Engineering & Technology · Civil Structures & Structural Engineering

Structural systems, load paths and built-environment needs

A floor needs more than strong material beneath it. Its load must reach supports, connections, foundations and the ground through a continuous physical system. At every transfer, the receiving part must resist the relevant force and deformation. Drawing that route exposes assumptions that a photograph of the finished building can hide.

Use Statics & Solid Mechanics, Materials Selection & Degradation and Measurement, Units & Engineering Evidence. Floor pressures, member properties, oscillator inputs and remaining areas are hypothetical teaching assumptions. No site-specific hazards, current code values, construction permissions or qualified structural designs are supplied.

Back to the chapter

After reading, you can

Trace gravity and lateral forces through a structural system while separating the model from the building’s full performance requirements.

Begin with the space and service the structure supports

A structural system creates usable space and carries actions while limiting unwanted deformation. A school, a bridge and a water-retaining structure have different geometry, contents and consequences of interruption. Required spans, access, vibration performance and future changes help define the problem before a member size can mean anything. Structure also meets the envelope, services and foundations at physical interfaces.

A load path describes how an action is transferred between connected parts. For gravity, a simple route might be floor to beam to column to foundation to soil. A wall can receive distributed floor reactions instead of a column’s concentrated force. At each boundary, draw the forces exchanged between parts; an internal action for the whole building becomes external when one component is isolated.

Relate form to the force it can transfer

Beams commonly transfer transverse load through bending and shear. Ideal pin-jointed truss bars carry axial tension or compression. Cables require tension and an appropriate geometry; arches transfer compression with horizontal reactions that their supports must accommodate. A real assembly can combine these behaviours. Naming a shape does not establish that its joints or restraints realize the assumed model.

Lateral actions need their own route. Floor or roof diaphragms can distribute in-plane forces to bracing, structural walls or moment-resisting frames, which transfer them toward foundations. Openings and discontinuities change that transfer. The presence of a gravity-supporting column does not establish lateral resistance. Connections and nonstructural attachments participate in the path even when their appearance is visually minor.

A 6 m by 4 m floor spans one way across the 4 m dimension to two simply supported 6 m edge beams. The plan uses the same scale in both directions. Each beam receives a 2 m tributary width: 3 kN/m² × 2 m = 6 kN/m, or 36 kN per beam. Each beam end transfers 18 kN to one of four corner columns. Total column force is 72 kN, equal to floor pressure times 24 m². Beam self-weight and other load paths are excluded; the plan contains no in-plane gravity arrows.

Check equilibrium at several scales

Start with the entire system, then isolate a floor, beam or joint. Sum forces and moments consistently. A tributary-area model assigns an area’s load to a support using an explicit spanning assumption. It is an accounting idealization, not a universal rule that every slab divides its load into equal rectangles. Two-way action, continuity and differing stiffness can change the distribution.

Foundation reactions close the superstructure model, but they begin a ground-response problem. Soil stiffness, bearing, settlement and groundwater can affect the boundary assumed above. A complete engineering assessment connects those models and their uncertainties. An equilibrium calculation establishes a force account; it does not establish member capacity, connection adequacy or acceptable settlement.

From a floor pressure to four column forces

Assume a 6 m by 4 m floor spans one way across 4 m to two simply supported 6 m edge beams. Supply a uniform downward load of 3 kN/m², excluding beam self-weight. Each beam receives a 2 m tributary width, so its line load is 3×2=6 kN/m. Over 6 m that is 36 kN, giving 18 kN at each beam end.

Four separate corner columns therefore receive 4×18=72 kN, matching 3×24=72 kN on the floor. The plan shows equal-scale geometry and the tributary split. Gravity acts perpendicular to that plan; it is not an arrow pointing toward one edge. Further floors, beam weights and lateral actions require additional accounts.

Try the reasoning

If the floor pressure doubles with the same ideal support model, what happens to the four reactions?
Show the answer and reasoning
Each doubles to 36 kN and their total becomes 144 kN. That linear account says nothing about available capacity.
Would the same tributary split necessarily apply to a slab supported on all four edges?
Show the answer and reasoning
No. Two-way plate action, restraint and relative stiffness require a different distribution model.
Does a foundation reaction of 18 kN mean the soil experiences a pressure of 18 kPa?
Show the answer and reasoning
No. Force must be related to contact area and distribution. The footing and ground model have not been specified.

What to carry forward

Trace each force across real interfaces and check both the total account and the assumptions at each transfer.

References

Teaching resources and research supporting this lesson’s explanations and examples.