Chinese JoineryChinese Joinery
中文
Angled and half crossings
踏步榫
tàbù sǔn

Stepped Tenon (阶梯榫, 踏步榫): How a Cross Beam Sits on a Round Purlin

Step tenon · 踏步榫 · 趴梁阶梯榫

By · Updated

Some beams in a Qing roof do not sit on columns: they lie across the purlins.

They hold on with a tenon cut in steps.

Chinese
阶梯榫 · jiētī sǔn
Also called
踏步榫
Joins
Cross beam (趴梁) to round purlin
Found in
Qing timber roofs, hip and corner framing
Signature parts
Three steps · ¼-radius depth · dovetailed top step
Drag to rotate. Slide to take the joint apart, or press Assemble to watch it go together.

How it works

A beam that lies across

Ma Bingjian describes the stepped tenon as used where a beam lying across (趴梁) or a diagonal corner beam meets a purlin, and where short and long cross beams meet.

Three steps

The tenon is usually made in three layers. The bottom layer goes into the purlin a quarter of its radius; the second is the same; the third may be dovetailed to pull the parts together, or straight. The tenon reaches no further than the purlin’s centre, and a quarter is left on each side as cover (包掩).

Proportions in the model are illustrative.

How it's made

  1. Across
    Step 1

    Across

    A beam laid across a purlin.

  2. First step
    Step 2

    First step

    Bottom step: a quarter radius deep.

  3. Second step
    Step 3

    Second step

    The second step the same.

  4. Top step
    Step 4

    Top step

    Top step dovetailed, to the centre.

  5. Socket
    Step 5

    Socket

    A stepped socket; sides left as cover.

  6. Drop in
    Step 6

    Drop in

    Dropped in from above.

Variations

Corner beam

A diagonal corner beam uses the same tenon cut at 45°.

Straight top step

The top step may be straight instead of dovetailed.

Can you CNC this?

Hard on a 3-axis machine as drawn

The socket is a stepped pocket in a round purlin, cut from above but with a dovetailed top layer, which is undercut. It may need a cradle for the round timber and a dovetail bit or hand work.

Free STL downloads (one file per part)

Before cutting or printing, read the CNC and 3D-printing notes on tool radius, scaling the STL files and their licence.

Questions

Why steps?

They seat the beam on the round purlin without cutting too deep.

How deep is each?

A quarter of the purlin radius.

How far does it reach?

No further than the purlin’s centre.

Further reading
  1. 马炳坚《中国古建筑木作营造技术》第二版（Ma Bingjian, Timber Construction Techniques of Traditional Chinese Buildings, 2nd ed.），第 127–128 页，图 3-15
  2. 马炳坚《中国古建筑木作营造技术》第二版（Ma Bingjian, Timber Construction Techniques of Traditional Chinese Buildings, 2nd ed.），第 125 页