Module 1: Introduction to Timber Engineering

This module is anchored in NSCP 2015 Chapter 6 — Wood. The NSCP update notes state that Section 616 — Design Provisions and Equations adopts NDS 2015 Chapter 3 almost in its entirety, Section 617 — Sawn Lumber adopts NDS 2015 Chapter 4 almost in its entirety, Section 618 — Structural Glued Laminated Timber adopts NDS 2015 Chapter 5 almost in its entirety, and the Section 619 connection tables were revised using NDS 2015.

Use Sections 615–619 as applicable to the actual product and check, with NSCP 2015 remaining the governing course basis. Use the official code tables or approved product data for project design values; do not substitute unsourced species averages.

Wood carries load most efficiently parallel to its fibers. Tension and compression parallel to grain, compression perpendicular to grain, and shear parallel to grain therefore use different design values and failure mechanisms. A connection detail that redirects force across the grain can become critical even when the gross member is large.

Knots, slope of grain, checks, shakes, splits, and wane disturb this ideal fiber path. Structural grading accounts for their effect by assigning a grade and corresponding reference design values rather than assuming clear-wood strength.

Below the fiber-saturation region, moisture changes cause dimensional movement and materially affect strength and stiffness. Longitudinal movement is usually small compared with radial and tangential movement, so wide panels, cladding, flooring, and exposed timber details need allowance for cross-grain movement.

Architecturally, moisture design affects roof overhangs, drainage, end-grain protection, façade interfaces, wet rooms, ground clearances, concealed cavities, and the detailing of rigid finishes around timber.

A structural designer does not select an allowable stress solely from the common name of a species. A review-ready design identifies the species/product, structural grade, size classification, moisture/service condition, and applicable NSCP table.

Reference design values commonly include bending, tension parallel to grain, compression parallel to grain, compression perpendicular to grain, shear, modulus of elasticity, and minimum modulus of elasticity. The exact set depends on the product and table.

Timber design starts with reference design values, not generic material averages. For sawn lumber, the governing table identifies the species/species group, structural grade, size classification, and the reference values for the applicable properties. Structural glued-laminated timber, structural composite lumber, I-joists, panels, CLT, and proprietary products use their own code tables or approved product reports.

Do not infer structural design values from density, common species name, architectural appearance, or a broad "typical strength" range. A worked problem may supply a value as input, but that value is valid only for that stated problem unless its code/product source is explicitly established.

For sawn lumber using ASD, the NDS-family applicability structure used by the NSCP basis is property-specific:

This matrix is a selection map, not a table of factor magnitudes. Obtain each required value from the adopted NSCP/NDS provision and verify its trigger before applying it.

The symbols are intentionally distinct: $C_t$ is the temperature factor, while $C_T$ is the special buckling-stiffness factor used only when its truss-chord conditions are satisfied.

For sawn lumber using LRFD, do not use $C_D$. The corresponding strength-resistance format applies the property-specific environmental/product factors together with the NDS LRFD factors:

$K_F$ is the format-conversion factor, $\phi$ is the property-specific resistance factor, and $\lambda$ is the LRFD time-effect factor where the applicability table permits it. In the 2015 NDS sawn-lumber table, $\lambda$ applies to $F_b$, $F_t$, $F_v$, $F_{c\perp}$, and $F_c$, but not to $E$ or $E_{min}$. These factors are not interchangeable with $C_D$. Use the adopted NSCP/NDS values and applicability rules for the selected property and load combination.

For ASD, select $C_D$ from the duration represented by the governing load combination, not from the member type:

Use the adopted NSCP/NDS load-combination rules to determine which duration category governs. $C_D$ does not apply to $E$, $E_{min}$, or $F_{c\perp}$, and special treatment/product provisions can impose additional limits.

NDS 2015 Table 2.3.3 groups properties by both temperature and wet/dry service condition. For sustained elevated temperature:

These factors are for sustained exposure up to $150^\circ\mathrm{F}$ ($65.6^\circ\mathrm{C}$). Do not infer a factor from a brief daytime temperature spike or reuse the dry row for wet-service strength values.

