מיתד - מונחים

כללי

מערכת עגינה מורכבת ממספר גורמים המשפיעים על איכות העגינה, לקבלת עגינה מיטבית יש לוודא:

  • את סוג התשתית, להתאים את המיתד הנכון לתשתית
  • לקדוח בצורה נכונה, במקדח ייעודי המתאים לתשתית
  • לוודא שיטת התקנה שתתאים לאפליקציה
  • מיתד שמתאים לשאת את העומס הנדרש
  • מיתד התואם את אופי הסביבה בה הוא מותקן

 

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התאמה לתנאי הסביבה

Corrosion protection

Corrosion is the destruction of metal by chemical or electrochemical reactions with the environment an a media, where the attack starts at the surface.

The material decomposition usually does not stop until the corrosive agent is able to damage the surface. For anchors in dry areas such as dwelling homes, offices, schools, and hospitals, with the exception of damp rooms anchors made from galvanized steel are suitable.

Due to its molybdenum alloy stainless steel of class A4 (material No 1.4401 and 1.4571) is resistant to industrial atmosphere, coastal atmosphere outdoor and damp rooms, provided no attacks from other corrosion influences are likely to occur.

                                        

Behavior in fire  

More and more frequently anchors are being used for passive fire protection. Special measures are required to ensure safe anchorage. Such preventive measures are things such as casings, coatings, or concrete covering of the steel components, or by using anchors that are approved and meet the requirements of fire regulations (e.g. IBMB). 

 

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תסבולת המיתד בעומס

Fixings are defined as devices consisting of one or more components, which are used for the attachment of elements to structural members. Load bearing fixings transfer the loads of the component into the structural member. In the event that anchor failure may result in loss of life or finance, it must be designed to be effective, economic, and safe in accordance with current standards and building regulations.

 

 


Typical anchor failure modes

 

In order to be able to judge the load-bearing capacity of an anchorage, the load direction of the outside load must be known. The different failure causes result from this.

 

Failure due to tension load

 

Steel failure

 can be predicted by calculating As x fuk where As=Stress sectional area of the bolt in question. fuk=nominal characteristic steel ultimate strength. Steel failure represents the upper limit of the achievable load bearing capacity

.

 

 

Pullout failure

 is characterized by withdrawal of the anchor from the drill hole, usually resulting in some spalling of the concrete near the surface. The load required to achieve this mode of failure is specific to the anchor type being used. The load can only be determined through testing.

 

 

 

Concrete cone failure is defined procedure, which is dependant essentially on the anchorage depth and the concrete compressive strength. If the anchors are close tighter or close to a free edge such that their zones of influence can overlap, a reduction factor must be applied to the load capacity.

 

 

 

Splitting failure

 is a special kind of failure that generally only occurs in slender building components. Splitting the concrete during anchor installation can be avoided by observing the recommended minimum anchor spacing, edge distance, and member thickness. Annex C to the ETAG gives a calculation method to predict splitting, this allows the designer to calculate the splitting load and eliminate it as a possible cause of failure.

 

Influence of anchor spacing and free edge distance

If the required anchor spacing and free edge distances are observed, the zones of influence will not be overlap and hence no reduction in anchor capacity will be required.

If the anchors are closer together and the zones of influence overlap, a reduction will have to be applied to the anchor capacities.

If the anchors are close to one or more free edge and the edge cuts into the zone of influence a reduction will have to be applied to the anchor capacities.

 

Failure due to shear loads

For anchors located in building components with the required thickness, anchor spacing, and edge distance the mode of failure should be steel failure. Before steel failure is reached local failure of concrete near the surface may occur.

 

Concrete pry out failure

Large diameter anchors with small anchor spacing and shallow embedment can fail by a concrete pry out failure at the side opposite the load direction.

 

 

 

Concrete edge failure

Anchors located close to a free edge of concrete member and loaded in the direction of that edge, can cause concrete edge failure. Edge failure is a defined procedure, which is dependant essentially on the edge distance and the concrete compressive strength. If the anchor zone of influence overlaps with the zone of of influence of another anchor, the load capacity must be reduced. The same applies to an anchor located close to the corner.

In order to achieve the highest efficiency form an anchor subjected to a shear load careful consideration should be given to the anchor spacing, free edge distance, building component thickness, bolt diameter, and steel quality.

 

Oblique tension load    

Combination of tensile and shear loading applied simultaneously. It is calculated as follows:

 

FR=√¯(Fz² + FQ²)

Bending load

Anchors are subjected to bending loads if the shear component of the applied load is introduced with a lever arm. Care should be given not to exceed the permissible/recommended bending moment.

