Just about every steel structure today will use bolts as the primary means of connecting the various parts together. For the uninitiated, the appropriate selection of suitable bolts can be somewhat intimidating – There are literally hundreds of possible bolt and nut combinations – not to mention, standards, material types, grades, classes etc. – add to that, the varieties of nuts, washers and thread types, then the possible options can make the head spin.
In this series of articles, we’ll try to simplify the bolt selection process and explain in layman’s terms the fundamentals of bolts and what, as a detailer, you should know and understand.
Firstly, we’ll look at some basic bolt terminology:
Thread – This applies to partially threaded bolts where the useable thread length is given from the end of the bolt.
Runout – This is the transition distance between the thread and the shank and though it may not seem important, the Runout distance will come into play when we look at determining the relationship between the bolt thread and the Shear Plane.
Shank – This is the smooth unthreaded portion of the bolt. The length of the shank is never given in the bolt catalogues because it’s variable depending on the runout.
Dia. – The bolt diameter is always measured at the shank
Grip – The grip is the effective thickness of the connected parts. If a washer is used, the grip is measured from inside the washer.
Now that that’s out of the way, we’ll look at how bolts actually work:
Bolts act in 3 ways: Shear, Tension, and Compression.
When the load is perpendicular to the bolt axis, the joint is said to be in Shear – the tendency is for the connected parts to slip until the bolt is bearing directly onto the inside face of the hole – in this instance, the bolt resists the force by Bearing and failure of the connection occurs when the force is greater than the shear capacity of the bolt.
The shear force is at its greatest at the contact planes of the connected parts – this is known as the Shear Plane.
Where 3 parts are bolted together, the joint is said to be in Double Shear which can transfer twice the force as a bolt in single shear.
Alternatively, the bolt could be tightened in such a manner that the force is resisted by the friction between the connected parts created by the bolt tension – The tension applied to the bolt is achieved by tightening to a predetermined torque value – These are known as Slip-Resistant connections
For these connections, the mating surfaces must be properly prepared in order to maximize the friction forces. Usually this would require cleaning, descaling, roughening and/or blasting.
Typical applications for Slip Resistant connections include:
- Moment Resisting Connections
- Joints that use oversized holes
- Joints that are subject to fatigue load with reversal of the loading direction
- Joints that use slotted holes with the applied load parallel to the direction of the slot
- Joints in which slip at the faying surfaces would be detrimental to the performance of the structure (e.g., sensitive machinery
- Joints that use partial length cover plates
Tension and Compression
When the load is parallel to the bolt axis, the joint is said to be either in Tension (Pulling apart) or Compression (squeezing together)
In both cases, there are no shear planes and no slip to consider. The capacity of the bolt is the same irrespective of the number of connected parts and only the tensile strength of the bolt is considered.
Strength of Bolts
Standard Bolt Diameters, Lengths, and Thread Types
For load bearing structural connections, Bolt Diameters and Thread Types are usually standardized and kept within a specific range – this not only simplifies the connection design but offers numerous practical advantages. The recommended Clearance Hole Diameters are a function of the bolt diameter and are key to ensuring the correct fitment at the connection.
Bolt Lengths are divided into Preferred Sizes – Usually, these are in 5 mm (1/4″) increments. These are basically standard lengths carried by suppliers – anything outside of the preferred range may require an additional lead time when ordering.
Do's and Dont's for Bolted Connections
here are certain things the detailer can do to make life easier for the workshops and site erection personnel,
- Never mix bolt classes or grades in a single structure – you can be sure at some point the wrong bolt will be placed in the wrong position, which could be a real problem. Choose one, and stick with it. Common practice is to use class 8.8 (Grade A307) bolts for diameters of M 20 (3/4″) and above.
Where slip-resistant bolts are required, it’s always a good idea to clearly mark holes in the workshop by painting a circles around the hole outside of the grip area.
- Try to limit the different bolt lengths used on a single structure. Bolt lengths are usually standardized in 5 mm increments, so try the graduate the bolt lengths used on the project in 10 mm increments – this will not only make the bolts easier to visually identify but it will conform to the recommended protrusion of between o and 10 mm.
The detailer could also consider using fully-threaded bolts, provided allowance has been made in the design for the reduced cross section area. This must be approved by the responsible engineer!