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Basic parameters:
tower crane model QTZ80A, foundation buried depth d = 3.8 m, bearing platform length Lc = 5.5 m, bearing platform width BC = 5.5 m, bearing platform thickness HC = 1.35 m, pile diameter d = 800 m, pile spacing a = 3.4 m, stirrup spacing S = 200 mm
dead weight calculation of pile cap:
where d foundation buried depth d = 3.8 M
LC cap length Lc = 5.5 m BC cap width BC = 5.5 m HC cap thickness HC = 1.35 M
After calculation, g = 25 × five point five × five point five × 1.35+20 × five point five × five point five × 3.8=3319.94 kN 
extended data:
for tower crane with pile foundation, the bearing capacity of pile foundation must be checked. According to the calculation and analysis, when the tower crane boom is perpendicular to the diagonal of the foundation surface, it is the most dangerous. When the connecting line of two diagonal piles is taken as the axis (Fig. 2-1), and the overturning moment is generated, the single pile will be stressed, and the stress of the pile is the most unfavorable situation
if the calculated P2 < 0, the pile will be pulled, and the pulling force is | P2 |
L -- the center distance of the pile
If the compressive bearing capacity of single pile is jointly borne by the side friction of pile, the compressive bearing capacity of single pile is as follows:the uplift bearing capacity of single pile is borne by the side friction of pile, and the uplift bearing capacity of single pile is as follows:
RK2 = up ∑ qsili (2-6)
where:
QP -- standard value of pile end bearing capacity, kPa
AP -- cross-sectional area of pile body, M2
up -- circumference of pile body, m
QSI -- standard value of skin friction of layer I soil of pile body, kpak
Li -- length of each section of pile divided by soil layer, m
compare P1 with RK1, | P2 | with RK2, if P1 & lt; RK1 and | P2 | & lt; RK2 can meet the requirements
Therefore, the bearing capacity meets the requirements. Hey, hey
Simple calculation of vertical bearing capacity: P = (4 * n * PI ^ 2 * e * I) / [(L / 2) ^ 2]
The calculated compressive stress is the compressive stress proced by the vertical pressure acting on the cross section of the square tube. This is relatively simple, that is, the pressure (in n) divided by the cross-sectional area of the square tube (in M2). As long as the compressive stress is less than the allowable stress of the material
extended data:
pile group effect
e to the interaction of pile cap, pile and soil, the bearing capacity and settlement behavior of pile group foundation under vertical load are often significantly different from that of single pile with the same setting method under the same geological conditions, which is called pile group effect. The bearing capacity of pile group foundation is not always equal to the sum of the bearing capacity of each single pile
pile group effect is shown in the following aspects: lateral resistance, end resistance, soil reaction of pile cap, load distribution on pile top, failure mode of pile group, settlement of pile group and its variation with load
for example, under the action of earthquake load, the pile foundation bears larger horizontal load. Due to the influence of pile group effect, the bearing capacity of each pile in pile group is much smaller than that of single pile, and e to the different influence scope of edge effect and shadow effect, the horizontal bearing capacity of front row pile (the first row pile in front of loading direction) is the largest compared with other rows of piles, However, the horizontal bearing capacity of other rows of piles decreases more than that of single pile
MAKRIS et al × 2. The influence of pile group effect on seismic response of pile group foundation under harmonic excitation (mainly inertial interaction) and S-wave excitation (mainly geometric interaction) at pile top. The results show that the influence of pile group effect is greater under inertial interaction. The influence factors of pile group effect
pile group effect is affected by many factors, including pile distance and number, pile diameter, soil quality, whether the pile top is embedded, load action mode and so on. The pile group effect and pile cap resistance are studied. It is pointed out that the pile spacing is the most important factor affecting the pile group effect and seismic response of pile foundation, and the soil type and density have no effect on the pile group effect and seismic response of pile group< br />
For some deep soil caves and karst caves, in order to prevent karst collapse, foundation piles are usually used to pass through the collapse pits and karst caves to transfer the load to the stable bedrock