 

Permissible loads on anchors

In this catalogue, the data of the permissible loads are taken from the appropriate approvals and can be taken as the maximum service loads for a single anchor. Anchors that have approvals based on characteristic loads and partial safety factor employ a safety factor of g=1.4 for combined dead and live loads, and a further factor for material depending on the mode of failure. In cases where the anchor spacing and free edge distances fall between the required and the minimum permissible spacing and edge distance the load bearing capacity must be reduced according to the designed method 06/93 of the German institute for civil engineering, or according to annex C of the European technical approval guideline 06/97. The approval requires that when anchors are close to a free edge, reinforcement must be present between the anchor and the free edge and not more than the value of hef (effective embedment depth) away from the anchor. The permissible loads given are valid for concrete with no reinforcement in the effective are of the anchor, (reinforcement distance < 15 cm).

 

 

 

Recommended loads for anchors

Recommended loads are manufacturers data concerning the loads bearing capacities of anchors, with the appropriate safety factors already taken into consideration. For the recommended loads the valid guidelines and regulation of the different countries apply. If due to the nature of the construction the building regulations are binding and the fixing has to be designed accordingly, then anchors with approvals have to be used. 

 

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אופן פעולת המיתד

Bonded techniques

Bonded anchors work by mixing different elements of quick setting resin mortar in a cleaned out drill hole before or during the installation of the anchor stud. The load transfer mechanism primarily associated with bonded anchors is the bond between the anchor stud and the mortar and between the mortar and the wall of the drilled hole. This stress free bonded technique allows for small edge distance with high load bearing capacity.

 

Expansion technique

Expansion anchors differ from other types of post-installed anchors in that they transfer tension loads to the concrete principally by friction. This method is not only used for heavy-duty anchors, it is also used for products in the program.

 

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שיטת ההתקנה

In place fastening

This method is applicable only to certain types of expansion, undercut, grouted and chemical anchors, which can be inserted through the fixture into the hole. When this method is used, the drill hole diameter in the base material is equal to the anchor diameter.

In place fastening is the most suitable method for fixing anchors in groups of 3, 4, 6 or more.

 

Pre-positioned fastening

With this method, the fastener is installed before the fixture is attached. It is applicable to pre-fixing and internally threaded anchor types. When this method is used, the drill hole diameter in the base material is different to the anchor diameter. This method is often used for overhead installations.

 

Stand off fastening

This method uses an anchorage assembly in which the fixture is secured at a distance from the surface of the base material. This method is applicable to pre-fixing and internally threaded anchor types. When this method is used, the anchors are subjected to additional bending stress.

 

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סוגי תשתית

The load capacity of any anchor system cannot exceed the load capacity of the base material into which it is fixed. Correct anchor selection is critical in insuring that maximum capacity of the base material is achieved. Incorrect selection can result in premature failure of the base material. If an anchor that is designed for use in normal concrete is used in aerated concrete, the aerated concrete in the anchoring zone could be damaged such that the load bearing capacity of the system is reduced. Consideration should always be given to the base material when selecting an anchor.

 

Note: for any given base material there are various choices of effective fixing methods.

The load bearing capacity of any base material is determined by the type and strength of the raw material used.

If there is no data available on the load bearing capacity of a particular base material we recommend on site testing to determine it’s strength.

In the following information, we list all commonly found base materials.

 

 

Concrete

Concrete is a non-homogeneous mixture of three components: cement, water, and different additives. Differentiation between light, normal, and heavy weight concrete is in accordance with the specific weight of the material.

 

Normal weight concrete

Normal weight concrete (dry raw density 2.0 to 2.8kg/m³) is a multi-applicable building material. The concrete’s form is created on site by the use of formwork and shuttering. Precast concrete components are manufactured in the factory and assembled on site. For fixing techniques of the concrete strength is crucial. The designations B20 or C20/25 mean that the concrete has reached a minimum cube compressive strength of 25N/mm² after a period of 28 days. This concrete has the minimum required strength class for external building components and therefore the most frequent concrete strength class.

Note: due to their load bearing capacity some anchor types for heavy-duty loads are only approved for concrete strength class B25 or C20/25.

Compared to other building components almost no deformation occurs in hardened concrete. There is a low tensile strength of 3.0N/mm² for hardened concrete in comparison to the compressive strength. The low tensile strength of concrete is over come by the inclusion of steel bars (reinforcement) that can take the tensile loads. The cured concrete becomes a bonded building material made of steel and concrete.

 

Reinforced concrete

Under normal economical use of reinforcement, the cracks up to a width of 0.3 mm occur as a result of tension stress. When using anchors it is significant that:

In cracked concrete only anchors approved for use in cracked concrete are allowed.

In non-cracked concrete it is permitted to use anchors with out such an approval, however arithmetical proof of the tension condition of concrete must be provided.

