the grillage strips form an L-shaped or T-shaped intersection, simply fastening the intersecting rods of the working reinforcement is not enough.
Here rods bent at right angles are placed, each part of which fits into one of the adjacent tapes and extends into it by at least 40 diameters.
Clamps are installed twice as often in these areas.
Manufacturing jobs
Reinforcement of the pile foundation grillage is carried out immediately after the construction of the formwork. A mandatory element of the latter must be transverse jumpers in the upper part, to which, ultimately, the reinforcement frame will be suspended.
Work on installing reinforcement in the future grillage can be divided into several operations.
Construction of the lower chord of the reinforcement cage
At the bottom of the formwork, special plastic bosses are placed on top of the waterproofing, on which the lower chord of the reinforcement cage will then be located.
If there are no bosses available, they can be replaced with fragments of brick or wooden blocks 40 - 50 mm high:
- the height of all pads must be the same so that the reinforcement bars take a strictly horizontal position;
- the spacing between the bosses or elements used as an alternative depends on the diameter of the working reinforcement: it must be such that the rods do not bend;
- it is necessary to lay the working reinforcement of the lower chord on the bosses with equal spacing. The distance from the outer rods to the side surfaces of the formwork should be 30 - 40 mm.
When constructing the frame, the contractor must be guided by the requirements of the documents describing the reinforcement of the pile foundation grillage: a design drawing with all the necessary instructions is given in the construction project.
If the grillage strip is of considerable length, each thread of the working belt is assembled from several reinforcing bars connected with an overlap of 1 m.
Transverse reinforcement bars or clamps are attached to the lower chord, combining transverse and vertical reinforcement.
Upper belt device
The working reinforcement of the upper chord is suspended on the formwork lintels, which were mentioned at the beginning of the section. The length of the hangers should be such that after pouring the grillage, a protective layer of concrete with a thickness of 30 to 40 mm is formed above the reinforcement.
The rods of the upper chord are connected to transverse and vertical reinforcement or to clamps, if any are used.
Then both working belts should be tied to the reinforcement protruding from the piles. The reinforcement frame can be considered ready.
Methods of knitting reinforcement
The most common fastening method is to tie the reinforcement together using a special wire. Electric welding is used very rarely and only for fittings marked with the letter “C”.
It is not allowed to connect conventional reinforcement by welding, since due to exposure to high temperatures it becomes less durable.
For knitting reinforcement, only annealed round wire with a diameter of 1 mm is used. Unburned wire is less flexible, so it bends poorly and breaks easily.
The fastest way to knit reinforcement is with a special gun equipped with a battery. But its purchase is only advisable for large volumes of work; moreover, it is not very convenient when tying reinforcement in hard-to-reach places.
Reinforcement of a pile-grillage foundation for a private house is mainly carried out using another tool - a special hook. Professionals prefer homemade hooks, but for one-time work, purchased ones will do.
On sale you can find both regular and screw hooks, also called semi-automatic.
The latter allow you to knit the reinforcement somewhat faster, but due to their design, after tightening the knot, they leave too long free ends of the wire, which often protrude from the concrete and begin to rust.
The simplest and most common types of knots are the so-called “loop” and “two loops”. The first is used when connecting reinforcement with overlaps, the second - for butt joints. In practice, the loop is often used not only for overlap joints, but also for corner joints.
At the final stage of installation of the reinforcement cage, the bosses on which the working reinforcement of the lower chord were installed should be removed. After this, the entire frame will be suspended on a wire wrapped around the upper formwork lintels. Now you can start pouring concrete.
Video about reinforcing the grillage of a pile foundation
