Cấu tạo thép tòa nhà - P2

Every building consists of a load bearing frame, usually created by reinforced con-crete, or by steel, or by the combination of both areas with usual earthquake activity. Although the load bearing frame is not visible after the completion of the construc

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There are several reasons that necessitate the existence of covering for ment:

reinforce-α The protection of reinforcement from oxidation

b The need for adequate adhesion between the steel and the concrete

c The need for fire precaution The reinforcement bars are protected by the crete covering them from possible deformations caused in case of excessive temperature development due to fire in the building,

con-d The need to create cable channels without harming the reinforcement It is very common for an electrician or plumber to engrave channels in the frame in order to integrate electric cables or pipes

In general, when the covering of reinforcement is guaranteed by the use of special plastic spacers, the cost is significant for the procurement, but very low for the im-plementation of these spacers

It is also possible to use ‘common’ materials for the creation of the covering, e.g marble particles, that have no procurement cost but require a significant labor cost for their implementation What is more, the main drawback of using such materials is the difficulty to be fitted properly since it should be implemented right after the instal-lation of reinforcement and just before the concreting process

In cases that we intent to leave the concrete visible, it is possible to use local or ear spacers made out of concrete reinforced with fibers or even use steel stands that have edges made out of concrete reinforced with fibers

lin-Lower reinforcement covering of slabs

The minimum covering that should be provided for the reinforcement of slabs is: In dry climates 2.0 cm

In seaside locations 3.0 cm

The covering is secured by the use of stands (called spacers) which are elements neutral to oxidation These elements are placed with approximately 1m spacing be-tween each other

Abstracts from the book of Apostolos Konstandinideis called ‘EARTHQUAKE RESISTANT BUILDINGS’, Volume A, chapter 2.6, published in low resolution in the site of pi-SYSTEMS International S.A www.pi.gr

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The lower reinforcement covering of the above slab is secured by the use of plastic spacers

The simplest solution of securing the covering is that of the plastic spacers as trated in the above picture

illus-The usage of steel spacers is forbidden since it will almost definitely be oxidized

When a steel rod is oxidized, it increases its volume and destroys the covering and then the plaster, thus decreasing the actual life span of the building and endangering the safety of the residents

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Securing the position of the upper reinforcement of slabs

The upper (or negative) reinforcement of slabs, that either connects two slabs or one slab and a cantilever, can only be properly secured with the use of specific spacers

In the above example that the slab is connected with a cantilever, the position of the upper reinforcement can be secured with the following techniques:

(α) Directly in the formwork, by using steel hystools (b) Indirectly, by using folded mesh spacers

(c) Indirectly, by using S-type wire spacers

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Direct, steel hystool

Steel hystools are constructed by hard drawn wire Hystools have plastic coated feet in or-der to eliminate any danger of rust staining in the area that the spacer is supported by the formwork

Indirect, S-type wire spacers it is available in the market in pre-cast packages of straight wire The S formation is implemented during the installation procedure

Indirect, mesh spacers It is easily created by folding the mesh in the required height The mesh is usually pre-fabricated and has a specific density, e.g Ǿ8/20 In cases of cantilevers, apart from working as a spacer, the mesh also satisfies the requirement for ‘hairpin bend’ reinforcement which secures the cohesion of the cantilever edges

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In cases that we use a mesh for the upper reinforcement, we can secure the position of the mesh by using an S-type spacer placed upon the lower re-inforcement, directly along the area of the plastic spacer

In cases that we use bended bars in the upper support areas, the bending of the bars will ensure that it will not move from its position Therefore, additional indirect spacers may not be required

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In the area of the left support of the slab’s continuation, we should place two S-type indirect spacers which are supported by the lower reinforcement mesh These spac-ers are placed above and along the plastic spacers

At the right continuation of the slabs-which is the cantilever- we should implement 2 rows of spacers: One row of indirect mesh spacers which are supported by two longi-tudinal spacers and one row of direct steel hystools

The side slipping of the reinforcement mesh is avoided by the use of local spacers These spacers must be placed after the implementation of the mesh and before se-curing the reinforcement bars of the slabs When wheel spacers are used, it is ad-vised to be placed vertically in order to avoid their drifting during the concreting proc-ess Regarding the ‘forehead’ of slabs, it is possible to place the wheel spacers hori-zontally (as illustrated in the following picture) since the concrete will not be purred directly over them

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Another optional solution, in cases that we use a lightweight steel mesh for the lower reinforcement, is to use indirect S-type spacers instead of direct hystools In these cases, it is more practical to use a ‘hairpin bend’ mesh spacer within which the mesh can be adequately fitted

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Finally, even in cases that we use a steel mesh for the lower reinforcement of the slab or the cantilever, it is possible to use steel hystools instead of the indirect S-type spacers

It is strictly forbidden to use spacers made out of steel bars that are supported by the framework The use of these old-fashioned extemporary spacers is also the most ex-pensive solution and should be avoided

