HSS members used as beams

The above URL contains the entire discussion. Below is a snippet.


I’ve noticed recently there is very little information out there that supports HSS members used as beams. My connection design software doesn’t support it and there are few connection design examples out there. AISC tabulates information (total UDL) for WF beams and other shapes but not for HSS shapes, just to name a few examples. Is there some reason why it is undesirable to use an HSS member as a beam?



HSS has several problems:

  • it can fill with water and then freeze, so if it is ever used for beams OR columns outdoors in cold environments, it must have adequate weep holes

  • it is a poor choice to use if you have to galvanize

  • it has an internal surface which can corrode from the inside out, and which cannot be easily inspected

  • it is an inefficient way to deploy steel if you know that the forces and moments will only act in particular directions, ever

HSS also has numerous advantages:

  • although the use of steel is inefficient in terms of pounds of steel to support a given load at a given distance, you can often use a shallower member and hence it can be more space efficient

  • attachment details between HSS members are relatively simple, if you use your brain when selecting the sections

  • its exterior surface is very easy to paint, and to inspect afterward

  • it provides lots of normal surfaces for attachment of secondary supports

  • it has incredible torsional strength, so is a wonderful way to support loads which may come from numerous different directions

We use HSS sections therefore almost exclusively to build the non-building structural frameworks for our modular chemical plants, used all over the world for decades. We do have clients who insist on galvanized bolted construction, and that system is (usually) superior in corrosion resistance, but comes at a huge cost adder.

Our plants aren’t designed for 50+ durability, so in practical terms we’ve never had an issue with a structural member corroding sufficiently to need replacement within the expected service lifetime of the plant.

Our module frames must be strong enough to be loaded and shipped laying down, then off loaded tilted up into final position. That’s a complex problem when you’re trying to design with members that are only strong in one direction, and invariably results in the excessive use of steel irrespective of which kind of section you use. But numerous problems encountered by the use of W and S and L and C sections are solved, trivially, by using HSS instead on our plants.

Without going too far down an unrelated path, the aviation industry started with welded tubing structures, and did very well for decades. Many lessons apply here.

To the OP’s point about drainage and accumulation of contaminants inside the HSS section, weep holes are one way but more often in aviation welded tubular structures larger cut-outs were made in the surfaces at the connection point - they would be covered when welded together so the opening can be much larger.

Protection of the inner surfaces is done in advance with coatings that flow when applied and harden with age. Of course the trick of filling your aircraft engine mount tubes with linseed oil and turning it over and over in your hands is nonsense when talking about building structures, but the general principle can still be applied with planning before construction.

The original design of a building, like many aircraft, is simple at first, and then gets complicated as other features, new owners, new missions are used. Having an over-designed framework makes it easier to make those changes later. The building (like the aircraft) enjoys a longer life of productivity.

1 Like