23. STEEL 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 are a method of joining structural elements together using bolts, which are screws with a head for tightening. They have become more popular than rivets and welds. Rivets require a lot of labor to install, making them less efficient, while welds can create weak points in steel if not done correctly. Bolted connections provide a reliable option to join steel beams and columns in construction.

Think of bolted connections like using screws to put together furniture. It's usually easier and faster than using adhesive (like welding), and you can easily take it apart if needed, unlike rivets which are permanent fixes.

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

The two most common types of high-strength bolts used in steel connections are the A325 and the A490. The A325 bolt is made from heat-treated medium carbon steel, which means it's strong and can withstand heavy loads. The A490 bolt is even stronger as it is made from an alloy of steel. These bolts come in various diameters, with 3/4" and 7/8" being typical for building construction, while larger bolts like 1" are often used in bridges due to the greater loads they bear.

Imagine A325 bolts like standard screws used in your furniture, while A490 bolts are like heavy-duty screws intended for building wooden decks. The heavy-duty screws can support much more weight, just as A490 bolts can support larger steel structures.

There are two types of bolted connections: 1) Bearing type, which transmits the load by a combination of shear and bearing on the bolt (Fig. 23.2). 2) Slip-critical, which transmits load by friction (Fig. 23.3). In addition to providing adequate at ultimate load, it must not slip during service loads.

Bolted connections can be categorized into two types: bearing-type and slip-critical connections. Bearing-type connections rely on the bolts to support the load through both shear (the force acting parallel to the surface) and bearing (the force pushing straight down through the bolt). On the other hand, slip-critical connections rely on friction to hold the parts together. For slip-critical connections, it’s crucial that they do not slip under normal use, meaning they must be tightly scuffed against each other to avoid movement.

Think of a bearing-type connection as stacking plates on a table, where the weight is distributed through the bolts like pillars holding each plate. In contrast, a slip-critical connection is like squeezing together two pieces of wet clay – once they bond through friction, they shouldn't move apart easily.

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

The nominal strength of a bolt in tension, which indicates how much load the bolt can safely bear, is calculated using the formula R = FbA. Here, Fb represents the bolt's tensile strength and A is the area of the thread that bears this tensile load, referred to as the tensile stress area. The area used in this calculation is slightly smaller than the total area of the bolt, leading to a ratio of 0.75 to 0.79, meaning that only about 75% to 79% of the bolt's total area effectively contributes to its strength.

If we think of a bolt like a rope holding up a weight, the tensile strength represents the maximum weight the rope can hold before it breaks. The effective area through which this weight is distributed helps to understand how much 'pull' the rope can withstand before it fails, ensuring we don’t exceed the safe limit during use.

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