How to calculate the axial force of foundation
Calculation of truss axial force with node method:
two unknown forces can be obtained from a node equation, generally starting from the support node and proceeding in turn. For a node, if the member is removed and replaced by force along the member direction, it can be assumed that it is tensile force (if the force is negative, it means pressure). The equilibrium equations in X and Y directions are listed respectively (the forces are projected into the equilibrium equations in X and Y directions respectively): ∑ x = 0 ∑ y = 0. The specific form may be as follows: f1cosa + f2cosb + acosc = 0, f1sina + f2sinb + asinc = 0, where a represents the known force, F1 F2 is the unknown force. The unknown forces F1 and F2 can be obtained by solving the equations. The positive value is the tensile force and the negative value is the pressure
The axial force of AC section is - 20KN, not - 10kN. Because - 10kN acts on point C, the AC is disconnected, the left part is taken as the isolator, and only the left end bears the axial force of - 20KN, so the axial force is - 20KN. Similarly, the axial force of CD segment is - 10kN, and that of de segment is + 10kN
For columns with large slenderness ratio, the initial eccentricity caused by various accidental factors cannot be ignored. With the increase of the load, the lateral deflection also increases. The compression deformation and bending deformation of the member occur at the same time. Finally, the member is destroyed under the combined action of axial pressure and additional bending momentfirstly, the concavity compressive concrete is crushed, the longitudinal reinforcement is bent and bulged out, and the concrete cover is peeled off; At the same time, when the convex surface is under tension, the concrete will proce horizontal cracks, the lateral deflection will increase sharply, and the column will be damaged
Extended data:
for short columns with longitudinal bars and stirrups, the strain of the whole section is basically uniform under axial load. When the load is small, the concrete and steel are in the elastic stage. As the load continues to increase, the lateral deformation of the concrete increases, the fiber stress at the edge of the section first reaches the tensile strength of the concrete, and micro cracks begin to appear in the column
Afterbecause the elastic molus of steel bar is greater than that of concrete, the stress of steel bar increases rapidly, the stress of column longitudinal bar first reaches the tensile strength of steel bar and is crushed, and micro cracks begin to appear in the column
when estimating the area of foundation bottom, the factor of foundation buried depth should also be considered
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Calculation of pre added axial force of steel support in deep foundation pit: firstly, add the pre added force to zero to calculate an anchor cable tension, which can ensure the stability of foundation pit against overturning, and then multiply it by 70 ~ 95% of the specification to get the pre added force
generally, the preloaded axial force of steel support in deep foundation pit is not more than 450kn, because the pile anchor structure is mostly sand, silt, or clay, and the uplift force of this stratum and anchor cable is limited. The preloaded force of 450kn means that the design tensile force reaches 450 / 0.95 ~ 450 / 0.7, which is basically the limit of ordinary anchor cable in this stratum. If it can not be met, it is necessary to consider densifying the anchor cable spacing
extended data:
basic requirements of deep foundation pit support:
1. The construction scheme of deep foundation pit support must be determined according to the design requirements, depth and on-site environmental engineering progress. After spinning, it shall be approved by the chief engineer of the unit and submitted to the chief supervision engineer for approval, and the construction can be carried out only when it meets the requirements of specifications and laws and regulations
2. The underground water level must be solved in the construction of deep foundation pit. Generally, light well point pumping is used to lower the underground water level to 1.0 meters below the bottom of foundation pit. A special person must be responsible for pumping water for 24 hours on ty, and the pumping records should be made. When the open ditch drainage is adopted, the drainage shall not be interrupted ring the construction period. When the structure does not have anti floating conditions, it is strictly forbidden to stop the drainage
3. During the excavation of deep foundation pit, the distance between multiple excavators should be more than 10m, and the excavation should be carried out layer by layer from top to bottom, and no deep excavation is allowed
The ladder or support should be g up and down the deep foundation pit, and it is forbidden to step on the support, and safety railings should be set around the pit5. When manually lifting earthwork, the lifting tools should be checked to see if they are reliable, and no one should stand under the bucket
When stacking materials and moving construction machinery on the side of deep foundation pit, a certain distance should be kept from the excavation edge. When the soil quality is good, it should be 0.8m away, and the height should not exceed 1.5m During the construction in rainy season, drainage measures must be taken to prevent rainwater and surface water from flowing into the deep foundation pit. During the excavation in rainy season, 15-30cm soil should be left above the elevation of the foundation pit, and the excavation should be carried out after sunny daysformula: FCD = 0.7· β H · FTD · um · h
FCD -- the maximum concentrated return force of concrete< br /> β H-for the thickness less than 300 mm, take 1
FTD -- axial tensile stress (C30 = 1.39 MPa)
um -- height conversion ratio = 2 · (a + b) + 4h, a = 20cm, B = 60cm (a, B are the width and length of wheel trace respectively)
H -- thickness
bearing capacity of Foundation (calculation target value): as the bearing capacity of concrete pavement is mainly analyzed, and the ternary structure (15cm loess cushion and 15cm sand gravel cushion) designed by the design institute generally meets the foundation requirements, the foundation in the calculation is considered as infinite width (rigid) Foundation (solved according to the thickness)
concrete grade: the grade in concrete is directly proportional to the stiffness, that is, the greater the grade, the greater the stiffness of concrete. Therefore, the selection of too low grade concrete will lead to the network crack of the whole pavement, while the selection of too high grade concrete will lead to the excessive stiffness of the whole pavement, which is brittle, that is, easy to crack as a whole, Therefore, the correct choice of grade is also an important factor for the long-term good condition of concrete pavement, so the concrete grade in this paper is C30 grade designed by the design institute
