Video from my YouTube channel

Subscribe to My You tube channel

Types and Classifications of Bolts and Bolted Connections in Steel Structures

By -

Bolts are one of the most essential components in steel construction. They connect individual structural members, transfer loads, and ensure overall stability. Understanding the different types of bolts, their strength grades, and connection classifications is critical for safe and efficient structural design and erection.

1. Classification of Bolts

Bolts used in steel structures are generally divided into two main categories:

1.1 Unfinished (Common) Bolts

These are low-strength bolts commonly used in secondary structural elements such as bracing, purlins, and temporary connections. They are economical and suitable where loads are relatively small and precision tightening is not critical.

1.2 High-Strength Bolts

High-strength bolts are designed for primary structural connections such as beam-to-column joints, truss connections, and bridge girders.
According to the ASTM (American Society for Testing and Materials), they are mainly classified as:

  • ASTM A325: Medium carbon steel, heat-treated, commonly used in standard structural applications.

  • ASTM A490: Alloy steel, heat-treated to a higher strength, used where greater load-carrying capacity is required.





The grade or strength of bolts is defined by several international standards, such as ASTM, ISO, and EN.

  • ASTM Classification:

    • A307 (Grades A and B): Common bolts used for non-structural or secondary connections.

    • A325 and A490: High-strength bolts used in primary structural connections, as mentioned earlier.

  • ISO / EN Classification:
    These standards classify bolts by property classes, such as 8.8, 10.9, and 12.9, which correspond to specific yield and tensile strengths.
    The table below (to be added) can summarize the relationship between ASTM and ISO/EN classifications, including their yield strength, ultimate tensile strength, and common applications.

Standard

Bolt Grade

Type

Yield Strength (MPa)

Tensile Strength (MPa)

Common Use

ASTM

A307 Grade A

Low-Carbon Steel, General Use

—

414 min

General hardware, light structural work

A307 Grade B

Medium-Strength, Flanged joints

—

515 min

Pipe flanges, castings

A325 (replaced by F3125)

Medium-Strength Structural Bolt

660

830

Structural steel joints

A490 (replaced by F3125)

High-Strength Structural Bolt

895

1040

Heavy structural steel, bridges

F3125 Grade A325

Medium-Strength Structural Bolt

660

830

Bridges, buildings

F3125 Grade A490

High-Strength Structural Bolt

895

1040

High-load bridge joints

ISO

8.8

Metric High-Strength Bolt

640

800

Structural joints, machinery

10.9

High-Strength Structural Bolt

940

1040

High-stress applications

12.9

Very High-Strength Bolt

1100

1220

Automotive, aerospace, tools

EN 14399

HR Class 8.8

Preloaded Structural Bolt

640

800

Preloaded bolted connections in steelwork

HR Class 10.9

High-Strength Preloaded Bolt

940

1040

Bridges, steel towers

Bolted connections are also classified according to the degree of bolt tightening, as follows:

  1. Snug-Tight Connections:
    In this method, bolts are tightened sufficiently to bring the connected steel parts into firm contact, typically until no further tightening can be achieved with a regular wrench.

    • This type of joint is referred to as a bearing-type connection.

    • Load transfer occurs primarily through the shear strength of the bolts.

    • Not recommended where the connection is subjected to significant tension.

  2. Pretensioned (Slip-Critical) Connections:
    In this type, bolts are tensioned to a specified level of tension or torque, verified using one of the following methods:

    • Calibrated wrench method

    • Turn-of-the-nut method

    • Direct Tension Indicator (DTI) washers

    The connection’s resistance depends on the friction between the faying surfaces, maintained by the bolt pretension. Both the frictional resistance and bolt bearing contribute to the connection’s total capacity. The table below provides guidance on the minimum required bolt pretension (kN) for different bolt diameters and grades (as per AISC or ASTM specifications).

