Structural Bolts

Structural bolts play a crucial role in ensuring the stability and safety of any construction project. These specialized bolts are designed to withstand heavy loads and provide a secure connection between structural components. Whether you are building a bridge, a skyscraper, or even a small residential structure, choosing the right structural bolt is of utmost importance. In this comprehensive guide, I will take you through everything you need to know about selecting and installing the perfect structural bolt for your project.

Different Types of Structural Bolts

Structural bolts come in various types, each with its unique features and applications. The most common types include:

1. High Strength Bolts

a. A325 Bolts

A490 bolts are similar to A325 bolts but are made from alloy steel. They are used in heavy construction projects that require high tensile strength.

b. A490 Bolts

A490 bolts are similar to A325 bolts but are made from alloy steel. They are used in heavy construction projects that require high tensile strength.

c. TC Bolts

These bolts feature a spline end and a unique installation method that ensures proper tensioning. They are commonly used in structural steel connections.

2. Machine Bolts

a.A307 Bolts

A307 bolts are low-strength bolts made from low carbon steel. They are generally used for erection purposes. A307 are generally called as erection bolts, unfinished bolts, machine bolts or soft bolts.

A325 and A490 Bolts Specifications

I. Comparison between A325 and A490 Bolts

		A325	A490
Chemical Composition		High strength carbon steel bolts	Quenched and tempered alloy steel bolts
Strength and Cost		Strength and cost both are less compared toA490 bolts	Strength and cost are high compared to A325 bolts
Washers	Standard, Oversized and Short Slotted Holes	Standard ASTM F436 washer for all the bolt diameters	Standard ASTM F436 washer for up to 1″ diameter bolt
	Long Slotted Holes	5/16″ thick plate washer or continuous bar	ASTM F436 with 5/16″ thick washer.3/8″ thick plate washer or continuous bar bolt diameter greater than 1″

II. Bolt Length Availability

Both A325 and A490 bolts are available in ½” to 1 ½” diameter and length generally up to 8”

III. Each strength grade can be ordered as:

• Type-1: Medium carbon steel for A325 and Alloy steel for A490

• Type-2: Type-2 has been removed from RCSC (The Research Council on Structural Connections)

• Type-3: Atmospheric corrosion resistant steel for both ASTM A325 and A490

Bolt Length Calculation

• Bolt Length = Grip + Thickness of Washer + Coeff.
• Grip = Thickness of piles connected by Bolt
• Tw = Number of washers x Thickness of washers
• Coeff. = Thickness of the nut + Bolt stick out

Washer Requirements for Bolted Joints with Oversized and Slotted Holes in the Outer Ply

^a This requirement shall not apply to heads of round-head tension-control bolt assemblies that meet the requirements in Section 2.7 and provide a bearing circle diameter that meets the requirements of ASTM F1852.

^b Multiple washers with a combined thickness of 5/16 in. or larger do not satisfy this requirement.

^c The plate washer or bar shall be of structural-grade steel material but need not be hardened.

^d Alternatively, a 3/8-in thick plate washer and an ordinary thickness F436 washer may be used. The plate washer need not be hardened.

ASTMDesignation	NominalBoltDiameter,d b , in.	Hole Type in Outer Ply
		Oversized	Short-Slotted	Long-Slotted
A325 orF1852	½-1½	ASTM F436a		5/16-in.-thick platewasher orcontinuous barb, c
A490	<= 1
	> 1	ASTM F436 with 5/16 in.Thickness b, d		ASTM F436 washer with either a 3/8-in.-thick structural grade plate washer or continuous barb

RCSC – Table 6.1

Table for the Bolt and Nut dimension

  RCSC – Table C-2.1

Bolt Stick out

Acceptable bolt stick out is established by the bolt and nut combination. Below is a chart that details general guidelines for acceptable bolt stick out. Note that any protrusion beyond these guidelines may still be suitable.

Types of Bolt Joints

1. Bearing Joint

In a bearing joint the connected elements are assumed to slip into bearing against the body of the bolt. If the joint is designed as a bearing joint the load is transferred through bearing whether the bolt is installed snug-tight or pretension.

2. Slip Critical Joint

• In a slip-critical joint the bolts must be fully pretension to cause a clamping force between the connected elements.
• This force develops frictional resistance between the connected elements.
• The frictional resistance allows the joint to withstand loading without slipping into bearing against the body of the bolt, although the bolts must still be designed for bearing.

Slip critical joints to be used as per the below cases:

• Joints that are subject to fatigue load with reversal of the loading direction     (not applicable to wind bracing).
• Joints that utilize oversized holes.
• Joints that utilize slotted holes, except those with applied load approximately perpendicular to the direction of the long dimension of the slot.
• Joints in which slip at the faying surfaces would be detrimental to the performance of the structure.