Do not transfer the sawn-lumber matrix unchanged to another wood product.

• Structural glued-laminated timber: use the glulam chapter/table. Bending can require the volume factor $C_V$, curved members can require the curvature factor $C_c$, and other glulam-specific provisions can apply. For glulam beam stability, $C_L$ and $C_V$ are not applied simultaneously; use the lesser factor as required by the NDS 2015 glulam provision. Do not substitute the sawn-lumber size factor $C_F$ for $C_V$.
• Structural composite lumber: use its chapter or approved product report; size/volume and stability rules can differ from sawn lumber.
• I-joists, wood structural panels, CLT, proprietary engineered products, and connectors: use the applicable NSCP/NDS chapter and approved product data. Manufacturer evaluation reports can govern properties or adjustments that are not represented by the sawn-lumber matrix.

The reference value and the adjustment procedure must come from compatible editions. Do not combine a current product value with an older factor table simply because the symbols appear familiar.

The ASD chain uses load-duration factor $C_D$ where permitted. The LRFD chain uses the LRFD format-conversion, resistance, and time-effect framework required by the adopted wood provisions instead of simply inserting $C_D$ into an LRFD calculation.

Choose the design method first and keep the load combination, adjustment factors, and available resistance on that same basis through the entire problem.

For a sawn-lumber beam requiring beam stability, first form the bending value before the stability factor:

Then establish $E_{min}'$, determine the effective unbraced geometry, calculate $F_{bE}$ and $C_L$, and form the final bending value with the applicable flat-use condition:

For a sawn-lumber compression member, first form

and

Then determine effective length and slenderness, calculate $F_{cE}$ and $C_P$, and finally obtain

This order matters because $C_L$ and $C_P$ depend on already-adjusted intermediate values. They are not interchangeable with ordinary environmental or size multipliers.

NSCP Chapter 6 includes requirements for decay and termite protection because biological deterioration can remove load-carrying section long before a strength equation becomes relevant. Architectural detailing should keep timber dry, ventilated where required, separated from soil or persistent wetting, and accessible for inspection when exposure warrants it.

Preservative or fire-retardant treatment can affect strength values or adjustment factors. Use manufacturer/code-approved values for treated products instead of applying a generic percentage reduction.

• Glulam: laminations bonded primarily parallel to member length; suited to long-span beams, columns, and curved members. Use the glulam provisions and tabulated values applicable to the product.
• LVL and structural composite lumber: manufactured veneers or strands oriented to provide predictable properties; design values come from recognized product data and applicable code provisions.
• CLT and other mass-timber panels: useful for floors, roofs, and walls, but do not assume that every modern mass-timber design method is contained in NSCP 2015. When a project relies on a product or system outside the explicit course provisions, identify the approved standard/evaluation basis separately.

Heavy timber can retain a load-carrying core as the outer layer chars, while unprotected steel rapidly loses strength and stiffness as temperature rises. That does not make timber automatically fire-safe. Fire-resistance design depends on member dimensions, connections, protection, penetrations, required rating, and the governing fire/building provisions.

For architecture, exposed timber connections deserve special attention because concealed metal plates and fasteners may behave differently in fire from the surrounding char-forming wood.

Before choosing timber as an exposed structural material, review: span and depth, available grades and sizes, moisture exposure, termite risk, fire rating, finish system, connection visibility, movement at glazing/façade interfaces, concealed services, erection sequence, and future inspection.

The best structural timber detail makes the load path legible while preventing water traps, cross-grain restraint, splitting, and inaccessible deterioration.

• 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 Chapter 6 timber quality, durability, design values, framing, engineered wood products, and connections; consult the official licensed code for complete requirements.
• American Wood Council — 2015 NDS package and Design Values for Wood Construction — External technical cross-check for the NDS-family timber provisions underlying the NSCP wood framework; do not mix editions.