    Since slip resistance depends on the frictional force between faying surfaces, these surfaces must be properly prepared and roughened. They should not be painted or coated, as this reduces friction. The shear capacity developed through friction can be expressed as:

    V=T×μV

    where:
    V = slip resistance (shear)
    T = bolt pretension force
    μv = coefficient of friction of the faying surface

Bolt holes are typically made slightly larger than the bolt diameter to allow for easy installation. The standard clearance is about 1/16 inch (≈3 mm). However, depending on construction and erection requirements, different hole types are permitted:










Comments

Post a Comment

Videos from My Youtube channel

Popular posts from this blog

Pile cap

By -
Image
Pile cap used to transfer the loads from superstructure to the piling. The pile cap is thick concrete mat rests on piles. It is part of the foundation and used to distribute the loads over the piles. Piles used when the soil bearing is not enough to carry loads of the structure. Figure 1  construction management: concrete construction bridge construction:How to become a bridge engineer Pile cap construction procedure Excavation around piles: After 7 days of the casting of the pile the concrete will gain approximately, 70% of it is compressive strength. The soil around piles will be removed to the required levels and dimension that allows construction of the pile cap. A step shall be made if the excavation is more than 1.5 m to prevent soil collapse. Figure 2 Pile head cutting: Concrete will be removed up to cut off level. If concrete under the cut off level is unsound. It shall be removed, and the pile will be repaired...
Read more

Bridge Diaphragm

By -
Image
The main function of diaphragms is to provide a stiffening effect to deck slab in case bridge webs are not situated directly on top of bearings. Therefore, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings.On the other hand, diaphragms also help to improve the load-sharing characteristics of bridges. Diaphragms also contribute to the provision of torsional restraint to the bridge deck. Diaphragm will be used also to resist stresses that occurs during the post-tensioning process. Post-tensioning hardware are casted inside the diaphragm. Figure 1 construction management: concrete construction bridge construction:How to become a bridge engineer Types of diaphragm: Diaphragm at the span end: this type of diaphragms will be located between two bridge spans or at the bridge abutments. Expansion joints will be used to connect bridge spans. This ...
Read more

Prestressing stands debonding

By -
Image
  Prestressing bridge concrete beams contributes to resisting the tensile stresses caused by the traffic loads and the beam's weight. Stressing concrete has various benefits, such as increasing concrete members' capacity, reducing the concrete member section, and increasing the span lengths for structures such as bridges and buildings. However,  The stressing of bridge beam ends can result in unwanted cracks. Therefore AASHTO calling for debonding a specific portion of strands at the beam end to minimize the stresses and reduce cracks. Figure 1 Debonding is achieved by wrapping the strands at the end of the beam with plastic. Wrapping the strand with plastic wrapping will prevent the strands from bonding with concrete, thus avoid transferring unwanted stress to the beam end. As a result, the unwanted cracks will be prevented by reducing the stress levels at the end of the beam.  
Read more

Moment Diagrams Constructed by the Method of Superposition

By -
Image
Shear and moment diagrams are very important to conduct a structural analysis for a member or structure. Also, the designer should know the shear and moment values applied to a member to determine the amount of reinforcement and member dimension. A structural engineer should have the ability to draw shear and moment diagrams . Superposition is one of the methods used to draw the moment diagram. In this method, the loads applied on beam taken individually. The moment diagram for each load drawn separately. The moment diagram for a beam is the sum of these diagrams for each load case. Figure 1 shows most of the load cases for a beam. Figure 1 To understand the principle of superposition let consider the simply supported beam in figure 2. the simply supported beam in figure 2 subjected to a distributed load of 4k/ft over half of the beam and moment of 300 k.ft at the middle of the beam. First, we will calculate the reaction at support A and B. for simpl...
Read more

Repairing of concrete bulging

By -
Image
Improper designing and installation of formwork for a concrete member can result in concrete Bulging. During the placing and compacting of concrete, forms may deflect largely in the weak areas. The deflection of forms will cause concrete to expand at this location, and this known as concrete Bulging. Concrete Bulging will spoil the appearance of a concrete member. Therefore we should always ensure that the forms are appropriately installed, and it is sufficiently supported to resist the concrete during placing and compacting. Bulging should be repaired in case we couldn't avoid it. The solution of concrete Bulging, offset, or surface irregularities are surface grinding. However, excessive surface grinding can weaken the concrete.  Figure 1 If the bulging or surface irregularities are small, grinding of concrete surface will be sufficient to remove surface irregularities. If the Bulging of concrete is significant, then the concrete should be chipped, and the surface...
Read more

