Also known as Working Stress Design. In this older philosophy, structures are designed so that the stresses produced by the expected service loads do not exceed a fraction of the material's failure strength (the allowable stress). It focuses entirely on the elastic range of materials and uses a single factor of safety applied to the material strength.

Also known as Load and Resistance Factor Design (LRFD) in the US. This is the modern philosophy required by the NSCP. It identifies all the possible ways a structure could fail (its "limit states," such as yielding, buckling, or excessive deflection) and applies statistically derived load factors to increase the expected loads, while applying resistance factors to decrease the expected material strengths. It is a probabilistic approach that ensures a more consistent level of safety across different materials and load types.

The permanent weights of the building materials. This includes the weight of walls, floors, roofs, ceilings, stairways, built-in partitions, finishes, cladding, fixed service equipment, and the structural frame itself.

Loads produced by the use and occupancy of the building. These loads are movable and transient. Examples include human occupants, furniture, movable equipment, and roof live loads due to maintenance or use. Live loads are typically specified as uniform loads (e.g., kPa) or concentrated point loads.

Loads caused by natural forces. These forces can be very complex to evaluate because their magnitudes are mostly unpredictable. Under the National Structural Code of the Philippines (NSCP) and ASCE 7, strict guidelines are provided:

Unlike steel or concrete, wood is a viscoelastic material. Its strength is heavily dependent on how long a load is applied. The allowable stress design of timber structures requires multiplying the base material strength by a Load Duration Factor ($C_D$). For example, a timber beam can carry a much higher load for 10 minutes (wind gust) than it can for 10 years (dead load) before failing.

Calculating environmental wind loads requires a Basic Wind Speed ($V$) based on the geographical location and risk category of the structure. This wind speed is converted into dynamic pressure ($q_z$) and adjusted for factors like:

Differential settlement occurs when different parts of a structure's foundation settle into the soil by unequal amounts. In a statically determinate structure, this simply causes rigid-body rotation without inducing internal stress. However, in a statically indeterminate structure, differential settlement forces the continuous members to bend and warp, inducing massive internal shear forces and bending moments that must be accounted for in design.

A core distinction in loading is how it applies over time:

Aside from gravity and environmental forces, structures can experience significant internal forces due to:

Also known as checkerboard loading or skip loading. Live loads are transient; they might not cover the entire structure simultaneously. To determine the absolute maximum internal forces (shear and moment) in continuous beams and frames, engineers must arrange live loads in specific patterns (e.g., loaded adjacent spans to maximize negative moment, or loaded alternating spans to maximize positive moment) rather than just applying a full uniform load everywhere.

When dynamic loads (like moving vehicles or cranes) are applied suddenly, they create internal forces significantly greater than their static equivalents. To simplify analysis, these are often treated as equivalent static loads multiplied by an Impact Factor ($I$).

Equivalent Static Load = Static Load $\times (1 + I)$