Chapter 4 - Bolted Connections
The primary structural fasteners used in steel construction have typically been rivets, bolts and pins. These fasteners can be field installed cheaper and with less problems than welds.
Bolts are generally installed so that they are either perpendicular to the force (i.e. the force causes shear in the fastener) or parallel to the force (i.e. the force causes tension in the fastener) that they are transferring between members. In some cases they have both shear and tension.
Structural fasteners, as we will see in the next section, transfer shear force between two members by both the shear strength of the fastener and by friction enabled by the clamping force the fastener creates between the connected parts.
Rivets have essentially disappeared from modern steel construction, however there are many existing connections that contain rivets so the modern engineer must be somewhat familiar with rivets, how they were installed, and how to determine the capacity of connections made with rivets.
One thing to note is that rivets provide a very inconsistent clamping force so determining friction capacity for shear transfer is problematic. The capacity of rivet connections is best done considering only the bearing capacity.
Since the SCM does not address riveted connection, we will not spend any more time on them.
Pins are generally smooth large diameter fasteners that are not threaded. These fasteners are not very common. Pins are always placed perpendicular to the load direction and are in shear. Since pins are not threaded, they do not clamp the connected members together and, consequently, do not enable friction based force transfer between the connected members.
In this chapter we will focus principally on the capacity of bolts as they are the preferred structural steel fastener. The three types of bolt steel that we will consider were discussed in BGSCM 2.3 and are:
The method in which bolts are installed is also important to how they transfer load.
Bolts may be installed as "snug tight" or "pretensioned". This would be a good time to look in Chapter J3 of the specification (SCM page 16.1-126). SCM J3.1 discusses the installation of bolts.
The definition of "snug tight" has gone through a number of revisions in the past decade, but was been returned to it's historical version in 2015: "The tightness that is attained with a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the plies into firm contact." (RCSC Specification errata, April 2015)
It should be obvious from the discussion in J3.1 that the snug tight installation requires less effort than the pretensioned installation and is, consequently, less expensive. The SCM points out that there are limitations on the use of snug tight installations.
The main difference between the two installation methods are that pretensioned bolts will create a significant clamping force between the connected parts where as bolts that are installed snug tight do not. This is significant in that the clamping force caused by pretensioned bolts allows the force between members to be transferred by friction between the plies and there is not movement between the members until the friction capacity is overcome. For snug tight connections, the plate will slip until they bear on the bolts. Slipping is not generally good if the bolt sees a reversing load. We'll discuss this more in the next section.
On the practical side, it is common to use a single bolt type (i.e A325 or A490) and a single bolt diameter throughout an entire project. One of the key design concepts is to keep things as consistent as possible so as to avoid complexity that may lead to errors in the field. It is a rare case when an engineer will knowingly violate this concept.
Designating Bolt Type and intended use/installation Information
When designating bolts to be used on a project, it is important that the engineer transmit certain information to those who will build the project. This is typically done giving the ASTM material specification followed by a dash then a set of letters. The common letter sets are:
For example, an A325-X bolt would be a bolt made to the A325 requirements and would be installed such that the treads are excluded from any shear plane(s). An A490-SC bolt is a bolt made to the A490 requirements and would be installing on a slip critical connection where the faying surfaces must meet special requirements.
The primary objective of checking all strength based limit states to ensure that the strength of the structural element is strong enough to handle anticipated forces exerted on them. In the case of bolts, this can be expressed as:
The FORCE on the bolt < the STRENGTH of the bolt
For bolts the forces applied to the bolts can be resolved into shear (perpendicular to the bolt axis) or tension (parallel to to the bolt axis) components.
Force on the Bolts
The force on any given bolt is the result of the forces being applied to the connection and the geometry of the connection. Principles of Mechanics and Structural Analysis are used to determine the force on any particular bolt in a connection. The next section discusses several commonly used methods for computing the forces on a bolt.
Strength of a Bolt
Bolts have one tensile capacity and two shear capacities. Typically the SCM denotes the nominal capacities of each as Rn. The capacities are defined by limit states.
Tensile capacity is controlled by the tensile rupture limit state which can be stated as:
The TENSILE FORCE on the bolt < The TENSILE RUPTURE STRENGTH of the bolt
Shear capacity is controlled by one of two limit states:
For the case of shear capacity, the limit states can be stated as:
The SHEAR FORCE on the bolt < The SLIP STRENGTH of the
It is common in connections with pins, rivets, and bolts to connect more than two members together with a given fastener. The force transferred by a fastener then becomes a function of the number of shear planes in the connection. As will be seen in the next section, a more convenient, or maybe more appropriate, way of expressing the shear limit states is:
The SHEAR FORCE on a SHEAR PLANE < The SLIP STRENGTH of
a SHEAR PLANE
The concept of shear planes is presented in the next section.