According to different pile forming methods, foundation piles can be divided into cast-in-place piles and precast piles. According to the characteristics of pile-soil interaction in the process of pile forming, the cast-in-place pile can be divided into non squeezing cast-in-place pile (such as bored pile, Luoyang shovel hole forming cast-in-place pile, manual digging cast-in-place pile), partial squeezing cast-in-place pile (such as punching cast-in-place pile), squeezing cast-in-place pile (such as sinking cast-in-place pile); There are two kinds of precast piles: ordinary reinforced concrete precast pile and prestressed reinforced concrete pile Generally speaking, bored pile is used to prevent karst collapse, and sometimes manual bored pile is also used. The design of bored pile is introced below Pile type design (1) determine the bearing behavior of pile (2) determine the working type of pile according to the grade, scale and load of building pile foundation and the nature and thickness of each rock and soil layer in the site. Generally, the upper part of Quaternary soil in karst collapse prone area is mainly sandy soil with small thickness, and the lower part is carbonate rock developed in shallow karst. Under such geological conditions, the bored pile tip needs to penetrate through the soil cave and shallow karst development zone to enter the complete and hard bedrock. Therefore, the bearing behavior of the pile mostly takes the end resistance as the bearing standard, that is, the type of pile is end bearing pile(2) select the material of pile
select the material and construction method of pile according to the local material supply, construction equipment and technical level, cost, construction period and site environment. For example, plain concrete cast-in-place piles can be used in small and medium-sized projects to save investment; Reinforced concrete piles should be used in large projects
(1) to determine the pile specification, it is generally necessary to select the complete and hard bedrock as the pile end bearing layer. The length of the pile depends on the aggressiveness of the Quaternary soil and the thickness of the shallow karst development zone. In addition, the pile top needs to be embedded into the pile cap. The pile length should be determined according to these factors in design The cross-sectional area ofpile is determined according to the pile top load, local construction equipment and construction experience. If large section piles are used in small projects, it will be wasted; Due to the low bearing capacity of single pile, the number of small section piles used in large-scale projects needs to be increased. It is not only difficult to arrange the piles, but also the pile cap size is large, and the driving cost is labor-consuming
(2) determine the vertical bearing capacity of single pile
(1) estimation of the number of piles1) when designing according to the technical code for building pile foundation (jcj 94-94), it can be estimated by the following methods:
under the action of axial vertical force, the calculation formula is
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where: n is the number of piles; F is the vertical design value acting on the top surface of pile cap (KN); G is the dead weight of bearing platform and its overlying soil (KN); R is the design value of vertical bearing capacity of single pile (KN)
Under the action of eccentric vertical force, the calculation formula is
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where: μ Is the eccentric compression coefficient of pile foundation, usually 1.1-1.2; The meanings of other symbols are the same as before
2) when designing according to the code for design of building foundation (GB 50007-2002), it can be estimated according to the following methods:
under the action of axial vertical force, the calculation formula is
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where f < sub > k < / sub > is the vertical force acting on the top surface of pile cap corresponding to the standard combination of load effect (KN); G < sub > k < / sub > is the standard value of self weight of pile cap and soil on the cap (KN); R < sub > a < / sub > is the characteristic value of vertical bearing capacity of single pile (KN); The meanings of other symbols are the same as before
When the eccentric vertical force is applied, the symbol meaning in the formula is the same as before (2) plane layout of piles after the number of piles is preliminarily determined, the plane layout of piles can be carried out according to the characteristics of superstructure and load properties1) pile center distance: generally, the center distance of bored pile should be 2.5D (D is the diameter or side length of pile). If the center distance is too small, it may affect the bearing capacity of the pile; On the contrary, if the center distance of pile is too large, the pile cap size is too large, which is not economical
2) plane layout of piles: the plane layout of piles is shown in Fig. 4-4. When arranging piles, the resultant point of bearing capacity of pile group should coincide with the center of gravity of long-term load; And make the pile foundation bear the direction of larger horizontal force and moment, that is, the long side of pile cap has larger section molus. The clear distance between the pile and the edge of pile cap shall not be less than D. Different types of piles should be avoided for the same structural unit
Fig. 4-4 pile layout plan
4. Checking calculation of single pile bearing capacity
(1) technical code for building pile foundation (JGJ 94-94) method
< P > under the action of central load, the actual load of each pile is required not to be greater than the design value of single pile vertical bearing capacity, Check calculation according to the following formula:
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where n is the design value of external force borne by single pile in pile foundation (KN); F is the design value of vertical force acting on the pile foundation (KN); G is the design value of self weight of pile cap and the standard value of soil self weight on pile cap (KN); R is the design value of vertical bearing capacity of single pile (KN); γ< Sub > 0 < / sub > is the partial resistance coefficient of the subsoil of the pile cap; The meanings of other symbols are the same as before
In addition to equation (4-9), the following equation should be satisfied under eccentric load:
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where n < sup > Max < / sup > < sub > Min < / sub > is the design value of maximum or minimum external force (KN) on single pile in pile foundation; M < sub > x < / sub >, m < sub > y < / sub > are the design values of the external force acting on the pile group on the X and Y axes passing through the center of gravity of the pile group (KN · m); X < sub > I < / sub >, Yi are the distance from pile I to X and Y axes passing through the center of gravity of pile group (m); X < sub > Max < / sub >, ymax are the distance from the farthest pile to the X and Y axes passing through the center of gravity of pile group (m); λ< Sub > 0 < / sub > is the importance coefficient of building pile foundation, which is 1.1, 1.0 and 0.9 respectively according to the safety level I, II and III of building pile foundation; The meanings of other symbols are the same as before
(2) under the action of axial vertical force in code for design of building foundation (GB 5007-2002), the bearing capacity of single pile in pile group is required not to be greater than the characteristic value of vertical bearing capacity of single pile, which is checked by the following formula:
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where r < sub > a < / sub > is the characteristic value of vertical bearing capacity of single pile; N is the vertical force of < sup > Single Pile under axial vertical load corresponding to load effect standard combination; When FK is corresponding to the standard combination of load effect, the vertical force acting on the top surface of pile cap < sup > < sub > GK is the standard value of self weight of pile cap and soil on pile cap; The meanings of other symbols are the same as before
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where n < sup > Max < / sup > < sub > min < / sub > is the maximum or minimum vertical force of single pile under eccentric vertical load corresponding to load effect standard combination; The meaning of other symbols is the same as before
(1) the role of pile cap the role of pile cap includes the following three items:1) connect multiple piles into a whole to bear the upper load together
The load of superstructure is transferred to the top of each pile through pile cap The pile cap is a cast-in-place reinforced concrete structure, which is equivalent to a shallow foundation. Therefore, the bearing capacity of pile cap is similar to that of shallow foundation, that is, pile cap effect(2) pile caps are divided into high and low pile caps. The pile cap with pile top at a considerable height above the ground is called high pile cap. The pile cap with the pile top below the ground is called the low pile cap. Usually, the pile cap bearing the building foundation belongs to this category. Like shallow foundation, the bottom of low pile cap is required to be buried below the local freezing depth
(3) material and construction of pile cap. Because the height and spacing of the top of each pile can not be very regular ring the construction of each pile, the pile caps can not be prefabricated because each pile is closely connected into a whole2) the concrete strength grade of bearing platform is not lower than C15
The reinforcement of bearing platform is determined by calculation. Rectangular cap should not be less than Φ 8 ＠ 200, and the stressed reinforcement shall be evenly distributed in both directions4) the thickness of protective layer should not be less than 50 mm
(4) the size of pile cap
the plane size of pile cap, according to the plane layout of pile, each side of pile cap extends from the periphery of pile not less than D / 2, and the width of pile cap should not be less than 500mm
The thickness of pile cap should ensure that the pile top is embedded into the cap, and prevent the punching failure of cap caused by concentrated load of pile. The minimum thickness of bearing platform should not be less than 300 mm. The thickness of pile caps in large and medium-sized projects should be determined by punching calculation. A building in Southwest China adopts pile foundation. Because the thickness of pile cap is too small, the punching failure of pile cap leads to the collapse of the whole building (5) internal force of pile cap the internal force of pile cap can be determined according to the simplified calculation method, and the strength of local compression, punching, shearing and bending can be calculated according to the code for design of concrete structures (GB 50010-2002), so as to prevent the damage of pile cap and ensure the safety of the project