23.1. Bolted Connections

Bolted connections, Fig. 23.1, are increasingly used instead of rivets (too labor intensive) and more often than welds (may cause secondary cracks if not properly performed).

Bolted connections have become a preferred method in construction and engineering because they are easier to install compared to rivets, which require more labor and time. Additionally, bolted connections are favored over welded connections as welds can lead to weaknesses in the material, known as secondary cracks, if not executed correctly. Therefore, the simplicity and reliability of bolted connections engage a lot of attention in structural applications.

Imagine a puzzle where pieces snap together with clips (bolted connections) instead of being glued (welded connections). Using clips allows you to easily take apart and reassemble the puzzle, while glue can sometimes result in broken pieces if not applied carefully.

Most common high strength bolts are the A325 and A490. A325 is made from heat-treated medium carbon steel, min. Fb 81 92 ksi, Fb = 120 ksi. A490 is a higher strength manufactured from an alloy of steel, min. Fb 115 130 ksi, and Fb = 150 ksi. Most common diameters are 3/4", 7/8" for building constructions; 7/8" and 1" for bridges.

High strength bolts, specifically A325 and A490, are essential for ensuring structural integrity. The A325 bolts are made from medium carbon steel and have specified tensile strengths, which are the maximum load allow for them before they fail. A490 bolts, on the other hand, are even stronger and are crafted from an alloy, making them suitable for high-load applications. Knowing the right type and size of bolt is crucial for construction, particularly in buildings and bridges where safety and durability are paramount.

Think of these bolts as different types of shoe laces for various sports. A325 bolts are like general fitness laces meant for casual running, while A490 bolts are akin to specialized laces designed for professional athletics—stronger and more durable for high-stakes situations.

The nominal area refers to the effective cross-sectional area of the bolt, which plays a crucial role in determining the load a bolt can handle. This chart shows various bolt diameters along with their respective nominal areas, which can be used for calculating the strength of a bolted connection. For instance, a larger diameter generally means a larger area that can withstand more tension and shear forces.

Imagine comparing water pipes of different diameters. A larger pipe can carry more water—just like a wider bolt can handle more stress. So, knowing the nominal area helps engineers design buildings and bridges that are safe and efficient, much like ensuring plumbing can support enough water flow.

There are two types of bolted connections: Bearing type which transmits the load by a combination of shear and bearing on the bolt, Fig. 23.2. Slip-critical transmits load by friction, Fig. 23.3. In addition to providing adequate ultimate load, it must not slip during service loads.

Bolted connections serve two main functions: the bearing type relies on both shear (the force acting parallel to the surface of the bolt) and bearing (the compressive load pressing on the bolt) to transfer the load effectively. The slip-critical connection relies solely on friction to resist the loads it experiences, and it is essential for the connection to maintain grip to prevent slipping under service loads. Understanding these types is vital for engineers to choose the right connection for different applications.

Consider a hand grasping a mug. When you lift it (bearing type), your grip (like shear and bearing) holds it securely. Now, if you are sliding something on a smooth surface, like pushing a box (slip-critical), your grip must prevent it from moving. In both cases, the method of load transfer is different, impacting how safely and efficiently the load is handled.

The nominal strength R of one fastener in tension is R = FbA, where Fb is the tensile strength of the bolt and A is the area through the threaded portion, also known as ensile stress area. The ratio of the tensile stress area to the gross area A ranges from 0.75 to 0.79.

The nominal strength of a bolt can be calculated using a straightforward formula: R = FbA, where R is the nominal strength, Fb is the tensile strength (maximum load) and A is the area through the bolts' threads. This calculation is vital for determining whether the bolt can handle the loads it will encounter. The mention of the tensile stress area ratio indicates that the threaded section is less than the entire bolt area, which is important for safety and design purposes.

It's like using a thread in sewing. If the thread is strong (high tensile strength) but too thin (small area), it might break under pressure. As a craftsman, you need to balance the thickness and strength to ensure your stitching holds together, just like engineers must ensure bolts are capable of withstanding expected forces.

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