10/15 10 — Building Structure and Structural Systems
A progressive guide to how buildings carry gravity and lateral forces, how major structural systems work, how foundations complete the load path, and how structure shapes architectural space and expression.
Structural principles and equilibrium
A building structure is an organized system of members, materials, connections, and foundations that carries forces safely to the ground. It makes architectural space possible by supporting floors and roofs, creating spans and openings, resisting wind and earthquakes, and maintaining stability.
A useful way to understand structure is as a device for channeling loads to the ground. Every applied force should have an understandable and continuous . That path may pass through a roof deck, beams, columns or walls, footings, and soil. If any link is missing, poorly connected, or unable to carry its share of the force, the whole system can be compromised.
Three requirements for structural performance
is the capacity to resist failure.
is the capacity to limit deformation, vibration, or movement.
Stability is the capacity of a complete system to remain in equilibrium rather than overturn, slide, buckle, or collapse.
These requirements are related but not interchangeable. A member can be strong enough not to break while still being too flexible for comfortable use. A collection of strong members can also become unstable if connections are inadequate or if the system lacks bracing.
Equilibrium and internal actions
A building at rest satisfies three basic equilibrium conditions:
These conditions mean that the sums of horizontal forces, vertical forces, and moments are zero. For example, a beam supported at both ends remains in equilibrium when its support reactions balance its applied loads.
Structural members respond through several common actions:
Compression: pushing forces shorten or squeeze a member.
Tension: pulling forces elongate a member.
Bending: loads cause a member to curve.
Shear: adjacent parts tend to slide past one another.
Torsion: a member twists around its longitudinal axis.
Buckling: a slender compression member bends sideways suddenly under load.
Most members experience combinations of these actions. A column may carry compression while also bending because of wind or an eccentric load.
Takeaway: Structural design coordinates , , and stability through a continuous path from applied forces to the ground.
Loads and continuous load paths
Loads are forces applied to a building. Identifying their direction, duration, and location helps explain how the structural system must be arranged.
Gravity loads
Gravity loads act generally downward. They include:
Dead loads: the permanent weight of structure, floors, roofs, walls, finishes, mechanical equipment, and other fixed components.
Live loads: changeable loads from people, furniture, stored materials, vehicles, and movable equipment.
Environmental gravity loads: snow, rainwater, and ponded water.
A typical roof is:
A floor may transfer loads from people and furniture through a slab or joists, then to beams, columns or walls, footings, and soil. The path should remain continuous. When a beam supports a column without an adequately aligned member below it, the condition is a transfer condition. Transfer beams and transfer floors can resolve the discontinuity, but they are often deeper, heavier, and more expensive than direct alignment.
Lateral loads
Lateral loads act primarily sideways. Important sources include wind pressure and suction, earthquakes, soil and water pressure, impact, blast, thermal movement, and differential settlement.
A typical lateral is:
Wind acts on the building envelope and can produce sway and overturning. Earthquake forces result mainly from the inertia of the building’s mass as the ground moves. In general, greater mass produces greater inertial force for the same ground acceleration.
A distributes lateral forces across a floor or roof plane. A concrete slab can act as a stiff horizontal plate. Collectors, also called drag struts, gather forces within the and deliver them to shear walls or braced frames.
Openings, irregular floor plans, weak connections, and discontinuous walls can interrupt a lateral path. Effective planning aligns vertical resisting elements, limits abrupt changes, and provides reliable connections between floors, walls, frames, and foundations.
Takeaway: A structural system is understandable when both gravity and lateral forces can be traced continuously to the soil.
Walls, frames, trusses, and space frames
Major structural systems differ in how they arrange supports, span space, and resist compression, tension, bending, and lateral movement.
Bearing walls and masonry
A uses walls to support floors and roofs as well as their own weight. Stone, brick, and adobe are especially effective in compression but comparatively weak in tension. Openings interrupt the wall, so lintels, arches, or reinforced beams are needed to carry loads around doors and windows.
Arches redirect loads along curved compression paths. Buttresses resist the outward thrust of vaults and arches, while reinforcement improves resistance to tension, shear, and earthquake forces. The Pantheon demonstrates how a concrete and masonry dome can channel loads through compression toward supporting rotunda walls. Coffers reduce weight, and the oculus reduces material at the crown.
Masonry systems are durable, fire-resistant, thermally massive, and capable of expressing solidity and permanence. Their limitations include high weight, limited tensile capacity, restrictions on wall thickness and openings, and vulnerability to damage when unreinforced in seismic conditions.
Post-and-beam and frame systems
A uses discrete columns and beams rather than continuous bearing walls. This arrangement allows exterior walls to become lighter enclosure systems. Columns primarily carry axial compression, beams resist bending and shear, slabs distribute floor loads, and connections transfer forces between members.
Braced frames use diagonal members. Triangular geometry allows braces to carry tension and compression efficiently.
Moment frames use rigid beam-column connections to transfer bending moments and resist lateral forces. They support open planning but often require heavier members and carefully designed connections.
Shear-wall systems use relatively solid vertical walls to resist lateral shear and overturning. Reinforced concrete and reinforced masonry are common materials.
The Farnsworth House illustrates how a minimal steel frame can elevate a transparent living space and make columns and floor plates part of the architectural identity.
Trusses and space frames
A truss is an assembly of slender members connected to form triangles. In an idealized truss, members carry primarily axial tension or compression rather than bending. Greater depth generally improves spanning efficiency. A space frame extends this principle into three dimensions, distributing loads in multiple directions and covering large column-free areas.
