Permanent, variable and environmental loads

Describe what produces the action

Permanent actions include a structure’s own weight and specified fixed components. Variable occupancy actions can change with use and position: people, stored items or moving vehicles do not always fill the same area. Environmental actions include wind, snow, water and ground motion. These categories help organize assumptions; they do not supply a numerical value for a particular site.

Some actions arise from an imposed change rather than a freely applied force. Restrained thermal expansion, material shrinkage and differential settlement can generate internal forces because connected parts cannot move as they would separately. A perfectly free uniform expansion can produce a displacement without thermal stress. The restraints and compatibility therefore belong in the load model.

Keep force, pressure, line load and moment distinct

Weight is a force. Unit weight in kN/m³ multiplied by volume gives kilonewtons; multiplied by a uniform layer thickness it gives kN/m². An area pressure becomes a beam line load only after assigning a contributing width. A line load in kN/m becomes a resultant force after integration over length. Do not add these different units directly.

For a resultant, retain its line of action. A uniform load’s resultant can reproduce a whole-body reaction calculation, while discarding information needed to find local bending or shear. A lateral resultant also has a moment arm above the supports. Wind pressure varies over surfaces and can act as suction as well as inward pressure; a single resultant is a deliberately reduced description.

State which actions act together

A load case describes a chosen spatial pattern and time or operating condition. A combination states how specified cases are considered together for a particular assessment. Full maxima of all actions are not automatically simultaneous, and a smaller stabilizing gravity force can sometimes be more adverse. Requirements and coefficients depend on the applicable design basis; the classroom values here are supplied assumptions.

Quasi-static analysis neglects inertia when appropriate to the changing action and structural timescale. Rapid or repeated excitation can require dynamic response, which the later lesson develops. Construction conditions and changed use also require their own cases. An original occupancy label or an old calculation cannot establish that a newly concentrated storage load is acceptable.

Gravity and a lateral force give unequal support reactions

Consider an abstract rigid body supported at two points 4 m apart. Permanent and variable pressures of 3 and 2 kN/m² over 24 m² give a centered downward resultant of 120 kN. Supply a simultaneous 12 kN rightward lateral force at height 3 m. Its clockwise moment is 36 kN m.

Taking moments about the left support gives 4 Rright−120×2−12×3=0, so the upward right reaction is 69 kN. Vertical balance gives the left reaction 51 kN. Horizontal reactions total 12 kN leftward; their split is unspecified. The plot varies only lateral-force height. These inputs are not a prescribed building load combination or foundation design.

Try the reasoning

1. What happens to the vertical reactions when the same lateral force reverses?

   Show the answer and reasoning

   Its moment reverses: the reactions swap to 69 kN left and 51 kN right under the same centered gravity.

2. Can a compression-only soil contact supply a negative upward reaction from an extrapolated formula?

   Show the answer and reasoning

   No. That would indicate lost contact in that model. Contact, restraint and equilibrium would need a revised analysis.

3. Can equal total gravity loads create different bending demands?

   Show the answer and reasoning

   Yes. Distribution and position matter; a point load and a spread load with equal resultants need not produce the same internal-force diagrams.

What to carry forward

A useful load account preserves origin, units, geometry, simultaneity and the conditions that justify its time model.