 


Classification of normal weight concrete 

In national and international regulations, concrete is classified by its compressive strength. The concrete strength class is one of the initial considerations taken when calculating the static capacity of an anchor. The following table shows the concrete strength classes according to Eurcode (2 EC 2) in comparison with DIN 1045.

 

Concrete strength classes

 

Compressive strength (N/mm²)

Acc. to EC 2

Acc. To DIN 1045

fck,cyl 1)

fck,cube 2)

C 12/15

B 15

12

15

C 16/20

 

16

20

C 20/25

B 25

20

25

C 25/30

 

25

30

C 30/37

B 37

30

37

C 35/45

B 45

35

45

C 40/50

 

40

50

C 45/55

B 55

45

55

C 50/60

 

50

60

  1. Characteristic compressive strength of cylinder with diameter of 150 mm and height of 300 mm.
  2. Characteristic compressive strength of a cube with a 150 mm edge length.

 

Light-weight concrete

The density of the normal weight concrete and its low thermal insulation can be unfavorable for certain applications. This led to the development of lightweight concrete with a dry density of <2.0 kg/m³.

Different applications apply:

-          Thermal insulating lightweight concrete (additives: natural pumice, foam lava, artificial pumice, flying ash, brick gravel, expanded clay and slate).

-          Structure lightweight concrete is similar to thermal lightweight concrete only without some of the additives, (typically expanded clay and slate).

-          Aerated concrete belongs to lightweight concrete.

 

Masonry construction

A large variety of bricks and blocks (clay units, dense or lightweight aggregate concrete units, autoclaved aerated concrete units, calcium silicate units, natural stone) are used for the construction of structural and partition walls. The mechanical characteristics of these units are very scattered, since they depend on parameter such as:

-          The materials used for production (quality varies from country to country and from region to region within a country).

-          The quality control of productions

-          The configuration of voids if any

-          The direction of loading (parallel or perpendicular to the perforations).

Thus, the characteristic compressive strength of masonry units commonly used in Europe varies between 2.0 and 60.0 Mpa.

For all the reasons mentioned above it is difficult to provide data for anchor performance in all the various types of masonry. The best way to determine load capacities for different masonry material is to carry out on site tests.  

 

Panels and boards

Panels and boards are thin walled building materials and are generally of low strength. They are either fixed directly onto another building component (e.g. ceilings or walls) or they are fixed at a distance form the surface of the base material.

There are different types of panels and boards.

 

Plaster board panel

Core made of plaster, both sides covered with fibrous paper sheet.

 

 

Fibrous concrete board

Thin boards made of a fibrous material bonded with concrete.

 

Plywood

Veneer boards: core made of wooden blocks laminated together, both sides covered with thin wooden sheet.

 

Chipboard

Pressed board made from wood chips and synthetic resin as a bonding agent.

 

Fireboard

Pressed or non-pressed boards made from mechanically fibered wood, bonded with glue or synthetic resin.

 

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שיטות קידוח

For correct anchor installation according to regulations and approvals the correct drill, setting and installation method is as important as the correct anchor selection. Incorrect drill, setting, and installation can lead to a drop in anchor performance and may even result in anchor failure. Therefore, it is important to read the assembly instruction carefully before installing the anchor. The manufacture details such as recommended anchor depth, edge distance, anchor spacing, and thickness of building components, tightening torque etc. should all be followed correctly. Often the tolerances of the drill hole diameters are exceeded if worn out or non-standard drill bit are used.

 

Rotary drilling

This method drills the hole using rotary action only. No hammer action is used. Rotary drilling is used frequently for thin walled or porous building materials (e.g. aerated concrete). You need only apply low pressure to the drilling machine when drilling such material. Using a hammer action would result in local damage to the base material. If a percussion-drilling machine is used, the hammer action must be disengaged.

 

Impact drilling

This method drills the hole using rotary and hammer action. The tungsten carbide drill bit is subjected to a simultaneous series of short impacts and high rotary speed. This method is suitable for use on solid bricks and blocks.

 

Hammer drilling

This method drills the hole using rotary and hammer action. It different from impact drilling because in this method the drill bit is subjected to a simultaneous series of fewer more powerful impacts and low rotary speed. The drilling machines are called drilling hammers and have a special drill chuck (e.g. SDS plus and SDS max). This method is suitable for fast drilling in concrete and other high strength building materials.

 

Diamond drilling method

This method enables drilling in concrete using rotary action only. This method is particularly suited to drilling large diameter holes as well as holes thought reinforced concrete. When using this system to drill for anchors, the anchor performance should be checked with the manufacture, as this method is not yet covered by the anchor approvals. This method must normally be used with a water supply however dry drilling is also an option in some cases.

 

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