Calculating and Designing Floor Panels

Introduction and purpose of the work

Multi-storey buildings are not only residential buildings, but also industrial, administrative, domestic and public buildings. Such buildings are most often made frame from precast reinforced concrete. The construction of these types of buildings consists of the following parts:

A skeleton is a spatial skeleton that carries vertical and horizontal loads and is assembled from individual elements: columns, girders, floor panels, and stiffness links.

Floor panels (slabs) - directly perceive the loads on each floor from the weight of the floor, equipment and people. These loads, together with the own weight of the panels, are transmitted to the girders; the latter rest with their ends on projections (cantilevers) of columns. Moreover, the column of each floor takes up the load from the columns of the overlying floors. Therefore, the most loaded are the columns of the first floor, they are installed on the foundations, through which all the load from the building is transferred to the base (soil).

In addition to vertical loads, the building also has horizontal loads: wind pressure, from braking of internal transport, as well as accidental impacts that are not always measurable. The combined action of vertical and horizontal loads can lead to a loss of overall stability of the building if you do not provide spatial stiffness, that is, stiffness in three planes: two vertical and horizontal. You can do this in two ways:

The first is to create rigid units for coupling girders with columns, which, unlike hinged ones, are able to perceive not only longitudinal and transverse forces, but also bending moments. Such frames are called frame frames. The second is to connect part of the columns with special stiffening links, preserving the hinged support of the girders on the column console. The role of such connections can be played by inter-room reinforced concrete partitions - they are called stiffening diaphragms. A similar type of skeleton was called a link.

In both cases, the horizontal links are floor panels which form rigid discs either by welding them to the girders or by tightly sealed longitudinal and transverse seams between the structures. Each system has its own application. For example, frame frames are more labor-intensive and material-intensive, but the floor spaces in them are not blocked by diaphragms, so they are preferred for industrial buildings. Communication frames are used where, for planning reasons, partitions are not an obstacle: institutions, schools, hospitals, some industrial enterprises. In such buildings, the loads on the floors are relatively small, so the structures are lighter here - they can use the so-called "hidden column consoles that do not protrude beyond the dimensions of the girders, which increases the volume of rooms and improves their interior (and 1).

The purpose of my work is to design load-bearing structures of a three-span three-story building. In the project, we calculate only some of the most characteristic elements: the row floor panel, the middle span girder and the middle column of the first floor. Each structure should be calculated by strength (and the floor panel also by stiffness and crack resistance) and drawings should be developed.

The initial data for the design are: the dimensions of the building in plan along the external axes L and L, the distances between the longitudinal and transverse layout axes l X l (column grid), the number and height of floors, the useful normative load per 1 m2 of coating and floors (including permanent, long-term and short-term) and working reinforcement classes. The above data, however, is not sufficient to start designing constructions directly. First, it is necessary to assemble the building, determine the dimensions of each structure and design spans. Drawings shall be executed taking into account the requirements of the Construction Design Documentation System - PAS [6, 7].

Layout of the building, determination of dimensions and design spans of the structure.

It is required to determine the overall dimensions of the load-bearing structures of a three-story, three-span frame building of the connected type and draw out the layout of the frame elements: the floor plan of the first floor, a cross-section, nodes and specification.

Initial data: plan dimensions on external axes 18 × 56 m, column grid 6 × 7 m, floor height in axes 5.4 m, floor panels are hollow, the construction area of ​ ​ Kurgan (III snow district [1]), floor loads are given in Table 1, to cover Table 2, building of normal reliability level.

The solution. To assign the size of the section of the columns, it is approximated, without taking into account the own weight of the girders and columns, to determine the force from the calculated load in the column of the first floor. As per Table 1, the design load on the floor is 11.34 kPa. With two intermediate floors and a load area of ​ ​ the column 6 × 7 = 42 m2, the force in the column will be 2 * 11.34 * 42 = 952.56 ≈ 953 kN. As per Table 2, the design load on the coating is 6.84 kPa, the force in the column from it is 6.84 * 42 = 287.28 ≈ 287 kN. Total force in the column: 953 + 287 = 1240 kN, which is less than 2000 kN. We take the section of the columns 300 × 300.

Since the reference of the extreme columns is axial, the design length of the girders

l = 600030040=5660. The cross-sectional dimensions of the girders are assigned b × h = 200 × 500 mm, with a shelf width of bf = 400 mm. Then the design length of the panels taking into account the gaps l = 7000 - 200 - 20 = 6780 mm. With a distance between the longitudinal (letter) axes of the columns of 6000 mm, the nominal width of the rows is assigned equal to 1200 mm, the average inter-column panels - 1200 mm, and the extreme inter-column ones - 600 mm (the actual design width taking into account the tolerances will be 10 mm less - 1190, 1190 and 590 mm, respectively). The columns are received with floor-by-floor cutting, the joints of the columns are located at a distance of 650mm from the top of the girders.

