Based on the manner in which the connection bears load, bolts are classified into two types: standard bolts and bolts intended for use in reamed holes. Based on head shape, varieties include hexagonal heads, round heads, square heads, countersunk heads, and others; among these, the hexagonal head is the most commonly used. Generally, countersunk heads are employed in applications where a flush surface is required at the connection point.
U-bolts (known in English as "U-bolts") are non-standard fasteners. Due to their distinctive U-shape, they are also referred to simply as "U-bolts." Both ends feature threads designed to engage with nuts. They are primarily used to secure tubular objects-such as water pipes-or flat objects, such as automotive leaf springs. The name "U-bolt" (literally "riding-horse bolt" in Chinese) derives from the fact that the manner in which it secures an object resembles a person riding a horse. Based on thread length, U-bolts are categorized into two types: fully threaded and partially threaded.
Based on thread profile, bolts are classified into two types: coarse-pitch threads and fine-pitch threads; the coarse-pitch designation is typically omitted from the bolt's markings. Bolts are further classified into eight performance grades: 3.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9. Bolts of Grade 8.8 and higher (including 8.8) are manufactured from low-carbon alloy steel or medium-carbon steel and undergo heat treatment (quenching and tempering); these are collectively referred to as "high-strength bolts." Bolts below Grade 8.8 (excluding 8.8) are collectively referred to as "ordinary bolts."
Ordinary bolts are classified into three grades-A, B, and C-based on their manufacturing precision. Grades A and B are considered "refined" (precision-machined) bolts, while Grade C is considered a "coarse" (rough-machined) bolt. For connection bolts used in steel structures, unless otherwise specifically noted, the standard choice is the ordinary, coarse-grade (Grade C) bolt. The manufacturing processes differ across these grades, typically corresponding to the following methods:
① Grade A and B bolts feature shanks that are precision-machined on a lathe, resulting in smooth surfaces and precise dimensions. They typically possess a material performance grade of 8.8. However, their complex manufacturing and installation requirements, combined with their higher cost, mean they are rarely utilized.
② Grade C bolts are manufactured using unmachined round steel bars; consequently, their dimensions lack high precision. They typically possess a material performance grade of 4.6 or 4.8. While they exhibit greater deformation when used in shear connections, they offer ease of installation and low production costs, making them widely used in tension connections or for temporary fastening during assembly.