Bolted connections are used for various purposes within the industry. Some of the applications can be made up according to product standards or bespoke specifications. These specifications shall then suffice to ensure the integrity of the connection giving an appropriate safety margin.
However, in many cases the bolted connection is subjected to factors not covered by the specification, or gives a reference to other stanards not necessarily pertinent to the components in question. A typical problem is then to determine an appropriate factor of safety. A thorough understanding of the mechanism may thus be required to verify the integrity of the connection. A scenario where this may apply could be a structural connection supporting a pressure vessel.
All engineering should start with an assessment of possible failure mechanisms. Failing of bolted connections can e.g. be:
| Overloading of bolts | |
| Gap between surfaces | |
| Sliding of surfaces in connection | |
| Loss of gasket seating pressure | |
| Damage to gasket | |
| Damage to flange seal faces | |
| Corrosion induced failures |
The number of possible failure modes are usually only limited by the designers imagination. The most common failure modes are however covered by the design methods built into the design checks of relevant design codes. Even though the failure modes may not be described explicitly, the standards say do this.. and check that..., then everything should be OK. Anything not covered by the code/specification is then up to the engineer, and requires a good understanding of both joint behavior and design codes used.
Some of the failure mechanisms can be quantified such that it is possible to indicated a safety margin on them, e.g. the probability for overloading of the the bolts. It is primarily these areas which are covered below.
The selection of the material and surface coating to be used is probably the single most important decision to make when designing a connection. The selection will be primarily based on the environmental conditions around the bolted connection which will govern the likelyhood for e.g. galvanic corrosion or hydrogen cracking. Very often these environmental conditions can be very difficult to establish, and the specifications of the materials to be used can thus more often be based on earlier experience than the assumed conditions for the new joint. The field of material engineering is very complex, and will thus only be mentioned in particular cases.
The safety margin is usually incorporated into the requirements of the equipment/construction to be provided, and the methods to be used to verify its integrity. For general structures or pressure vessels, the two most common calculation methods are linear elastic design using either the allowable stress method or the partial coefficient method.
The partial coefficient method appears more flexible than the allowable stress method as it allows the use of Limit State Design Methodology. The Limit State Design Method is however, primarily used within structural engineering, and not yet adapted in the most used pressure vessel codes which currently use the allowable stress method.
The Limit State Design method is a recognized method for obtaining the proper safety margin for various failure modes.
For bolted connections the codes are not quite as simple. The allowable utilization of the bolts may be based on the general methodology of the design code, but also on experience and testing, resulting in a practical approach to a safe design. An example of this can be found in EN1993-1-8 for pretensioned connections (with EN1090-2 as the execution code). Bolts are allowed to be pretensioned up to 0.7x the ultimate tension strength of the bolts, but the methodology implies that there may be significant yielding in the bolt (considering uncertainty in tensioning, but also that the bolt proof stress which is the allowable limit for visible yielding during testing is approximately 0.93 x the bolt yield stress). Thus yielding is inherent in the methodology of the code, but considered not to jeopardize the integrity of the connection. This can be important for the designer if hardening of the material affects the corrosion resistance of the bolt.
A recognised standard for structural design is EN1993-1-8. Bolted connections in this standard shall be categorized as one of the following:
| Shear Connections | Bearing type | |
| Slip resistant at servicability limit state | ||
| Slip resistant at ultimate limit state | ||
| Tension Connections | Connections with non-preloaded bolts | |
| Connections with preloaded bolts |
Note that this is a structural standard, and does not include bolted connections for pressure vessels. Also the categorization is in reality a selection of which checks are necessary to ensure that the connection works as intended.
EN1993-1-8 refers to the bolt materials in ISO 898-1 and nut materials in ISO 898-2.
A recognised standard for structural design is ANSI/AISC 360. This specification refers to "Specification fo Structural Joints Using High-Strength Bolts", which has several similarties with EN1993-1-8, but also som differences making it difficult to give a general comparison. A general contractual requirement is to use the most conservative code.
The high strength bolting material is specified in ASTM F3125 (conventional bolts formerly ASTM A325 and ASTM A490)..... to be continued