Responsibilities part two - connections with high tying forces

Continuing on with our investigation into responsibilities in connection design, we will take a quick look at the the first item from our list: connections with high tying forces.

What are tie forces?

Tie forces can best be thought of as OH NO forces. As well as being designed for the expected forces a connection should see in its lifetime, connections on all buildings are also designed for another force, one trying to rip all the bits of it apart caused by something unexpected and catastrophic happening elsewhere in the building. These forces manifest as tension in the connection: acting in a different direction to the forces it must ordinarily resist in its usual operating life, which necessitates some additional checks.

What’s the problem then? Why can’t the fabricator just design for the forces?

The problem comes when tie forces are high. When the tie forces are high, the connection is forced to become chunky and therefore stiff. When a connection is chunky and stiff, it is no longer a pin. You are instructed to design one thing - a pinned connection that can resist the forces required - but the design parameters necessarily force you to provide something else.

I still don’t get what the problem is - so it’s not a pin: why do I care?

The problem is that the chunky connections invalidate the design assumptions of the overall frame design. If the connections aren’t pinned, they are (to some extent) fixed. If they are fixed, they bend the columns, and the columns aren’t designed for this bending*, and could potentially fail. It is the responsibility of the original frame designer to ensure that their design philosophy doesn’t suffer this paradox. To paraphrase David Brown, the frame designer should be aware of the form of their connections, even if they aren’t doing the detailed design of them.

To put it in my own words: A frame designer should have awareness enough to at least eyeball their connection loads against the tables provided in the Green Book. If your loads exceed the tying capacity for the standard connections, you know that you are in the realms of bespoke connection design and ought to be doing two things:

  1. Checking your columns for extra bending

  2. Providing a note on your GA drawings to say that bespoke connections are expected, and that the connections provided need not be strictly pinned.

*Note - the columns are in fact designed for a good deal of bending. The bending induced by over-stiff connections is in addition to this and can potentially over-stress the column, leading to yielding or buckling failure.

How was it resolved in our real-world project?

Having got the theory out of the way - where did this leave us on project? Our project was littered with connections falling into this category: in fact more connections than not were of this variety.

I would love to tell you a tale of the resolution of this conflict as an epic battle of one team’s collective wit pitted against another; a competition in which only one team would be able to hold their heads high after, where the loser was condemned to wander the streets with their faces buried shamefully in their hands, but it would be a complete fabrication (groan).

In truth, when we received the loads for the connections, the consultant and I had the briefest of polite conversations in which I told him that just by glancing at the magnitude of the loads I could tell that many of his connections on this job would not be able to classed as strictly pinned, and would he kindly check his columns out for some additional bending. He swiftly replied that he was perfectly content without needing to check his model, because he knew that he’d massively over-designed the columns in the first place.