Bolt Bearing Type Connection (Bolt Shear)

Bolt Bearing Type Connection (Bolt Shear)

A discussion of the bolt bearing type connection, which places the bolts in a joint under shear loading. This page is divided into the following sections:

  1. Introduction
  2. What is a Bearing Type Connection?
  3. Failure Modes of a Bearing Type Connection
  4. When to Avoid Loading Bolts in Shear
  5. How to Avoid Loading Bolts in Shear
  6. Maximising Shear Capacity
  7. Note of Caution

References

  1. EN 1993-1-8:2005 – Eurocode 3 – Design of Steel Structures – Part 1-8 – Design of Joints
  2. E. Moore, F. Wald – Design of Structural Connections to Eurocode 3 – Frequently Asked Questions
  3. P. S. Green, T. Sputo, P. Veitri – AISC – Connections Teaching Toolkit – A Teaching Guide for Structural Steel Connections
  4. G. L. Kulak, J. W. Fisher, J. H, A. Struik – AISC – Guide to Design Criteria for Bolted and Riveted Joints, Second Edition

1. Introduction

As noted on the page describing the nominal shear capacity of bolts, it is widely considered in precision mechanical engineering applications (and certainly for high integrity applications in the nuclear industry), that using bearing type connection and loading bolts in shear is not ideal and should be avoided where possible. Having said that, many structural engineers do like to design joints that transfer shear load across bolts and in fact they are very common in typical building structures. This type of joint is permitted in the Eurocode and Clause 3.4.1 of EN 1993-1-8 [1] refers to this types of bolted connection as a “Category A: Bearing Type Connection”.

2. What is a Bearing Type Connection?

In a bearing type connection, the forces are transferred by bearing of the connected parts against the shank / outer surface of the bolt. Consequently, the bolts experience shear across the bolt section. A bearing type connection is assembled such that it is at least “spanner-tight”. I.e. the bolt clamping force is sufficient to produce enough friction force between the connected parts to transfer a small load without the joint slipping. Once the applied load on the joint is increased during service / erection then the friction is overcome and permanent slip occurs. This slip closes the clearance between the bolt(s) and the hole(s). The outer surface of the bolt(s) comes into contact with the part(s) being bolted and the joint slips no further. When this process happens suddenly, the joint can give off an almighty bang, like a gun shot. This phenomenon is imaginatively referred to as “bolt banging” and is generally regarded as harmless on statically loaded structures.

When additional load is applied to the joint, there is an elastic response until plastic deformation starts in either the outer surface of the bolt, the connected part, or in both simultaneously. In most connections the bolts are generally stronger and harder than structural steel; therefore it is the material being connected that yields first [2]. As the bolts are subject to equal and opposite bearing loads, they are now also experiencing a shearing load and often, some bending moment. The magnitude of the bending moment is dependent on the distance between the point of application of the shear forces and is occasionally considered negligible.

2.1 Lap and Butt Joints

Not to be confused with a late night venue…  In engineering terminology, lap and butt are the words used to describe different joint configurations for the connection of loaded members:

  • In a lap joint, the two loaded members are connected by overlapping them. The connection between the two parts and the transfer of load is made using a bolt, pin, rivet, dowel or similar. Single and double lap joints are illustrated in Figure 1 (a) and (b), respectively.
  • In a butt joint, the two loaded members a placed end-to-end (abutting) with one or more additional splice plates fastened on one or both sides of the joint to transfer the load. The splice plate is often also referred to as the fish plate or cover plate. Single and double butt joints are illustrated in Figure 1 (c) and (d), respectively.

Bearing type connection - Lap and butt joints

Figure 1 – Lap and butt joints

 

2.2 Single-Shear Connection

A single shear connection is one in which the bolt, pin, rivet, dowel or similar is only able to resist the separation forces across a single shear plane. A very simple illustration of a single-shear lap joint connection is presented in Figure 2, where: (a) illustrates a single-shear lap joint under equilibrium loading; (b) illustrates the failure mode in which the rivet is sheared (separated) across the shearing plane; and (c) illustrates failure of the connection due plastic bending of the splice plates.

The bending failure illustrated in Image (c) occurs due to a moment induced in the splice plates by virtue of the offset of the applied forces. This is a considerable disadvantage of the single lap connection under high loads. In fact, bending of the splice plates can lead to a prying action across the bolt and subsequent failure of the fastener due to tensile loading.

