AD 298: Guidance on the use of Quicon slotted hole connections
This AD presents guidance on the use of the Quicon slotted hole connection system based on experience gained during a recent application of the system. This guidance is supplementary to the design information presented in P338, which is still valid. The following issues have been identified as needing additional guidance:
- Twisting of primary beams
The moment-rotation behaviour of a Quicon cleat when connected to a column is very similar to that of a fin plate, exhibiting significant rotational stiffness compared to a true pin. However, when connected into the web of a beam that is itself free to twist, there is less end restraint and the mid-span beam deflections approach the predicted simply supported values. The SCI recommends that Quicon should be treated like any other type of simple connection when predicting beam deflections, allowing for any flexibility in the supporting member. A beam supported by Quicon connections cannot deflect by more than its theoretical simply supported value for a given magnitude and distribution of load.
Designers should note that there has been a tendency in recent years to place the concrete in floor slabs to the required level, irrespective of the slab thickness. When deflections in the metal decking, secondary beams and primary beams are taken into account, the additional weight of concrete can be very significant, leading to deflections well in excess of those based on a uniform slab thickness. This is the case irrespective of the connection type.
It is common practice to precamber long span secondary beams to either 2/3 or 3/4 of the simply supported dead load deflection, which assumes that the supports provide some stiffness to reduce the deflection. This approach is still recommended where Quicon connections are used, provided that the support is sufficiently stiff to prevent rotation of the cleat. This will always be the case with beam-to-column connections and should also be true for many beam-to-beam applications. Where the supporting beam is susceptible to twist (e.g. an edge beam), it may be advisable to increase the precamber to the full dead load deflection. However, all of the secondary beams, including those that connect directly to the columns, will need to be precambered by the same amount to avoid potential difficulties on site.
Where a primary beam supports a secondary beam on one side only, it will be subjected to torsion due to the unbalanced loading. The resulting twist in the primary beam will depend on the magnitude of the torsion and the torsional rigidity of the primary beam and its connections to the supporting columns. In this respect, there are two issues that could result in greater twist when using Quicon compared to other simple connections:
- Where a Quicon cleat with 2 columns of studs is used in beam-to-beam connections, the increased eccentricity associated with the second column of bolts results in increased torsion in the primary beam, thereby increasing the twist.
- As the studs in a Quicon connection are not tightened against the cleat (only against the beam web), there is sufficient slack in the connection to permit a small amount of twist between beam and cleat. This could contribute to the twist experienced by the primary beams when Quicon connections are used between the primary beams and the columns.
Designers are advised to consider the use of flexible end plates for the connections between the edge beams and the columns in cases where significant twist is expected.
Where the construction programme calls for the concrete slab to be poured on one side of a line of primary beams before the steel has been erected on the other side, the primary beams in question should be considered as edge beams in the temporary condition. It is important that twisting of these beams is limited in order to facilitate the erection of the adjacent secondary beams.