Bolted Parts

1. Connected Plies

All connected plies that are within the grip of the bolt and any materials that are used under the head or nut shall be steel (uncoated, coated or galvanized). Compressible materials shall not be placed within the grip of the bolt. The slope of the surfaces of parts in contact with the bolt head and nut shall be equal to or less than 1:20 with respect to a plane that is normal to the bolt axis.

2. Faying Surfaces

Faying surfaces and surfaces adjacent to the bolt head and nut shall be free of dirt and other foreign material.

a. Faying Surfaces of Snug-Tightened Joints and Pretensioned Joints:

b. Faying Surfaces of Slip-Critical Joints:

The faying surfaces in slip-critical joints require special preparation. The faying surfaces of slip-critical joints including those of filler plates and finger shims, shall meet the

following requirements:

i Uncoated Faying Surfaces:

Uncoated faying surfaces shall be free of scale, except tight mill scale, and free of coatings, including inadvertent overspray,in areas closer than one bolt diameter but not less than 1 in. from the edge of any hole and in all areas within the bolt pattern.

The faying surfaces of snug-tightened joints and pretensioned joints are permitted to be uncoated, coated with coatings of any formulation or galvanized.

ii. Coated Faying Surfaces:

Coated faying surfaces shall first be blast cleaned and subsequently coated with a coating that is qualified in accordance with the requirements in Appendix A as a Class A or Class B coating.

iii. Galvanized Faying Surfaces:

Galvanized faying surfaces shall first be hot-dip galvanized in accordance with the requirements of ASTM A123 and subsequently roughened by means of hand wire brushing. Power wire brushing is not permitted. When prepared by roughening, the galvanized faying surface is designated as Class C for design.

Faying surfaces of slip-critical connections painted with unqualified paints

Bolt Shear Plane

The shear plane is the plane between two or more pieces under load where the pieces tend to move parallel from each other, but in opposite directions.

Types of shear planes

1. N-Type

In N-type the threads are included in the shear plane.

2. X-Type

In X-type the threads are excluded in the shear plane.

The capacity of a bolt is greater with the threads excluded from the shear plane 

Bolt Holes

1. 1. Standard Hole
2. 2. Oversized Hole
3. 3. Short-slotted Hole
4. 4. Long-slotted Hole

The nominal dimensions of standard, oversized, short-slotted and long-slotted holes for high-strength bolts shall be equal to or less than those shown in the belowTable-3.1. Thermally cut bolt holes shall be permitted if approved by the Engineer of Record. For statically loaded joints, thermally cut surfaces need not be ground. For cyclically loaded joints, thermally cut surfaces shall be ground smooth.

RCSC-Table 3.1. Nominal Bolt Hole Dimensions

1. Standard Hole

In the absence of approval by the Engineer of Record for the use of other hole types, standard holes shall be used in all plies of bolted joints.

2. Oversized Holes

When approved by the Engineer of Record, oversized holes are permitted in any or all plies of slip-critical joints.

3. Short-Slotted Holes

When approved by the Engineer of Record, short-slotted holes are permitted in any or all plies of snug-tightened joints, and pre-tensioned joints, provided the applied load is approximately perpendicular (between 80 and 100 degrees) to the axis of the slot. When approved by the Engineer of Record, short-slotted holes are permitted in any or all plies of slip-critical joints as defined in Section 4.3 without regard for the direction of the applied load.

4. Long-Slotted Holes

When approved by the Engineer of Record, long-slotted holes are permitted in only one ply at any individual faying surface of snug-tightened joints, and pre-tensioned joints as defined in Section 4.2, provided the applied load is approximately perpendicular (between 80 and 100 degrees) to the axis of the slot. When approved by the Engineer of Record, long-slotted holes are permitted in one ply only at any individual faying surface of slip-critical joints as defined in Section 4.3 without regard for the direction of the applied load. Fully inserted finger shims between the faying surfaces of load-transmitting elements of bolted joints are not considered a long-slotted element of a joint; nor are they considered to be a ply at any individual faying surface.

Minimum Bolt Spacing

Distance between the centers of standard, over sized or slotted holes shall not be less than 2 2/3 times the nominal diameter, d. of the fastener.A distance of 3d is preferred.

Minimum Bolt Edge Distance

1. Standard Hole

The distance from the center of the standard hole to an edge of a connected part in any direction shall not be less than the value from the AISC table J3.4

2. Slotted and Over-sized Holes

The distance from the center of an over-size or slotted hole to the edge of a connected part shall not be less than the required for a standard hole to the edge of a connected part plus the applicable increment C2 from the AISC table-J3.5

Bolt Tightening Methods

1. Snug Tight

The snug-tightened condition is typically achieved with a few impacts of an impact wrench application of an electric torque wrench until the wrench begins to slow or the full effort of a worker on an ordinary spud wrench. More than one cycle through the bolt pattern may be required to achieve the snug-tightened joint.

