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Correct Understanding of High Strength Anchor Bolt

High-strength anchor bolts are the most common construction component in steel construction, and all steel structure engineers will find it familiar and think they know it well. 

High-strength anchor bolts are the most common construction component in steel construction, and all steel structure engineers will find it familiar and think they know it well. However, this may not be the case. Today we start with the most basic concepts and bring you a new understanding of high-strength steel bolts.

What is a High-strength Anchor Bolt

High-Strength Friction Grip Bolt, literally translated as: high-strength friction pre-tightening bolt, English abbreviation: HSFG. It can be seen that the high -strength bolts mentioned in Chinese construction are short for high-strength friction pre-tightening bolts . In daily communication, the words “Friction” and “Grip” are simply abbreviated, but it has caused many engineers and technicians to understand the basic definition of high-strength bolts, which has caused misunderstanding.

Bolts with material grades exceeding 8.8 are “high-strength anchor bolts”?

The core difference between a high-strength anchor bolt and a normal bolt is not the strength of the material used, but form of force. The essence is whether to apply pre-tightening force and use static friction to shear. 
In fact, in British standard, high-strength anchor bolts (HSFG BOLT) mentioned in American standard are only 8.8 and 10.9 (BS EN 14399 / ASTM-A325 & ASTM-490), while ordinary bolts contain 4.6. 5.6, 8.8, 10.9, 12.9, etc. (BS 3692 11 Table 2); thus , material strength is not key to distinguish between high-strength bolts and ordinary bolts.

Correct Understanding of "High Strength"

According to GB50017, calculate the tensile strength and shear strength of a single ordinary bolt (Class B) 8.8 and high strength anchor bolt 8.8.

Strength Grade Designation4.64.85.65.86.66.88.810.912.914.9
Tensile Strength ( R m), Min.40405050606080100120140
Yield Stress (R e ), Min.243230403648
Stress at Permanent Set Limit (R 0.2 ), Min.6490108126
All stress and strength values are in kgf/mm 2 units.

Through calculations, we can see that the design values of tensile strength and shear strength of ordinary bolts are higher than those of high-strength bolts under same level.

So how are high-strength bolts called "high tensile "?

In order to answer this question, we must start from the design working state of 2 bolts, study the law of elastoplastic deformation, and understand limit state of design failure.

Deformation Curve of High Tensile Bolt

Stress-strain curve of ordinary bolt and high-strength bolt under working condition

Common Bolt: screw itself plastically deformed beyond a design allows the screw to be cut bad. 
Ordinary bolt connection, relative slippage occurs between connecting plates before shearing force is started, and then bolt rod and connecting plate are in contact, and elastic plastic deformation occurs, and shearing force is received. 
High-strength Bolt: The static friction between effective friction surfaces is overcome, 2 steel plates are relatively displaced. The design consideration is damage. 
High-strength bolt connection, friction is firstly subjected to shearing force. When load is increased until friction is insufficient to resist shearing force, static friction is overcome and connecting plate is relatively slipped (limit state). However, although it is broken at this time, bolt rod and connecting plate are in contact with each other, and elastoplastic deformation of bolt can be utilized to withstand shearing force. 

High-strength Anchor Bolts Have Higher Bearing Capacity

From calculation of a single bolt, design strength of high-strength bolts is lower than that of ordinary bolts. The essence of its high strength is that node does not allow any relative slip during normal operation, which is, elastoplastic deformation is small and joint stiffness is large.

Compare of High Strength Bolt and Ordinary Bolt

It can be seen that the node designed with high-strength bolts does not necessarily save number of bolts used in the case of a given design node load, but its deformation is small, rigidity is large, and safety reserve is high. It is suitable for main beam, and position where the node stiffness is required to be large, which is in line with basic seismic design principle of “strong node and weak member”. 

High-strength Bolt Strength is not the design value of its own bearing capacity, but the rigidity of design node, high safety performance and strong resistance to damage.

Anchor bolts shall be commercially smooth and free of burrs, laps, seams, cracks, and other injurious manufacturing defects that would make them unsuitable for the intended application.

Washers used on anchor bolts, installed in holes with dimensions greater than oversize or short slot as defined by the Research Council on Structural Connections, require design consideration. (For guidance refer to Specification for Structural Joints Using ASTM A 325 or A 490Bolts.)

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