Connections are crucial because forces must transfer between members. Structural depth must also be coordinated with ceilings, services, and other building systems.
Takeaway: The choice of structural system affects not only the force path but also enclosure, openness, span, material use, and architectural character.
Curved, shell, and tensile structures
Curved and tension-based systems use geometry to achieve efficient spans and distinctive spatial effects.
Arches, vaults, and domes
An arch carries loads primarily through compression along a curved path. Its supports must resist vertical reactions and outward horizontal thrust. A vault is an extended or intersecting arch, while a dome is a curved shell generated around a central axis.
Gothic cathedrals combine pointed arches, ribbed vaults, and flying buttresses. Ribs organize the vault geometry, and buttresses carry outward thrust to the ground outside the main wall. The visible structure communicates a force path through piers, ribs, and buttresses.
Shell structures
A shell is a thin, curved surface that carries loads through its geometry. Curvature provides and can allow a large span with less material than a flat plate of similar span. Shell forms include domes, barrel vaults, hyperbolic paraboloids, and folded plates.
Shell behavior depends on geometry, edge conditions, openings, imperfections, and construction accuracy. A thin shell may be materially efficient but vulnerable to buckling if it is poorly supported or irregularly loaded. The State Farm Center illustrates how a thin concrete shell can create a rigid, low-profile dome over a large interior.
Tensile and membrane systems
A tensile structure carries loads primarily through tension. Cables, fabric membranes, and thin rods can span long distances while remaining lightweight. Because cables cannot resist compression, they must be anchored or combined with compression elements such as masts, arches, or towers.
A cable takes a curved shape called a catenary under a uniform load per unit length. Tensile systems often use prestress, which introduces tension before service loads are applied. Prestress helps maintain shape and reduces unwanted movement.
In a suspension bridge, the deck transfers loads to vertical suspenders, then to main cables, towers, anchorages, and foundations. Main cables carry tension, while towers carry compression. This combination of tension and compression also appears in large-span roofs and canopies.
Takeaway: Curvature and prestress are not merely visual effects; they are ways of organizing forces so that a system can span efficiently.
Foundations and ground behavior
The completes the building’s structural system by transferring forces into soil or rock. It must distribute loads without excessive settlement, sliding, overturning, or bearing failure. Design depends on building weight, structural layout, soil , groundwater, frost, seismic conditions, and nearby construction.
Shallow foundations
Shallow foundations transfer loads near the ground surface. Common forms include:
Strip footings: continuous supports beneath bearing walls.
Spread footings: individual bases beneath columns or piers.
Combined footings: one footing supporting two or more columns.
Raft or mat foundations: large reinforced slabs supporting many columns or walls.
Deep foundations
Deep foundations transfer loads to deeper, stronger soil layers or develop resistance through friction along their length. Piles and drilled shafts are used when near-surface soils are weak, compressible, or highly variable.
performance
A must address:
Bearing capacity: whether the soil can support the applied pressure.
Settlement: downward movement caused by soil compression.
Differential settlement: unequal movement that can crack walls or distort frames.
Sliding: horizontal movement caused by lateral forces.
Overturning: rotation caused by wind, earthquakes, or eccentric loads.
Uplift: a tendency for part of the structure to rise under wind or overturning forces.
The is not an isolated technical layer. It is part of the complete , and a well-designed superstructure can still fail if its forces are not transferred safely into the ground.
Takeaway: A structural concept is incomplete until the forces have a safe and compatible route into the soil or rock.
Structure as architectural expression
Structure becomes architectural when its form, material, rhythm, or construction logic contributes to the experience of a building. may be direct, as with a visible steel frame, or indirect, as with a concrete shell whose curvature communicates its spanning action.
Useful design questions include:
Is the structure visible or concealed?
Does the structural grid organize rooms, circulation, and furniture?
Are materials expressed according to their structural behavior?
Does the building reveal how forces travel to the ground?
Does the system support adaptability and future change?
How does the structure shape the relationship between mass, light, enclosure, and space?
Examples of architectural expression include:
Thick masonry walls that communicate mass and permanence.
Classical columns that establish post-and-lintel order.
Gothic ribs and buttresses that reveal vertical and lateral force paths.
Steel frames that express industrial production and modular coordination.
Concrete shells that communicate continuity and geometric efficiency.
Cable roofs that express lightness, tension, and long-span movement.
A method for reading structure
Identify the primary system: bearing wall, frame, shell, arch, truss, tensile system, or combination.
Locate columns, walls, piers, arches, masts, and other supports.
Trace gravity loads from roofs and floors to foundations.
Trace lateral loads through diaphragms, collectors, and resisting elements.
Observe which materials primarily resist compression, tension, bending, or combinations of actions.
Examine connections to determine whether the system is pinned, rigid, suspended, or continuous.
Look for discontinuities such as transfer beams, cantilevers, large openings, setbacks, irregular plans, and abrupt changes.
Relate the system to ceiling height, column spacing, enclosure, daylight, movement, and flexibility.
Decide whether the structure is hidden, revealed, exaggerated, or transformed into architectural form.
The strongest usually corresponds to real structural behavior. Spatial form, materials, construction, and structural action are most convincing when they are developed together rather than treated as separate layers.
Final takeaway: To understand a building, connect what it looks like to what it carries, how its members connect, and how every force reaches the ground.