Today it’s very rare for the engineer to design each and every flexible connection, often he or she will not even select a connection type, preferring instead to leave it up to the fabricator to select such connections best suited to preference provided they meet the design requirements. The fabricator will often delegate the selection to his in-house engineer, but smaller workshops will not always have the luxury of such input and will instead leave it up to the detailer to take on this responsibility.
It should be noted here that there is a difference between ‘Design’ and ‘Selection’. Design is the responsibility of the engineer, it is he or she that carries the ultimate responsibility for the overall integrity of the structure, including the connections, so, delegating the task of connection ‘Design’ does not necessarily relieve the engineer of the responsibility.
Connection design is really quite complex; a detailed analysis requires the following:
- Check the shear capacity of the bolt-group, which must be greater than the end-reaction of the beam
- Check the shear and bearing capacity of the end-plate or cleat.
- Check the shear capacity of the beam web, especially if the beam is notched (or Coped)
- Check the capacity of the fillet-welds connecting the end-plate to the beam-end, or in the event of a Fin-Plate, the capacity of the fillet-weld connecting the plate to the supporting member
- For end-cleats, check the shear capacity of the shop-bolts
- Check the local shear capacity of the supporting member web
- The structural integrity requirements, which involves checking the tension capacity of the connection, which must be greater than the tie-force.
In selecting a connection, the detailer would apply the ‘Select-and-Check’ method, whereby the connection is selected according to preference and checked that it will fulfill the design requirements.
Checking the Connection
Fortunately, in checking the connection it’s not necessary to conduct a full design analysis each and every time, experience in the use of such connections and continuing research has meant that empirical values have been given to the connection components meaning that - provided the connection has been detailed to conform to these values in a vast majority of cases it will prove to be adequate.
The beam-end shear reactions required to check the connection should be provided by the responsible engineer, if not, then it’s customary practice to assume the maximum allowable uniform load of the beam which is a function of the beam size and its unsupported span. The draftsman then only needs to ensure that the shear value of the ‘standard’ connection is not less than the designed reaction.
Tabulated shear resistances for connections having standard dimensions are printed and distributed in tabular form by all the recognized steel construction institutes.
The ‘Select-and-Check’ principle does have one drawback – and that is the connection is not purpose designed to suit each specific condition, and the resultant ‘selected’ connection will often be over-designed using perhaps more bolts than necessary adding to the cost. But, this additional cost must be weighed against the benefits of the time saved by the detailer and the accrued benefits to the fabricator and erector of the ability to standardize
Other factors to take into account is the preferences of the fabricator:
The fabricators preferences must always be taken into consideration and is a key factor in correct connection selection. Most fabricators have them and they are usually determined by the facilities available.
These facilities can be summed-up by the levels of automation. Many workshops today employ CNC (Computer Numerically Controlled) technology, and much of this equipment is designed for specific operations such as, drilling or punching, profile-cutting, welding, etc. which can mean that certain workshops are more efficient in, but not necessarily limited to one operation over another. Workshops which are not so automated are generally more flexible and able to accommodate a wider selection of connection types, but even these will have preferences. The draftsman, especially if he or she is contracted to the fabricator, should be aware of these preferences and prepare the drawings and details accordingly.
The benefits of standardization cannot be over-stated. Connections will be found to make up less than 5% of the total mass of a typical steel structure but will be found to account for as much as 35% of the total fabrication costs, so any benefit that can be gained by streamlining the connection design and selection process will have a significant impact on the financial viability of ant construction project. Standardization is achieved in two steps:
- General Standards
- Standardizing the connection components.
The following are simple guidelines:
- Limit the connection options. Choose one and stick with it for the entire project, whether it be End-Plates, Cleats, or Fin-Plates. It’s never a good idea to mix-and-match connection types, standardization creates repetition which is beneficial to the fabrication process, it will also go some way in reducing the ‘learning-curve’ apparent at the beginning of the site-erection process. There may be instances where it’s unavoidable, but this should be regarded as an exception.
- Wherever possible, use standard ‘Flats’ for End-Plates and Fin-Plates, meaning they can be simply cut to length, reducing off-cuts and wastage. If the project is detailed using 3D Modelling and Detailing software then this may not necessarily be a great advantage - the software should be able to optimize the cutting and profiling.
- Choose one bolt-diameter and stick with it as far as possible for the entire project. This will mean the workshop can set-up for production without having to continually stop to change drills or punches, it will also benefit the erector who won’t have to have multiple wrench sizes on-hand in order to tighten joints.
- Never mix bolt-grades. It will inevitably mean that at some point the wrong bolt will be placed in the wrong position. Where slip-resistant or HSFG bolts are to be used it’s a good idea to mark the bolt group with a marker or dab of paint being careful not to contaminate the joint mating surfaces.
- Consider using fully-threaded bolts, it will ensure fewer variations on bolt length.
Standardizing the Connection Components
Standardizing the connection components is of incalculable benefit to the fabricator, it opens the door to ‘Production-Line’ fabrication whereby the workshop can be set-up to manufacture standard components. In doing so, the various fabrication processes can be completed as concurrent activities thus streamlining the entire process.
Standardizing the connection components begins with the following:
- Standardize the bolt-diameters, which in this publication have been set at M16 – M20 and M24. These bolt diameters together with their imperial equivalents are tabulated under ‘Standard-Connections’
- Standardize the Edge-Distances which are a function of the bolt diameter. For connections designed to resist shear only, the vertical edge distance should not be less than 2 x the bolt-diameter, while the horizontal distance should not be less than 1.5 x the bolt-diameter.
- Standardize the horizontal bolt-hole Pitch, which has been set at - 140 / 90 and 70 mm
- Standardize the vertical bolt-hole pitch, which, like the gauge, is a function of the bolt diameter. For flexible connections it should not be less than 3 x the bolt-diameter
- Standardize the Notch or ‘Cope’ This will enable the determination of the distance between the top of the beam and the first row of bolts. The notch depth and length are a function of the dimensional properties of the supporting beam. (Sometimes but rarely, they may be a function of the connecting beam)
Before we take this standardization further, it’s important that the detailer understands the various connection options and the relative practical advantages and disadvantages of each. The Connection Guide, will, it’s hoped give some insight into the various options.