Professional guidance of secondary structure: calculation method and key points of gravity load

(A) the calculation method of gravity load

(1) The practice of high-rise building structure shows that complex high-rise buildings always have a certain number of standard floors. The accurate calculation of gravity load should start from the standard floor, and take standard units (such as houses, hotels) or standard blocks (such as office buildings) in the standard floor for accurate calculation, so as to be accurate and complete. Roof layer, equipment layer and podium layer are calculated according to the same principle respectively.

(2) The concrete calculation steps should be carried out in the order of plate, beam and column (wall), that is, the evenly distributed surface load of plate should be calculated first, then the linear load of beam should be calculated, and then the concentrated load of main girder and the load on Du (wall) should be calculated. There are two points to point out here:

The uniformly distributed surface load of 1) slab refers to the arithmetic average of all slab loads (total load/slab area), including the linear load of the infilled wall, and is not used to calculate the bearing capacity of slab and the converted uniformly distributed live load within the local range of reinforcement. If the latter is used as the uniform surface load of the plate, the value is often too large, which is inconsistent with the reality.

2) In the whole calculation process, the uniformly distributed surface load of the calculation plate is transferred to the beam (wall) as a line load, and the line load of the beam is transferred to the main beam as a concentrated load, and the line load (self-weight, etc.) of the main beam. ) and the concentrated load is transferred to the column (wall) as a concentrated force, which can generally be carried out by simple supporting method, regardless of the continuity of the actual structure, in order to simplify the calculation. This is because the influence of the continuity of the horizontal roof structure of high-rise buildings is often concealed by the elastic compression, condensation shrinkage and creep of the vertical members under the gravity load of high-rise buildings.

(2) Calculation points of gravity load

(1) For common cast-in-place concrete structures, when calculating the weight of beams, attention should be paid to deducting the plate weight of the overlapping parts of beams and slabs, especially in flat beams and wide flat beams. Because usually for the convenience of calculation, the plane uniform load of the plate is transferred to the beam and divided according to the center line of the beam grid. The calculation shows that the error of total gravity load increase caused by neglecting to deduct the plate weight of the overlapping part of beam and slab is usually about 10% ~ 20%. At present, some automatic load guidance softwares for high-rise structures do not pay attention to this point, resulting in a large calculated value of gravity load, which needs special attention in design. At the same time, it should be noted that the surface load transfer distribution of the plate cannot be calculated repeatedly.

(2) When calculating the self-weight of the wall, we should pay attention to deducting the weight of the repeated part of the wallboard in the same way. The calculation shows that the error of the total gravity load increase caused by this overlapping part is usually about 5%.

In the final analysis, the calculation of standard cell gravity load is to calculate the total gravity load of columns and walls in a standard cell, including the white weight of columns and walls.

(3) Pay attention to reduction when calculating live load. According to the Code for Load of Building Structures (GB 50009-2001), in order to simplify the calculation, it is suggested to uniformly take the reduction factor of the standard value of floor live load in Table 4. 1.2 when calculating the total gravity load of columns and walls and determining the section of walls and columns, in which the low value >: 20-story building takes the high value.1.2.