wing wall arrangement and orientation for abutments

By -
Image
wing wall main function is to retain the embankment near abutment. the common slope is 2:1. greater than 2:1 is not allowed. figure no: shows a wing wall for the abutment. Figure 1  there is three arrangement for wing wall: wing wall parallel to abutment: easy to build. construction can be done in a short time. this type of wing wall will not disturb the existing embankment and utilities but it is not the most economical arrangement.   figure 2 construction management: concrete construction bridge construction:How to become a bridge engineer wing wall at an angle with abutments: this type is most economical among the three arrangement. Figure 3 wing wall perpendicular to abutment: in this arrangement. wing wall provides a continuous alignment with bridge deck which can be used to support parapets. this arrangement will disturb existing embankment and utilities adjacent to the structure. for curve bridge, this type o...
Read more

Flakiness Index and Elongation Index of Coarse Aggregates

By -
Image
Flaky and elongated aggregates can adversely affect concrete. Flaky and elongated aggregates are difficult to compact. Therefore using flaky and elongated aggregates will reduce concrete workability, which required more amount of water to produce a workable mix. Increasing water content will reduce concrete strength and durability. Moreover, these aggregates are not robust, which leads to lowering concrete strength. Figure 1 construction management: concrete construction bridge construction:How to become a bridge engineer The Flakiness Index of aggregates is the percentage by weight of particles whose least dimension (thickness) is less than three- fifths (0.6times) of their mean dimension. This test does not apply to sizes smaller than 6.3mm. The Elongation index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than nine-fifths (1.8times) their mean dimension. Flakiness and elongation tests ar...
Read more

Prestressing strand manufacturing

By -
Image
  Prestressing strands is high tensile strength steel. The tensile strength of prestressing strands can reach up to 1860 Mpa. the process of manufacturing prestressing steel differs significantly from the conventional reinforcement.  Figure 1 The process of manufacturing strands will begin by de-scaling the raw material (wire rod). The purpose of de-scaling is to remove the iron oxides (mill scale) from the surface of the wire rod. after removing the mill scale, the wire rod's surface is coated with a textured coating to enhance the adherence of lubricants to the raw material surface. The most commonly used coating is zinc phosphate.  Figure 2 The wire rod then passed through a series of dies. The process of reducing the wire rod diameter is known as cold-drawing. In this process, the wire rod is drawn through a die with the required diameter. To prevent the wearing of dies, the wire rod will be drawn through a lubricant box. the lubricant will coat the wire with a thin l...
Read more

shear key

By -
Image
The purpose of the shear key is to improve the bonding between concrete surfaces. The shear key is usually made at the construction joint location to enhance the adhesion between the old and new casted concrete. Construction joint is considered a weak point because the concrete is not casted at one time, in large structures where the concrete can not be casted for some members at one time, such as a slab. We usually divide the slab and cast it at many stages. The large quantities of concrete will make it impossible to cast the slab at one time. Therefore, the casting will be done in several stages. The location of the construction joint should be selected carefully. Construction joint shall be made at the location of minimum shear and moment.  The bonding between old and new concrete can be improved by roughening the old concrete and forming a shear key on the concrete surface while the concrete is still plastic. The shear key can be formed by pressing timber into the concrete surf...
Read more

stress-strain diagram for aluminium alloy and rubber

By -
Image
figure no:1 showing the stress-strain diagram for aluminum alloy. we can notice from the stress-stress diagram that aluminum has a considerable ductility. despite the absence of explicit yield point as in structural steel . the initial portion of the stress-strain curve is linear with a recognizable proportional limit. the proportional limit for aluminum alloy ranges from 70 to 410 Mpa (10 to 60 Ksi). aluminum alloy undergoes large strain before failure. ultimate stress ranges from 140 to 550 Mpa (20 to 80 Ksi). construction management: concrete construction bridge construction:How to become a bridge engineer Figure-1 Yield stress for aluminum alloy can be determined by drawing offset line parrel to the linear portion of the stress-strain curve. the straight line offset by a standard strain such as 0.002. the intersection point with the stress-strain curve is the point of yield stress. figure no:2 illustrates the concept of offset method. yield stress acquire...
Read more