concrete thickness (generally 18cm-30cm): 25cm, 28cm and 30cm are substituted according to the formula. Taking the 25cm thick C30 concrete as an example, the axial compressive strength of C30 is 20.1mpa = 20.1n/mm2 = 20.1 × 1000000 n / m2, equivalent to 20.1 × 100000 kg (five zeros divided by 10, acceleration of gravity), which is 20.1 × 100 tons, 2010 tons, that is, 2010 tons / m2. Because it is 25cm thick concrete, it needs to be multiplied by 0.25. Therefore, it is estimated that the design compressive capacity of 25cm thick C30 concrete per cubic meter is about 502.5 tons / m3 Preliminary calculation, C30, 25cm thick, can only bear 63.245 tons)
Foundation bearing capacity = 8 * N-20 (n is the number of hammers)
the characteristic value FAK of foundation bearing capacity is the empirical value directly determined by load test or indirectly determined by its correlation with in-situ test. It is corresponding to the pressure value corresponding to a specified deformation in the linear deformation section of the pressure deformation curve of the foundation ring the load test, and its maximum value should not exceed the proportional limit value on the pressure deformation curve
extended data
subgrade bearing capacity is the bearing potential of foundation soil per unit area with the increase of load, commonly used in kPa, which is a comprehensive term to evaluate the stability of foundation
It should be pointed out that the bearing capacity of foundation is a practical professional term for evaluating the strength and stability of foundation, not the basic property index of soil. The shear strength theory of soil is the theoretical basis for studying and determining the bearing capacity of foundation The methods to determine the bearing capacity of foundation are as follows:(1) in situ testing method: it is a method to determine the bearing capacity through direct field test. It includes (static) load test, static cone penetration test, standard penetration test, pressuremeter test and so on
(2) theoretical equation method: it is a method to determine the bearing capacity according to the theoretical formula calculated by the shear strength index of soil
(3) code table method: according to the indoor test index, field test index or field identification index, the bearing capacity can be obtained by consulting the tables listed in the specification. Different specifications (including specifications of different departments, instries and regions) will not have the same bearing capacity, so it is necessary to pay attention to their respective service conditions
(4) local empirical method: it is a method to determine the bearing capacity by analogy based on the regional experience. It is a macro auxiliary method
The design value of foundation bearing capacity should be calculated as follows: F = FK + η b γ b－3＋ η d γ 0 (d-0.5) (5.1.3)
where f is the design value of foundation bearing capacity
FK --- standard value of foundation bearing capacity
η b、 η D -- foundation bearing capacity correction coefficient of foundation width and buried depth, according to the soil type under the base, refer to table 5.1.3
γ --- The gravity of soil is the natural mass density of soil below the base ρ The proct of G and G is the effective gravity below the groundwater level
b --- the width of the foundation bottom (m), when the foundation width is less than 3m, it shall be considered as 3M, and when the foundation width is greater than 6m, it shall be considered as 6m
γ 0 -- weighted average weight of soil above foundation bottom, and effective weight below groundwater level
d --- the embedded depth of Foundation (m), generally calculated from the elevation outside the room
extended data:
determination method
(1) in situ testing method: it is a method to determine the bearing capacity through direct field test. It includes (static) load test, static cone penetration test, standard penetration test, pressuremeter test and so on
(2) theoretical equation method: it is a method to determine the bearing capacity according to the theoretical formula calculated by the shear strength index of soil
(3) code table method: according to the indoor test index, field test index or field identification index, the bearing capacity can be obtained by consulting the tables listed in the specification. Different specifications (including specifications of different departments, instries and regions) will not have the same bearing capacity, so it is necessary to pay attention to their respective service conditions
(4) local empirical method: it is a method to determine the bearing capacity by analogy based on the regional experience. It is a macro auxiliary method
Explanation of foundation bearing capacity calculation formula: F = FK+ η b γ( b-3)+ η d γο( d-0.5) &# 8194;
FK standard value of bearing capacity of soft soil layer at the bottom of cushion (KN / m2)
0 η b、 η D -- correction coefficient of bearing capacity of foundation width and buried depth, respectively
B -- foundation width (m) &8194
D -- buried depth of Foundation (m)
d γ—— The weight of the bottom of the base (KN / m3) & # 8194
γ 0 -- average weight of foundation bottom (KN / m3)
engineering application:
when the area and buried depth of foundation bottom are determined according to the bearing capacity of foundation or the number of piles is determined according to the bearing capacity of single pile, the load effect transmitted to the bottom of foundation or pile cap shall adopt standard combination according to the limit state of normal use, The corresponding resistance limit value adopts the modified characteristic value of foundation bearing capacity or the characteristic value of single pile bearing capacity. That is s ≤ C, C is the limit value of resistance or deformation; PK ≤ fa (Foundation); QK ≤ RA (pile foundation). At this time, the characteristic values FA and RA are the resistance design values under the serviceability limit state
when the height of foundation or pile abutment, the section of retaining structure, the internal force of foundation or retaining structure, the reinforcement and the strength of material are determined according to the material properties, the load effect from the upper structure and the corresponding base plate should be combined according to the basic load effect under the ultimate bearing capacity state, i.e γ When 0s ≤ R, the foundation reaction P, the reaction Ni under the pile top and the active earth pressure EA are the corresponding design load values, and the corresponding partial coefficients should be adopted
< H2 > reference: Network -- foundation bearing capacityf=fk+bc(b-3)+audio(d-0.5)