Module 2: Timber Tension and Compression Members

Use NSCP 2015 Section 616 — Design Provisions and Equations together with Section 617 — Sawn Lumber or the appropriate engineered-wood section. Connection-related net-section and tear-out behavior must also be coordinated with Section 619 — Timber Connectors and Fasteners.

Tension parallel to grain can be efficient in clear, straight-grained wood, but knots, slope of grain, holes, and connection zones are highly influential because timber is relatively brittle in tension parallel to grain. Evaluate the member away from connections and then review the connection region separately.

A complete tension-member review includes the applicable adjusted tension design value, gross/net section, local stress concentration introduced by connection geometry, and timber-connection failure modes.

The member can be adequate in simple axial stress yet fail at the connection. Review net-section tension, local crushing/bearing, row tear-out, group tear-out, splitting perpendicular to grain, fastener strength, and any eccentricity introduced by one-sided connection geometry.

Architecturally exposed knife plates or side plates should be detailed to avoid placing large tension perpendicular to grain near a member end.

For mechanics and preliminary design checks, the following idealized end conditions provide useful reference values. They describe perfect boundary conditions, not automatically the restraint of a real timber frame.

How to use the table: the theoretical values explain Euler buckling behavior. For design, use the effective-length procedure permitted by the adopted NSCP/NDS basis; where Appendix G's ideal-condition approximation is used, prefer the recommended design values rather than automatically taking the lower theoretical values. Partial rotational restraint, frame sidesway, weak surrounding members, or uncertain bracing can require a larger $K_e$.

A short stocky member may be governed primarily by crushing parallel to grain; a slender member is increasingly governed by instability. NSCP timber design represents this transition through the column-stability procedure rather than by using unrelated short/intermediate/long-column equations as competing design methods.

What short, intermediate, and long mean

These are behavioral classifications, not three separate NSCP design equations:

There is no single universal $L_e/d$ number that separates all three behavioral labels for every wood product. The code procedure uses the unified $C_P$ equation across the permitted range.

The axial formulas in this module require property-specific adjusted values, not an unexplained $F_t'$ or $F_c'$ copied from a previous example.

For sawn lumber in ASD:

For compression parallel to grain, first form the pre-stability value

and the stability modulus

Only after the effective-length and slenderness checks are complete should the column-stability factor be applied:

For local bearing,

with $C_b$ included only when the bearing geometry satisfies the provision that permits the bearing-area adjustment. Do not use $C_D$ for compression perpendicular to grain merely because the surrounding load combination contains a short-duration load.

The stability equation includes the product constant $c$. The commonly used NDS-family values associated with the NSCP wood framework are:

Use the value corresponding to the actual product and adopted code provision. Do not leave $c=0.80$ simply because the first worked example used sawn lumber.

Timber is much more deformable perpendicular to grain than parallel to grain. Beam seats, column caps, sill plates, and concentrated reactions therefore require a local bearing check using the applicable adjusted compression-perpendicular design value and bearing-area provisions.

Bearing deformation can also be an architectural/serviceability issue where exposed framing must remain aligned with partitions, glazing, or finish tolerances.

For sawn-lumber compression perpendicular to grain, the NDS-family bearing-area increase is limited to small interior bearing areas. For a bearing length $b<6\text{ in.}$ located at least $3\text{ in.}$ from the member end, the permitted factor is

with $b$ in inches. The direct metric equivalent uses $9.525\text{ mm}$ in place of $0.375\text{ in.}$.

Do not apply $C_b$ to a bearing at the member end, to a bearing $6\text{ in.}$ or longer, or outside the provision's scope. In those cases, use $C_b=1.0$ unless another governing product provision explicitly states otherwise.

Column size is not determined by area alone. Increasing the dimension about the weak buckling direction can be more efficient than simply adding the same amount of material elsewhere. In exposed timber, this affects visual proportion, bay planning, wall integration, connection concealment, fire-exposed sacrificial section, and the space needed for steel plates or fasteners.

• Association of Structural Engineers of the Philippines (ASEP) — National Structural Code of the Philippines, C101-15, Volume I, Seventh Edition (2015) — Governing course reference for timber tension, compression, bearing, stability, and connection-region checks; consult the official licensed code for complete Sections 615–619 provisions.
• American Wood Council — 2015 NDS package — External technical cross-check for column stability, product constants, and timber design-value provenance; NSCP 2015 remains the governing course basis.