Holdfast Training Services Trial Blog

I remember with joy the moments in the Phase 1 lectures when the Lecturers outline various design approaches (let’s call them easy, complicated and devilish), and then tell us to draw a line through complicated and devilish, “you’re a fool if you choose those options, we’ll focus on the simple option for this tutorial”. Brilliant news.

Looking back through my Steel connection notes this is exactly what I’ve done and all of my Ex Steel joints were designed as simple connections (i.e not moment connections). However, it’s not always possible to draw a line through the difficult options.

One of the projects I’m working on over the next few weeks is the back analysis of steel connections in a building which has experienced a failure during use (a loud bang at night as one of the connections failed). I’ve still got a lot of work to do but below are my initial musings from SCI P207/95.

Figure 1 – Failure of B33 to C13 connection

Figure 2 – Steelwork contractor calculation output

The frame was designed by my consultancy and a steelwork contractor designed the connections. The design of the connections was carried out using Masterkey software to BS 5950.  The BS states that when the design is handed from a Structural Engineer to a Steelwork Contractor the contract documents and design drawings must adequately define the design requirement for the connections.  Further guidance within the National Structural Steelwork Specification for Building Construction states this information should include:

• Design concept
• Member schedule and drawings
• Design standard to be used – in this case BS 5950
• Forces, moments and combinations at each connection – some raking/angled beams also have an axial force to consider.
• Factored or unfactored loads used
• Restrictions on joint details

My task is to back analyse the connections designed by the Steelwork Contractor, using the same software in order to check their connections for revised actions. The reason the actions have been revised is because regardless of whether the connections were designed as simple or moment connections (the two simpler design approaches), the reality is that there will be a degree of deformation at the connection, therefore the joint will really be acting as semi rigid with some degree of rotational deformation. If the connections is partially deforming then part of the moment will be transferred into the adjacent beam, in the case of a double beam to column connection, and part into the column.

Figure 3 – Classification of moment connections as per SCI P207/95

Initial investigations have shown that some of the “simple” connections may be too robust to act as a simple connection, and will in reality transfer some of the moment through the connection. Take note McClure – you shouldn’t always go big early.