End Cleats are usually used to connect beam-to-beam - they may also be used for beam-to-column connections but there are limitations, which we'll look at later in this article
Cleats are usually fabricated from standard hot-rolled angle sections and may be configured either as ‘Single’ or ‘Double’. Single cleats offer many practical advantages, the most obvious being the fact that only half of the material is used, in addition, their configuration makes them relatively easy to erect, and they can be useful when erecting beams close together and space is limited. But questions have been raised about the eccentricity of the connection, and the tendency to ‘twist’ during erection making the tightening of the bolts sometimes difficult, as a consequence, they have not gained widespread acceptance and the ‘double’ configuration remains the de-facto standard.
One exception is when considering cleats for channel section beams, often one cleat is sufficient which may be situated either on the inside or outside of the web.
The big advantage with End-Cleats is that they can be loosened, or removed altogether on site to facilitate erection, and being adjustable allow a little more tolerance to the fabricator. To make the most of this advantage the Cleat should protrude from the beam end by approximately 5-mm.
They are also very adaptable in that they may be used on all connections from the largest to the smallest, and being fully bolted connections, are resistant to fatigue. They are therefore commonly used on structures which may be subject to high deflections and vibration.
To accommodate all the various load conditions, cleats may be classified as (rather unimaginatively) Small, Medium, and Large.
Small Cleats would typically be used on small beams (less than 200 mm deep) where only one row of bolts can be accommodated. To prevent hinging of the connection a double row of shop-bolts will be needed connecting the cleats to the beam-web. They are typically formed from unequal-leg angles.
Medium Cleats are generally considered suitable for beams greater the 200, rising to approximately 900 mm deep. Standard cleats will make up a great majority of connections likely to be encountered. They are typically formed from standard equal-leg angles.
Large Cleats, using a double row of bolts in both directions and would apply to extra-heavy beams and plate-girders typically exceeding 1000 mm in depth. The bolts connecting the cleat to the beam web may be either in-line or Offset. When offsetting the bolts, it may be possible to reduce the back-marks on the angles which will most likely require the pitch of the bolts to be increased. In the interests of standardization, bolts will be assumed to be in-line.
They are typically formed from standard equal-leg angles, or for heavier duties may be fabricated from plates with internal stiffeners.
To Determine the Bolt-Hole Gauge
Establish the Minimum Back-Mark
The first step in determining the gauge is to establish the minimum back-mark on the angle cleat – this is measured from the heel of the angle to the centre-line of the bolt-hole (bk) - remember, this is the minimum value and will be rounded up to take into account the beam web thickness. This you will see when we look at Standard Cleats
The main concern is to ensure there is sufficient clearance between the shop-bolts, which connect the cleat to the incoming beam and the wrench socket used for tightening the bolts during site erection.
The following illustration and tables are based on the following:
- The Socket Diameter (a) is based on ISO standards, which may vary according to the manufacturer
- The value for (b) is calculated by adding the cleat leg thickness (which is taken as the bolt diameter / 2), the nut thickness (assumed to be a class 8.8 precision bolt) and the nominal protrusion. See Table below. - Note that no washer has been allowed.
- The protrusion has been assumed at 6 mm. Note that the thread will be inside the shear-plane of the bolt - therefore, the reduced shear value must be assumed. See Fasteners > Shear Plane
- The clearance dimension (c) is assumed and will need to be adjusted to conform to a preferred standard bolt length. See Fasteners > Preferred Lengths
The minimum back-mark (bmk) has been calculated as - b + c + a/2 (Table 1 below)
Standard Gauge vs Standard Back-Mark
Option 1 - Maintain the Back-Mark
The back-mark is the distance between the heel of the angle and the centre of the bolt-hole (bk), by keeping this dimension constant the cleats will all be the same. The advantage with this is that the cleats will all be the same, allowing the fabricator to pre-manufacture as many as required as a concurrent activity and simply adding them to the beam-end as and when required. There’s an obvious benefit here in both time and cost.
However, the drawback here is that the gauge of the bolt-holes will vary according to the web thickness of the beams, if the beams are all the same, then the problem goes away, but if they differ, it could lead to an issue with Shared Bolts.
If the beam is a channel section, issue becomes more pronounced. Good practice recommends that the gauge is measured from the heel of the channel, which is fine if there is only a single cleat (fastened to the heel), but if it’s a double cleat, then the gauge will not be symmetrical about the heel of the channel which, as we’ll see later, can cause a few complications.
Option 2 - Maintain the Gauge
Maintaining the bolt-hole gauge means that the Back-Mark will vary according to the beam web thickness, again, if the beams are all the same then there is no issue, but if they vary widely the cleats themselves will vary accordingly, negating the benefits of maintaining the Back-Mark. However, the advantage with this method is that any potential problems with Shared Bolts goes away.
Returning to the channel section, maintaining the gauge does present more difficulties because the back-marks will not only be variable, but they will differ from each other.
It can be argued that the differences will most likely be slight and that the normal fabrication tolerances will compensate for any misalignments, but the detailer should bear in mind that these tolerances belong to the fabricator and to ignore that would be considered bad, if not risky practice.
If the steelwork is accurately modelled and detailed using the Parabuild, then the built in automatic clash detection will compensate for any misalignments, but if it’s to be detailed manually then the detailer will need to be wide awake.
At the end of the day it boils down to a matter of choice, both options work and whichever one is chosen will have no impact on the integrity of the connection. In the interests of Standardization, we’ll consider maintaining the gauge as standard – the standard gauge will fall into line with that proposed for Welded End Plates, which will allow for a certain amount of interchangeability, which though not generally recommended, can be useful in certain circumstances.
One last fact to be consideded when comparing the options is that by standardizing the back-mark it's possible to use a smaller angle cleat - See Table 3 below. This will be looked at in more detail under Standard Cleats
Vertical Bolt Pitch
To determine the vertical bolt-hole pitch the distance between the top of the beam and the first row of bolts (v1) must be established - this is a function of the Notch Depth (whether there is one or not) and the vertical edge distance between to top of the cleat and the centre of the first row of bolts.
Dimension (d1) is the distance between the top of the beam flange and the top of the cleat. In most cases (d1) will equal the notch depth, but there are circumstances where they will differ - which we'll look at.
(e1) is the vertical edge distance which should not be less than 2 x the bolt diameter.
Refer to Tables 4 and 5
There will be instances when two adjacent beams with different calculated notch depths connect to a common web thus sharing bolts. In such cases, the cleat may be lowered (on the smaller beam) to match that of the greater. Alternatively, the notch may be deepened.
It will be found that a great majority of Notch Depths will fall into the 0 - 50 mm range - so though this will always be a consideration, it should never be a major issue.
For more information on standard notch depths see Notches.
End Cleats Connecting to a Column
It must be said that Cleats are not ideal for connecting to columns because the bolt-hole gauge is pre-determined by the column size. When connecting to the column flange it's not such a problem provided the flange with is not less than 200 mm. This will enable the use of Standard Cleats.
Connecting to the column web is another issue. To establish the bolt-hole gauge you must work backwards from the cleat size - the measurement from 'over-cleats' must be less than the depth of the column web between the root radii (hw)
For this reason cleats will not work if the depth of the column is less than 250 mm - which may have to be increased if heavy -H- sections are used having thicker flanges.
Alternatively, a plate may be welded across the toes of the column flanges to which the cleats are bolted.