Beam reinforcement covering

The minimum covering that should be provided for the reinforcement of beams is: In dry climate 2.5 cm

It is suggested that the stirrups in the lower area of the beam should be supported by the use of a uniform inert bar because all loadings from reinforcement are concen-trated in this area

The side covering of the stirrups’ cage is secured by using the appropriate plastic spacers In cases that we use industrial stirrup cages, the wheel spacers should be fitted above the connection bars of the cage in order to secure their position during the concreting process

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The side spacers have no loadings to support and thus it is not required to use heavy-duty spacers It is fitted after the implementation of the stirrup cage in the beam’s body and before the fixation of both the beam’s longitudinal bars and the ad-jacent slab’s reinforcement bars

The usage of longitudinal plastic spacers in the sides of the beam (identical to those fitted in the lower area of the beam) can create two problems: a) it can obstruct the proper fitting of the stirrup cage in the beam’s body and b) it can obstruct the proper concreting process

If we are using an industrialized stirrup cage than uses longitudinal bars to connect the stirrups then it is possible to implement small pieces of plastic spacers fitted in a vertical position

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Abstracts from the book of Aposto

chapter 2.6, published in low resolution in

Column reinforcement covering

The minimum covering that should be provided reinforcement of columns is: In dry climate 2.5 cm

It is quite simple to secure the covering of the reinforcement in columns We only need to use four wheel spacers in the upper area of the column In the base area, the reinforcement bars are secured into their position since it is steadily connected to the dowel bars

Especially for columns, the usage of the spacers in order to create the covering will also ensure the proper cen-tering of the vertical rein-forcement bars Thus, there will be no extra time and cost required during the concret-ing process in order to move the reinforcement bars into the correct position

The covering can be ensured either by the use of wheel spacers in the upper edge of the reinforcement bars (In these particular position there is no danger for the spacers to be carried away by concrete) or by the use of vertical wheel spacers fitted on the stirrups, or by the use of plastic pieces mounded vertically on the formwork Finally, the spacers should always be fitted after the stir-rup cage has been put into its position in order to sim-plify the implementation

los Konstandinideis called ‘EARTHQUAKE RESISTANT BUILDINGS’, Volume A, the site of pi-SYSTEMS International S.A www.pi.gr

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ment bars

Shear walls reinforcement covering

In order to ensure the proper covering of the integrated columns in the edges of the shear walls we should follow the same rules explained in the paragraph for columns In order to ensure the covering of the reinforcement of the body of the shear walls we should follow the following rules:

After the back of the shear wall is created, we should fit plastic rods (usually 2.0m in length) directly in the formwork

The two longitudinal plastic spacers are fitted in the formwork

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Then, the outer columns are created and the body’s reinforcement is implemented After all reinforcement has been properly fitted we can secure the plastic spacers of the opposite side directly in the steel mesh In this way, when the rest of the shear wall’s framework is fitted, the proper centering of the reinforcement will be ensured

After all reinforcement has been fitted and before the shear wall is created by the formwork, the plastic spacers are mounded directly in the internal steel mesh

The most effective way to ensure the proper positioning of reinforcement in shear walls is to fit it before the creation of the formwork In these cases, all spacers are mounded directly on the steel mesh

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The minimum covering that should be provided for the reinforcement of the tion elements is:

Foundation ‘sitting’ on a thick concrete layer 4.0 cm Foundation ‘sitting’ directly on the ground 7.0 cm

Generally, it is not allowed to construct the foundation directly in the ground’s surface except for special occasions The usage of a thick layer of concrete below the foun-dation has the following advantages:

1) Clean and comfortable area to work on

2) Capability of precise marking of the area of footings and columns

3) Provision of a stable ground layer where the spacers can be safely placed 4) Avoidance of a muddy foundation ground due to possible rain or wastewaters The covering can be secured with point spacers, or even better with linear spacers Due to the weight and loadings involved, it is advisable to use heavy duty spacers

The footing reinforcement covering is secured with plastic spacers

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The spacers in the sides of the footings are obligatory in order to keep the ment bars in their position These spacers do not have any loadings to support and can be sparse and lightweight, fitted vertically in order to avoid disruption during the concreting process

reinforce-Securing the position of the upper reinforcement of foundation slabs

In cases that we use a partial or complete rough foundation or we want to construct the base slab of a pool, we must use a steel mesh as upper slab reinforcement Just like in the common slabs of the structure, in the edges of foundation slabs it is possible to combine ‘hair pin’ reinforcement with open or closed steel mesh

In the area between the reinforcement we can use steel spacers placed on the lower steel mesh in order to secure the covering

The above picture illustrates the steel spacers placed on the lower steel mesh in der to support the upper steel mesh

or-Abstracts from the book of Apostolos Konstandinideis called ‘EARTHQUAKE RESISTANT BUILDINGS’, Volume A, chapter 2.6, published in low resolution in the site of pi-SYSTEMS International S.A www.pi.gr

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