15.1. Introduction

The main purpose of a structure is to transfer load from one point to another: bridge deck to pier; slab to beam; beam to girder; girder to column; column to foundation; foundation to soil.

Structures are built to support and distribute loads. This means that the weight and forces acting on a structure move from one point to another, which helps to maintain the integrity and stability of the entire building or bridge. For instance, in a bridge, the load from vehicles travels from the bridge deck down to the piers and then to the foundation below.

Think of this like a game of Jenga. When you pull a block from the bottom, the weight of the blocks above must be transferred downwards to maintain balance. If all blocks were not properly aligned and supported, the structure would collapse.

There can also be secondary loads such as thermal (in restrained structures), differential settlement of foundations, and P-Delta effects.

Secondary loads are additional forces that can affect a structure beyond the standard loads. For example, thermal loads result from temperature changes that cause materials to expand or contract. Differential settlement refers to uneven sinking of a building's foundation, which can cause stress and deformation. P-Delta effects occur in tall structures during lateral loading (like wind), where additional moments are created due to lateral displacement, leading to further stability concerns.

Imagine a tree. If a strong wind blows, the trunk bends slightly, which can put extra strain on certain branches. Similarly, when buildings sway in strong winds, they experience additional forces due to their own weight and movement.

Loads are generally subdivided into two categories: Vertical Loads (or gravity loads) and Lateral Loads.

Understanding the types of loads is crucial for structural engineering. Vertical loads include anything that acts downwards, usually categorized into dead loads (constant weights like the structure's materials) and live loads (temporary loads like people or furniture). Lateral loads, on the other hand, act horizontally on a structure, such as wind and earthquake loads.

Consider a book on a table. The book’s weight pressing down represents a vertical load (dead load). Now, if someone pushes the table, causing it to sway, that action illustrates a lateral load.

This distinction is helpful not only to compute a structure’s load but also to assign different factors of safety to each one.

By categorizing loads, engineers can calculate the different forces a structure must withstand and design it safely. Each type of load can have different safety factors based on how predictable they are—for example, dead loads are usually more stable compared to live loads, which can vary.

Think of it as planning for a road trip. You know how much fuel you need for the journey (dead load), but you also have to estimate unexpected stops or detours (live loads) and ensure you have enough resources for both situations.

For a detailed coverage of loads, refer to the Universal Building Code (UBC), (UBC 1995).

Building codes like the UBC provide essential guidelines and regulations for designers to ensure structures are safe and can handle various loads. Referring to these codes helps in maintaining standard practices in construction.

Just as a chef follows a recipe to make a dish successfully, engineers and builders rely on building codes to ensure safety and effectiveness in their designs.

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