Foundations, Sill Plates, and Pile Systems

Traditional Japanese timber construction (Zairai-Kōhō) is based on a framework of columns and beams.
Supporting this entire framework are the foundation, sill plate, and—when necessary—the pile system.

Understanding these three components reveals the entire structural logic of a timber building.

1. Foundation – The Structural Base

Foundations transfer building loads to the ground and play two main roles:

✔ Distribute loads evenly to the soil

Including building weight, live loads, seismic forces, and wind.

✔ Prevent differential settlement

Uneven settlement can cause cracks and door misalignment.

✔ Distribute loads evenly to the soil
✔ Prevent differential settlement

Main Types

Main Types

① Strip Foundation

Concrete placed along the perimeter and load-bearing walls.
Traditional and cost-efficient.

② Mat (Raft) Foundation

A full concrete slab supporting the entire footprint.
Highly resistant to seismic action and settlement.

2. Sill Plate (Dodai)

The sill is the first timber member placed on top of the foundation.

✔ Must be treated against decay and termites

Common materials: Hinoki, treated SPF, engineered lumber.

✔ Fastened with anchor bolts

Provides vertical and horizontal stability.

3. Connection Hardware

● Anchor Bolts

Standard fixings between sill and foundation.
Typical spacing: around 910 mm, depending on structural design.

● Hold-down Connectors

Prevent columns from being pulled out during earthquakes.

4. When the Ground is Weak: Pile Systems

Even a strong foundation is ineffective if the soil beneath is weak.
In such cases, piles transfer loads to deeper, stronger strata.

Piles are used when:

Low N-values in soil investigation
Ground improvement is insufficient
Medium–large buildings (stores, hotels)
Differential settlement must be avoided

5. Common Pile Types

① Steel Pipe Piles

High precision and quick installation.
Effective as end-bearing piles.

② PHC Concrete Piles

Prestressed high-strength concrete piles.
Used widely in commercial and residential buildings.

③ Cast-in-place Piles

Concrete poured on site after drilling.
Suitable for large structures.

④ Ground Improvement Methods

(Not technically piles, but often used instead)

Surface improvement
Deep mixing method

6. Relationship Between Piles and Foundations

The load path is:

Pile → Foundation → Sill → Column → Beam → Roof

If the pile or foundation is inadequate, the entire structural system becomes compromised.

7. Piles Cannot Be Inspected After Completion

Since piles are buried deep underground,
post-construction inspection and repair are extremely difficult.

Quality depends on:

Detailed soil investigation
Accurate pile length and location
Precise installation
Proper documentation during construction

Especially in hotel/retail renovations, existing pile information is essential.

8. Practical Points for Renovation Projects

Useful even for interior designers and renovation planners:

Foundation type (strip / mat)
Presence and type of piles
Depth to bearing stratum
Soil investigation logs
Condition of sill plates
Anchor bolt placement
Hold-down installation

These determine the building’s seismic behavior, vibration, and structural limits during renovation.

⚠ Required Disclaimer（English Version）

This article provides general information on foundations, sill plates, and pile systems in traditional Japanese timber construction.
Actual requirements vary by country, region, soil condition, and building age.
Always consult licensed professionals and local authorities for final decisions.

Traditional Japanese Timber Construction