5.3 Structural Materials: Concrete, Steel, Timber & Masonry

Reinforced Concrete: The Dominant Philippine Structural Material

Plain concrete is strong in compression but weak and brittle in tension - its tensile strength is typically only a small fraction of its compressive strength. Because nearly every structural member experiences some tension (a simply supported beam develops tension along its bottom face as it bends, for instance), plain concrete alone cannot safely carry bending loads. Reinforced concrete (RC) solves this by embedding deformed steel reinforcing bars ('rebar') in the zones where tension develops, creating a composite material in which concrete carries compression and steel carries tension. Columns add lateral ties or spiral reinforcement around the longitudinal bars to confine the concrete core and provide ductility - the ability to deform without sudden brittle failure - which is especially critical in Seismic Zone 4, where a structure must absorb and dissipate energy during shaking rather than fracture outright.

Reinforced concrete's cast-in-place, monolithic nature is precisely why it dominates Philippine mid-rise and high-rise construction: it can be detailed to form the ductile beam-column frames and shear-wall cores that Sections 5.1 and 5.2 describe as the backbone of Philippine seismic-resistant design. Compressive strength classes commonly specified in Philippine practice range roughly from 20.7 MPa (about 3,000 psi) for lighter elements up to 34.5 MPa (about 5,000 psi) or higher for heavily loaded columns, though the specific class is always a project-specific structural engineering decision.

Structural Steel

Structural steel's chief advantage is its superior strength-to-weight ratio compared with concrete: a steel member carries far more load per unit of self-weight, which is why steel is the preferred choice for long-span roof trusses over gymnasiums and terminals, and for bracing and moment frames in high-rise cores where reducing building mass also reduces seismic demand (recall from Section 5.1 that seismic force scales with mass). Steel's ductility under load also makes it attractive for ductile moment-resisting frame connections.

In the Philippine context, structural steel carries two practical tradeoffs an architect must plan for. First, steel loses strength rapidly when exposed to fire and therefore requires fireproofing (intumescent coatings or fire-rated enclosures) in most occupied buildings. Second, the Philippines' humid, coastal, typhoon-exposed climate accelerates corrosion, so exposed structural steel needs galvanizing, painting, or other protective coating systems, with particular attention near the coastline where salt-laden air is aggressive. Steel is increasingly common in pre-engineered warehouse, big-box retail, and industrial construction, where its speed of erection and long-span efficiency offset these added protective costs.

Timber

Timber is the Philippines' most traditional structural material, seen in the post-and-beam framing of the bahay kubo (the bamboo-and-nipa vernacular house) and the heavier timber-and-masonry hybrid framing of the bahay na bato ('house of stone') built during the Spanish colonial period. Native hardwoods such as yakal, guijo, apitong, and narra were historically prized for structural framing. Today, timber is used far less often for large-scale structural framing - deforestation, logging restrictions, and cost have pushed most mid-rise and high-rise work toward concrete and steel - but timber remains common in low-rise residential and vernacular-inspired construction.

Timber's two defining durability concerns in the Philippine climate are moisture and termites. Sustained moisture exposure causes fungal decay and rot, while subterranean termites are a severe and widespread risk in a hot, humid tropical environment. An architect specifying structural timber must detail for ground clearance (keeping wood off grade and away from direct soil contact), adequate ventilation to let framing dry out between rain events, moisture barriers at connections, and chemically pressure-treated lumber where appropriate - details that matter as much to a passing exam answer as they do to a building that survives its first decade.

Masonry: Concrete Hollow Blocks (CHB)

Concrete hollow blocks (CHB) are the single most common wall material in Philippine construction, used for everything from residential partitions to low-rise bearing walls. CHB is standardized under PNS ASTM C90:2019 (load-bearing) and PNS ASTM C129:2019 (non-load-bearing), which the Bureau of Philippine Standards adopted in 2019 to replace the older PNS 16:1984. Load-bearing CHB typically runs 150-200 mm thick, while non-load-bearing partition block runs thinner, around 75-100 mm.

CHB's critical structural limitation is that unreinforced masonry is brittle: on its own it has low tensile and shear capacity and fails suddenly rather than deforming gradually, which is precisely the wrong behavior in an earthquake. NSCP therefore requires reinforced masonry wherever CHB carries structural load: vertical reinforcing bars dowelled into the foundation and run through grout-filled block cores, combined with horizontal bond-beam or truss-type joint reinforcement tying the wall together. The prevailing Philippine practice - highly testable on the ALE - is confined masonry: reinforced concrete tie columns and tie beams framing every wall opening, corner, and intersection at regular intervals (commonly around every 3 meters), with CHB infilled and reinforced between them. Plain, unconfined, unreinforced CHB is never an adequate lateral-force-resisting system in Seismic Zone 4 on its own.

Choosing Among the Four Materials

A useful way to hold these four materials in mind together, especially for scenario-style ALE items, is to compare how each behaves under the loads covered in Sections 5.1 and 5.2:

No single material is 'best' in isolation; the ALE rewards recognizing which material fits a given span, loading, and maintenance context. A long-span gymnasium roof favors steel for its strength-to-weight ratio; a mid-rise condominium favors a reinforced-concrete frame with a shear-wall core for ductile lateral resistance; a low-rise vernacular house may reasonably use timber if moisture and termite detailing are addressed; and ordinary partition and infill walls default to CHB, provided it is confined or reinforced wherever it is asked to do more than divide space.