Bearing type connection - Single shear lap connection

Figure 2 – Single-shear lap connection

This type of connection is referred to as a single-shear connection as the bolt (or in this case rivet) resists the shear load across a single shear plane. The shear stress induced across the single shear plane is simply:

(1)   \begin{equation*} \begin {split} \tau & =\frac{F}{A} \\ & = \frac{4F} {\pi d^{2}} \end{split} \end{equation*}

2.3 Double-Shear Connection

A double-shear connection, is one in which the bolt, pin, rivet, dowel or similar is able to resist the separation forces across two shear planes.  A very simple illustration of a double-shear lap joint connection is presented in Figure 3: (a) illustrates a double-shear lap joint under equilibrium loading; and (b) illustrates the failure mode where the rivet is sheared across the two shearing planes.

Bearing type connection - Double shear lap connection

Figure 3 – Double-shear lap connection

This type of connection is referred to as a double-shear connection as the bolt (or in this case rivet) resists the load in shear across two shear planes. The shear stress induced across each shear plane of the rivet is given by:

(2)   \begin{equation*} \begin {split} \tau & =\frac{F}{2A} \\ & = \frac{2F} {\pi d^{2}} \end{split} \end{equation*}

We can see, intuitively, that the shear stress is halved as we have doubled the shear area (two shear planes). The shear capacity of the fastener is therefore doubled, although the load transmitted through the single plate remains the same (F).

The double-shear connection is a very common design feature, for example it is often seen in structural bolted connections for transferring shear loads and lug / clevis connections at the end of tension rods or hydraulic rams. The double-shear connection is generally preferable to the single-shear connection as it generates a balanced and symmetric load transfer with respect to the plane of the connected plates. I.e. there is no moment induced in the loaded members. It also produces double-shear conditions in the fasteners and thereby reduces the required number or size of fasteners.

2.4 Multiple-Shear Connection

It follows, of course, that a lap or butt joint can be formed with multiple overlapping members connected by the same bolt(s); thereby providing multiple shear planes and a commensurate reduction in the peak magnitude of shear stress in the bolt(s).

3. Failure Modes of A Bearing Type Connection

It should be noted that there are a number of possible failure modes that can affect a bearing type connection. The shear failure of the bolt itself is not the only failure mode of concern.

3.1 Shear Pull-Out / Edge Tear-Out

Shear pull-out, also referred to as edge tear-out is the failure of one (or more) of the connected parts through shear.  Instead of shearing across the bolt section, the failure is shearing of the material between the hole and the edge of the plate. The line of shear failure is nominally parallel to the direction of loading. Effectively, a slug of material is dislocated from the plate. The failure mode is presented in Figure 4. Image (a) illustrates a connection under loading and image (b) illustrates the characteristic appearance of an edge tear-out failure. The failure is rapid and generally not easily detected by inspection.

There are fairly simple rules of thumb that can be used to prevent tear-out. Many codes and standards stipulate a minimum distance from the edge of the bolt hole to the edge of the plate. An edge-to-hole diameter ratio of around 1.5 is considered reasonable. This will be covered in greater detail in a separate article [REFERENCE AND LINK: TBC].

Bearing type connection - Shear pullout failure

Figure 4 – Shear pullout failure

3.2 Net Section Failure / Tensile Rupture

Net section failure is characterised by tensile fracture across a line of bolts, perpendicular to the line of force action. Stress in a tension member is nominally uniform throughout the cross-section; however the loss of material due to holes or other discontinuities reduces the effective area for tensile resistance and generates stress concentrations. The failure mode is presented in Figure 5. Image (a) illustrates a connection under loading and image (b) illustrates the characteristic appearance of net section rupture.

Bearing type connection - Net section failure (tensile rupture)

Figure 5 – Net section failure

Prior to failure there will be some localised yielding at the reduced section; however there will be limited or no yielding across the main gross section. Tensile fracture will occur suddenly and with little warning. Design guidance to mitigate this failure mode will be covered in greater detail in a separate article [REFERENCE AND LINK: TBC].

3.3 Block Shear Rupture

Block shear rupture is a failure mode characterised by combined shear and tensile fracture local to a bolt pattern. The failure path includes an area subject to shear and an area subject to tension. The failure mode is presented in Figure 6. Image (a) illustrates a connection under loading and image (b) illustrates the characteristic appearance of block shear rupture.

