When designing any building, whether it be a steel frame, masonry, timber or concrete construction, it’s important to consider the minimum bearing requirements and practical installation limitations. Not only should adequate support be provided to suit the design standard, but it should be designed in a way that considers the buildability aspect of the specific details.
Due to the complexity of some projects and pace of modern construction, it can be easy to miss important bearing details or access requirements during the design stage and we have on occasion experienced details that are unsafe or impossible (due to site obstructions) to work with without remedial site works, which can be costly and time consuming for the contractor responsible for the support details – typically the steelwork contractor.
To help identify issues, SMD have invested heavily in the training and implementation of deck detailing using 3D Models (Tekla Structures and BIMSight). 3D models make it easier to highlight issues to our clients that may have gone unnoticed working with 2D models, avoiding costly remedial works on site.
Some of the more common details where support can be missed are:
- Missing support adjacent to core walls
- Deck unable to bear onto top flange of a beam due to steel connection plates
- Lack of support around beams supporting columns with baseplates on the top flange
- Lack of support to face of large columns, or columns that have no incoming beams where deck spans onto it
SMD Best Practice data sheet DATA/21 (extracted below) has been developed to help designers identify these situations.
A number of key dimensions MUST be adhered to when supporting metal decking on steel beams or angles, this varies depending on the type of support, deck support type (deck sheet end support or intermediate support) and if the beam is composite.
The minimum bearing for metal decking is 50mm on steelwork.
If the beam is designed as composite, this dimension must be increased to 60mm to allow for sheets to receive thru-deck welded shear studs – refer to our blog post ‘Shear studs and composite decking’ for more information on this topic.
Although a 102mm top flange beam is sufficient to support the deck at intermediate or end locations, it’s not adequate for composite beams where deck sheet ends butt together – we suggest that a minimum of 127mm top flange beams are used in all composite beam locations.
Shelf Angles or bottom flanges:
To reduce the structural zone, it’s sometimes necessary to install the decking onto angles fixed to the beam webs or bottom flanges. Where deck ends are supported on shelf angles or bottom flanges between beam webs, the shelf angle or bottom flanges must be designed to extend a minimum of 50mm beyond the toe of the beam top flange. This minimum dimension of 50mm is essential to enable the sheets to be physically positioned between the toes of the top flanges and provide access for a cartridge tool to be used to secure the decking into place.
Where the deck spans parallel to the beam web a structural support angle is the recommended detail. It may be possible depending on spacing of secondary beams to utilise a 2.0mm gauge flashing to avoid the requirement for a structural angle, however this will impact on the slab capacity for high concentrated loads locally in the region of the non-structural flashing.
Where the deck is to butt up against a concrete core or similar, shelf angles are to be installed by others to provide adequate end bearing for the metal deck.
When developing such a detail, consideration must be given to the height of any continuity reinforcement extending from the core to ensure it does not clash with the troughs of the deck profile.
The minimum bearing for metal decking is 70mm on masonry or concrete.
It’s suggested that a minimum block size of 140mm wide is used as, similar to the deck support type on steel supports, it’s often unknown if the wall will be an intermediate support, support a butt joint or be an end bearing support. In the instance of an end bearing support, an edge trim and 70mm bearing are also required.
Listening to our Feedback.
We hope the feedback we give to our clients is helpful and slowly grows industry knowledge, resulting in a good working relationship throughout the design team to develop efficient project designs whilst minimising site issues.
How can we help further?
We offer various pre-contract support solutions to clients and engineers. Our TGN Online is fast becoming the first point of reference to clients and specifiers and our CPD presentations delivered by our Technical team are now frequently being carried out across the country.