Structural Analysis Fundamentals / Structures, modelling and idealisation
What a structure has to do
Load paths, structural forms, and the journey from load to ground.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Checks needed for project use
- An independent structural reviewer must check remaining derivations, diagrams, sign conventions and the interactive models across their full input ranges.
- Model applicability, diagrams and implementation across all input cases remain subject to independent engineering review.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
Trace the load path before calculating member forces. It gives you a physical check on the model and helps you question results that do not fit the structural arrangement.
By the end of this lesson you should be able to
- Say what a structure is for, in plain terms
- Name the common structural forms and how each carries load
- Trace a load path from where the load lands down to the ground
What you should already know
- Basic ideas of force and weight
- No prior structural analysis is assumed
A structure has a simple job. It has to pick up the loads that land on it, carry them safely down to the ground, and not move so much that it becomes unusable or frightening.
That gives three things to worry about. It must be strong enough not to break. It must be stiff enough not to sag or sway too much. And it must be stable — it must not fall over or buckle sideways.
Loads arrive in different ways. Some are permanent, like the weight of the structure itself and everything fixed to it. Some are variable, like people, traffic, stored goods, snow and wind. Some are accidental, like an impact. Some are not forces at all: a temperature change or a settling foundation can push a structure just as hard as a load can.
Whatever the load, it has to get to the ground. The route it takes is called the load path. In a typical building the path runs: floor slab, then beams, then columns, then foundations, then soil. Each element transfers load through its connections to the next. Strength and stiffness need not increase at every step.
Predict first
A person stands in the middle of an upstairs room. Which of these is the most sensible load path?
Structural forms are the vocabulary engineers use. Each one carries load in a particular way, and each is efficient at something different.
A beam spans across a gap and carries load by bending. A column carries load along its own length by squashing, and its real enemy is buckling. A tie is a member that only pulls; a strut only pushes. A truss is a triangulated frame where, if the loads land at the joints, members carry axial force in the ideal model, provided the members are straight, joints act as frictionless pins and member loading between joints is neglected. A frame joins beams and columns rigidly, so the joints carry moment and the whole thing acts together. A slab is a plate that spans in one or two directions. An arch carries load mainly by compression, pushing outwards at its ends. A cable does the opposite: it carries load purely in tension and pulls inwards.
Supports are where the structure meets the rest of the world. They are the last link in the load path, and they decide what the structure is allowed to do. A support that stops movement in some direction must push back in that direction — that push is a reaction.
Check yourself
You need to span a 30 m gap with something light, and you can build strong anchorages at each end that can resist a big inward pull. Which form suits best?
Practice
An internal column in an office supports a floor area measuring 6.0 m × 5.0 m. The floor carries a total load of 5.0 kN/m². What load does the column carry from that floor?
Practice
A floor slab spans between parallel beams 4.0 m apart and carries 6.0 kN/m². Treating the slab as spanning one way, what uniformly distributed load does each internal beam pick up, in kN per metre of its length?
Practice
A steel tie 3.0 m long hangs a walkway and carries 80 kN. If a second identical tie is added so the two share the load equally, what force does each now carry?
Summary
- A structure must be strong, stiff and stable
- Loads may be permanent, variable or accidental, and some 'loads' are movements rather than forces
- The load path is the route from where load lands to the ground
- Each structural form carries load in its own way; triangles keep their shape and so make good trusses