HSS steel connections are usually made by welding a plate to the outside of the tube and bolting to the plate, or by welding the tubes directly to each other. The common details are shear tabs and through plates for beams, cap plates where a beam sits on a column, knife plates for bracing, end plates for splices, and cut-to-fit welded joints for trusses and frames. Because a hollow section has no access inside and relatively thin walls, HSS connections are designed differently from W-beam connections, and most of the work is done in the shop.
This guide walks through the details you will see on drawings and how they are fabricated. For the specific case of a W-beam framing into an HSS column, see our post on W-beam to HSS column connections.
Why HSS connections are different
Hollow structural sections (HSS) are square, rectangular or round tubes. They make efficient columns, braces and truss members, and they look clean when exposed. Their shape also creates three connection problems:
- No access inside. You cannot put a wrench on a nut inside a closed tube. Ordinary bolted connections have to go through plates welded to the outside, or use special fasteners.
- Thin, flexible walls. A load pushed or pulled on the flat face of a tube can bend the wall locally, especially near the middle of a wide face. The tube may be strong as a member and still weak at a connection to its face. Engineers check this local wall bending, often called face plastification.
- Through-bolts do not tighten properly. A bolt through both walls cannot be fully tightened without crushing the tube, so it works more like a pin. That suits some light connections, not others.
So the shape that makes HSS efficient as a member also shapes how it connects. Our comparison of W-beam vs HSS covers when each section makes sense.
Common HSS connection details
| Detail | What it is | Typical use |
|---|---|---|
| Shear tab on the column face | A single plate welded to the tube wall, bolted to the beam web | Simple beam to HSS column connections with light to moderate reactions |
| Through plate | A plate passing through slots in both walls of the column, welded on both faces | Heavier reactions where a single tab would overload a thin wall |
| Cap plate | A plate welded on top of the column, with the beam bearing on it and bolted down | Beams running continuous over a column, canopies, roof framing |
| End plate | A plate welded across the end of an HSS and bolted to a matching plate | Splices in columns and members, braces bolting to a frame |
| Knife plate (slotted tube) | A gusset plate slotted into the end of the tube and welded | Diagonal bracing |
| Directly welded joint | The end of one tube cut to fit against another and welded | Trusses, frames, railings, canopies |
| Base plate | A plate welded at the foot of the column with anchor rod holes | Every HSS column, from house posts to buildings |
Which one the design uses depends on the load, the wall thickness, the look the architect wants and what can be built. The choice between bolting and welding in the field is covered in bolted vs welded steel beam connections.
Beams to HSS columns
The simplest beam connection is a shear tab welded to the column face. When the reaction is too large for the tube wall, a through plate spreads the load to both walls. Where the beam can run over the top, a cap plate connection is often the easiest of all: the column gets a plate on top, and the beam sits on it and bolts down. Connections that resist moment are more involved and need a full design.
Splices and end plates
Long HSS columns and members are spliced with end plates welded to each piece and bolted together, or with welded splices. End plate splices work well in compression. In tension, the plates have to be much thicker to stop them bending, and the bolts sit outside the tube, which makes the joint bulkier.
HSS bracing connections
Bracing is one of the most common uses of HSS, and the end connection is usually the critical part of the brace.
- Slotted tube with a knife plate. The brace end is slotted, a gusset plate slides into the slot and is welded along both sides. The plate then bolts to a gusset on the beam or column. This is the classic HSS brace detail.
- Cap plate with a gusset. The brace end is closed with a cap plate, and a plate welded to the cap bolts to the frame gusset. It seals the tube but adds a part.
- Welded directly to a gusset or another member, where the connection is made in the shop or by a certified welder on site.
When a slotted tube carries load through only part of its perimeter, the designer has to check the reduced effective area of the tube end (shear lag). That is why slot lengths, weld lengths and any reinforcing plates are shown on the drawings and should not be changed in the shop or field.
Welded HSS framing and trusses
In trusses, canopies and frames, tubes are often welded straight to each other. The end of the branch member is cut to fit the face of the main member (the chord). Square and rectangular tubes are cut flat or bevelled; round tubes need a curved "fish-mouth" profile.
Joint types are named by their shape: T and Y joints have one branch, K and N joints have two branches meeting at the chord, either with a gap between them or overlapping, and X joints have branches on opposite sides. Gap joints are generally easier and cheaper to fabricate than overlapped ones, while overlapped joints can carry more load. The member sizes and wall thicknesses are chosen so the joints work, which is a design task, not a fabrication choice.
Fabricating HSS connections in the shop
On the shop floor, HSS work has its own checklist:
- Cutting. Tubes are cut square or mitred to length; branch members get the end profile they need to fit the chord. Slots for knife plates are cut to the length on the drawing.
- Fit-up. Plates are fitted and tacked square to the tube. On tubes, a tab off by a few degrees throws the beam or brace off line.
- Welding. Welds are made to CSA W59 by a CWB-certified shop. Fillet welds are preferred on tube joints where the design allows, because they are simpler than groove welds.
- Sealing. Columns and members are usually closed with welded cap and base plates, so water cannot get inside and freeze. Open ends exposed to weather are a problem.
- Galvanizing. If the steel is to be hot-dip galvanized, sealed tubes need vent and drain holes, located with the designer's approval. We can prime or galvanize to suit the job.
- Appearance. Exposed HSS often has architectural requirements for weld finish and alignment. Those need to be on the drawings before fabrication.
HSS sizes and wall thicknesses available in Canada are listed in our HSS size chart.
Shop drawings: where HSS connections get settled
Most HSS connection problems are cheaper to solve on paper. The structural drawings show the members and the connection forces or typical details. Connection design may be done by the engineer of record or delegated to a connection designer, as the contract specifies. The shop drawings then show every plate, slot, hole and weld for each piece.
When reviewing HSS shop drawings, check:
- Wall thickness of each tube matches the design, since connection capacity depends on it.
- Plate positions on the tube face, especially offsets from the centreline.
- Slot and weld lengths at brace ends.
- Cap and base plates that seal the tube, or vent holes if galvanized.
- Field connections are bolted or welded as the erector expects.
Structural steel framing of any kind needs a designer or engineer, and most new structures and alterations need a building permit.
HSS framing and connections from Procut
We fabricate HSS posts and framing with base, cap and connection plates welded on, under CWB certified welding, from shop drawings prepared in-house. Engineering services are available for projects that need them. We deliver and install across the GTA and Ontario.
Send your structural drawings for a free take-off and quote, or see our steel products and services.
Frequently asked questions
How do you connect to HSS steel?
Most HSS connections are made by welding a plate to the outside of the tube (a shear tab, gusset, cap plate or end plate) and bolting to that plate, or by welding tubes directly to each other. Bolting straight through the tube wall is possible but limited, because there is no access inside.
Can you bolt through an HSS column?
Through-bolts can be used for some light connections, but they cannot be tightened hard without crushing the tube walls, so they behave more like pins. Blind bolts made for one-sided installation are another option. The connection designer decides what is acceptable.
How is HSS bracing connected?
A common detail slots the end of the HSS brace and welds a gusset (knife) plate into the slot, which then bolts to a gusset on the frame. Another uses a cap plate on the brace end with a plate welded to it. The designer checks the plate, welds and the tube end.
Should HSS columns be sealed?
Tubes are usually closed with welded cap and base plates so water cannot get in and freeze. If the steel is to be hot-dip galvanized, vent and drain holes are needed instead, located with the designer's approval.