2. Pre-tensioning

1. Turn-off-nut pretensioning
2. Calibrated Wrench Pretensioning
3. Twist off type Tension control bolt Pretensioning
4. Direct Tension Pretensioning

a. Turn-off-nut Pre-tensioning

In this type the tightening is achieved by extra tightening of the head/nut as per RCSC table-8.2 from the snug tight condition, using an impact wrench. 

b. Calibrated Wrench Pre-tensioning

Achieved by using calibrated wrench by applying the minimum torque as per the RCSC table-7-1 from snug tight condition.

c. Twist off type Tension control bolt Pre-tensioning

ASTM F1852 & F2280 twist-off type tension control bolt assemblies have a splined end that exceeds the threaded portion of the bolt. During installation, this splined end is gripped by a specifically designed wrench chuck and provides a means for turning the nut relative to the bolt. The splined end will shear off after reaching the minimum pretension

d. Direct Tension Pre-tensioning

DTI are hardened, washer shaped devices incorporating small arch like protrusion on the bearing surface that are designed to deformed in a controlled manner when subjected to control load.

Bolt Head Markings

The ASTM requires structural bolts to be distinctively marked. These bolt head markings include the manufacturer’s mark, the grade of bolt (ex. A325, A490), and markings to classify the type of bolt (Type 1 or Type 3).

In addition to the mandatory markings, certain manufacturers may include additional markings to distinguish them from others.

RCSC – Figure C-2.1

Understanding the Importance of Choosing the Right Structural Bolt

The structural integrity of any building or infrastructure relies heavily on the quality and suitability of the bolts used. A weak or incorrectly chosen bolt can compromise the entire structure, leading to potential disasters. It is essential to invest time and effort in selecting the right structural bolt that meets the specific requirements of your project. By doing so, you ensure the safety of those who will use and rely on the structure in the future.

Factors to Consider When Selecting a Structural Bolt

Choosing the right structural bolt involves considering several important factors. These factors include:

Load Capacity

The bolt must have the required load capacity to withstand the forces acting upon it. This includes considering the weight of the structure, wind loads, seismic loads, and any other relevant factors.

Material

Consider the environmental conditions and the type of structure being built to select a bolt material that provides the necessary strength and corrosion resistance.

Size

Choose the appropriate bolt size based on the thickness of the materials being connected and the required grip length.

Corrosion Resistance

Depending on the environment in which the structure will be located, it is crucial to choose a bolt with appropriate corrosion resistance. This ensures that the bolt will not weaken or fail over time due to corrosion.

Material Compatibility

The bolt should be compatible with the materials being joined. For example, if the structure is made of stainless steel, it is important to use a bolt made of a similar material to prevent galvanic corrosion.

By carefully considering these factors, you can choose a structural bolt that meets the specific needs of your project and ensures its safety and strength.

Installation Guidelines for Structural Bolts

Proper installation of structural bolts is critical to maintaining the integrity of the structure. Follow these guidelines to ensure a secure and reliable installation

Preparation

Make sure the holes are clean, free from debris, and properly aligned. Use a template or guide to ensure accurate hole drilling

Torqueing

Use a calibrated torque wrench to tighten the bolt to the recommended torque value. Over-tightening can lead to bolt failure, while under-tightening can result in a loose connection.

Inspection

After installation, visually inspect the bolts to ensure they are properly seated and tightened. Check for any signs of damage or misalignment.

Documentation

 Keep a record of the installation process, including torque values, for future reference and maintenance.

Maintenance and Inspection of Structural Bolts

Regular maintenance and inspection of structural bolts are crucial for ensuring their long-term performance and safety. Here are some key practices to follow:

Visual Inspection

Regularly inspect the bolts for signs of corrosion, deformation, or damage. Address any issues promptly to prevent further deterioration.

Tightening Checks

Periodically check the bolt tightness using a calibrated torque wrench. Retorque any bolts that have become loose due to settlement or vibration.

Corrosion Protection

Apply appropriate corrosion protection measures, such as coatings or galvanizing, to prevent rust and deterioration.

Record Keeping

Maintain a comprehensive record of inspections, maintenance activities, and any repairs or replacements performed on the bolts.

Conclusion

Choosing and installing the perfect structural bolt is a critical aspect of any construction project. The safety and longevity of the structure depend on the quality and suitability of the bolts used. By understanding the different types of structural bolts, considering the necessary factors, and following proper installation and maintenance guidelines, you can ensure a secure and reliable connection. Remember to consult with experts, refer to industry guidelines, and choose a reputable supplier to obtain high-quality structural bolts for your project. Invest in the right bolt, and you’ll have peace of mind knowing that your construction is built to last.

Apply appropriate corrosion protection measures, such as coatings or galvanizing, to prevent rust and deterioration.

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