Bearing type connection - Block shear rupture

Figure 6 – Block shear rupture failure

This limit state is so named because the associated failure path tears out a “block” of material [3]. Design guidance to mitigate this failure mode will be covered in greater detail in a separate article [REFERENCE AND LINK: TBC].

3.4 Excessive Plastic Deformation at the Hole

Highly localised compressive (bearing) stresses are generated when bolts contact the edge of bolt holes. This bearing stresses can rapidly exceed the yield stress of the material, which results in local plastic deformation of the hole. Excessive plastic deformation at the holes is generally not of concern for structural stability / safety; however it does have an effect on serviceability. That is, excessive deformation results in larger holes and the potential movement of the joint out of tolerance and subsequent excessive global deflection of the structure.

Bolt bearing is applicable to each bolted ply of a connection. The failure mode is presented in Figure 7: image (a) illustrates a connection under loading; and image (b) illustrates the characteristic appearance of excessive plastic deformation at the hole edge.

Bearing type connection - Plastic bearing deformation at bolt hole edge

Figure 7 – Excessive plastic deformation at the hole edge

4. When to Avoid Using a Bearing Type Connection

It is necessary to avoid using a bearing type connection and loading bolts in shear in a number of cases.

4.1 – Precision Machines

In precision machine assemblies and structures with moving parts where tight tolerances are required it is not advisable to use a bearing type connection. We know that for bolts to be loaded in shear, the joint needs to “slip” slightly to take up the clearance between bolt and bolt hole. Therefore it is not really possible to use a bearing type connection when tight tolerances on positioning are required due to the movement of the parts under design load. It is not a good idea to use a bearing type connection in any form of machine assembly or structure with moving parts.

4.2 – Joint Subject to Variable Shear Load

It is inadvisable to use a bearing type connection where the joint experiences a variable shear load across it with changing direction (“sign”). In this case the clearance between the bolt and hole allows relatively large displacements to occur repeatedly. In other words, the joint will clatter about and make a terrible racket before eventually crashing to the ground around you in a heap of twisted metal and gross negligence claims.

4.3 – Joint Subject to Vibration

It is inadvisable to use a bearing type connection where the joint experiences vibration. When a joint is subject to vibration or small changes in loading, loosening of the bolts can occur. Bolt loosening is an undesirable occurrence where the level of undesirability spans an unpleasant spectrum from unsettling, to catastrophic.

4.4 – Safety Critical Applications

It is generally not possible to guarantee that the load is shared evenly between bolts in a large pattern due to fabrication and erection tolerances. In fact it has been empirically proven that when a lap joint is placed under load, the outer bolts in a bolt pattern (in the direction of load) come into contact with their holes first [4]. This type of observation makes nuclear inspectorates quite nervous. It is noted however, that aerospace and automotive manufacturers use bolts and rivets in shear; however these joints are generally precision bolts located in precision, tight fitting holes. My personal experience is more of heavy industrial structures and machines.

5. How to Avoid Using a Bearing Type Connection

Instead of using a bearing type connection, the following approaches can be taken to transfer shear loads across a joint:

  • Use tight tolerance shear dowels, pins, keys or stepped / serrated mating interfaces to transfer the shear loads.
  • Designed the joint as a pre-loaded (pre-tensioned) high strength friction grip joint, such that shear loads are transferred by friction between the mating parts.
  • Completely reconsider the design and rotate the plane of the joint face such that transverse (shear) loading is lower.

6. Maximising Shear Strength

Empirical studies have shown that for a bolt loaded in a double-shear configuration, the shear capacity is maximised when both shear planes cross the shank portion of the bolt. Shear capacity is minimised when both shear planes cross the threaded portion of the bolt [3]. This is intuitive, as shear capacity is directly proportional to shear area. This observation also agrees with the approach taken by EN 1993-1-8:2005 [1]. Therefore, to maximise the shear capacity of a bolted joint, ensure all shear planes cross the shank portion of the bolt. In any case, this is good design practice.

7. Note of Caution

Hopefully it is clear; however for the avoidance of doubt, just because the bolt(s) can take the shear loading, it does not mean that a joint is safe. All other failure modes (limit states) such as those listed above, must be assessed.