Draw layered framing feature layouts, including ground floor plan, cross section, subassemblies, and schedule (drawing QP1. LBC. 03 - 1 - LC). In this case, we provide one cantilever in the columns along axes A and D, in contrast to two cantilevers in the columns along axes B and C; columns located at the end walls are loaded less than the rest, so we assign them all different grades - from K1 to K4. Girders have two brands - single-shelf at the end walls (panels rest on them only on one side) and two-shelf others. We also mark the floor panels differently - ordinary, intercolon middle and intercolon extreme. stiffening diaphragms, staircases, external wall panels and other elements on the diagram are not conventionally shown. We fill the BOM after calculating our own mass of structures, that is, after the completion of their working drawings.

Constructs a panel.

Working drawings of the hollow panel are given on one sheet of A3 format. This sheet contains a formwork drawing, reinforcement diagram, specification and steel consumption list, it also shows grids, frames, installation loop and group specification of reinforcement. The text material reflects the features of reading drawings and the necessary instructions for the production of products .

Stressed rods are arranged symmetrically in section. The transverse reinforcement is combined into KR1 frames, and the longitudinal reinforcement in the compressed zone is combined into a C3 grid with cells of 200 × 250 mm. In addition, in the support sections of the mesh C1 made of class Bp1 wire, which protect concrete from splitting by pre-crimping, and with a panel width of more than 1.5 m, also the mesh C2 preventing the development of longitudinal cracks in the lower shelf from local bending (in the panel drawing, the mesh C2 is shown in the order of reference).

Four loops are designed to lift the panel, their diameter of 10 mm is determined from the reference materials of the tutorial bearing in mind that the own weight of the panel 2079 kg is distributed into three loops. The dimensions of the loops are also found from the reference materials of the tutorial.

When designing grids and frames, we take into account the structural requirement of norms: the length from the ends of the rods to the axis of the extreme intersecting rod must be at least the diameter of the protruding rod and at least 20 mm.

Constructs a column.

In the upper part of the column at the corners, recesses are provided for discharging the reinforcement with their subsequent welding with the outlets of the rods of the upstream column. After installation, the excavations are sealed with concrete.

The length of the column is determined taking into account its sealing below the floor elevation by 0.8 m and the location of the joint 0.65 m above the floor l = 5.4 + 0.8 + 0.65 = 6.85 m. Longitudinal rods, determined by calculation, are defined in two flat frames of CR 1, which are combined with the help of transverse rods into a spatial frame of CP 1 (see drawing).

The pitch s of the transverse rods shall be not more than 500 mm and not more than 20 ds, where ds is the diameter of the longitudinal rods. At [mu] = (As + As ")/Ab > 3%, the step s is reduced to 300 mm or 10 ds. In our case i = 3.1% ≈ 3%, we take s = 300 mm. According to the welding conditions, the diameter of the transverse rods should be at least 0.25 ds, from here 0.25 × 28 = 7 mm we take with a margin of d = 8AI.

According to the requirements of the norms, the protective layer of concrete to the working reinforcement must be at least 20 mm and at least ds, in our case - 28mm. The distance from the axes of the longitudinal rods to the outer faces is taken as equal to 45mm, taking into account the possibility of putting indirect reinforcement grids C 2 on KP 1. These grids, along with grids C 1, are installed at the top of the column to protect the upper part of the concrete from destruction during local compression, i.e. crushing (in the lower part they are not needed, since the column is made in a glass of foundation).

Mesh cell dimensions shall be in the range of 45 to 100 mm, but not more than 1/4 the smaller side of the cross-section of the element (300/4 = 75mm) grid spacing - in the range of 60 to 150 mm, but not more than 1/3 the smaller side (300/3 = 100 mm). At the length (from the end of the column) at least 10 ds = 10 × 25 = 250 mm, at least 4 such grids are installed. At the same time, the volume reinforcement coefficient should be μxu > 0.0125.

We pre-assign the spacing of the grids s = 80mm, rods d = 6mm AIII, with cells 45 × 45mm for C1 and 60 × 60dl C2.We limit the reinforcement coefficient for C2 μxy = (nx Asx lx + ny Asy ly )/( Aef .s) = (5 × 28.3 × 280 × 80 )/( 0,0 )/( 9000) Reduce the pitch: s = 70 mm, μxy = 0.126 > 0.0125, the condition is satisfied.

The volume of concrete of the column is 0.62 m3, its own mass is 1.55 tons. According to the tables of the training manual, two loops are provided from rods d = 12mm A-I. With a symmetrical section and reinforcement of the loop, it is advisable to position it at a distance from the ends of a = 0.21 × l = 1, 44m, then the positive and negative moments from their own weight are equal. To install the column, we provide an opening d = 40mm in it, into which a steel pin with rings will be inserted for insurance during installation.

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