Buildings

Building structures must carry gravity and lateral loads. Learn structural systems (load-bearing, frames, shear walls, cores, tube, outrigger), wind loads, drift limits, floor systems, foundations and the special challenges of tall buildings.

A building's structure is its skeleton: it carries floors and people down to the ground and resists wind and earthquakes trying to push it sideways. As buildings get taller, lateral loads rather than gravity start to govern.

Choosing the right structural system at the concept stage is the most important structural decision for any building.

An analogy

A tall building is like a vertical cantilever beam fixed in the ground: wind pushes on it like a load on a diving board, and it must be both strong and stiff enough not to sway uncomfortably.

Structural systems

Load-bearing masonry (low-rise), RC or steel moment frames (up to ~20 storeys), shear walls and braced frames (stiffer), core + frame (most mid/high-rise), framed tube and bundled tube (supertall), outrigger systems linking core to perimeter columns, diagrid exteriors.

Lateral loads and drift

Wind pressure grows with the square of speed and with height. Earthquake forces depend on mass and stiffness. Drift (sideways movement between floors) must be limited to protect partitions and cladding; occupant comfort limits accelerations in tall buildings.

Floors and transfer structures

Floor systems (flat slabs, beam-slab, post-tensioned, composite, precast) act as diaphragms that distribute lateral forces to walls and frames. Transfer beams/slabs redirect columns where the grid changes (e.g. podium parking) — heavy and critical elements.

Tall-building engineering

Wind-tunnel testing, tuned mass dampers (Taipei 101), differential shortening of columns and cores, P-Δ effects, foundation rafts and piles, and construction sequence analysis all matter. Performance-based wind and seismic design is common for supertall towers.

Formulas

• Wind pressure: q = ½ ρ V² (ρ ≈ 1.225 kg/m³)
• Wind force: F = C_f · q · A
• Drift limit: Δ/h ≤ 1/500 (wind), 0.004h (seismic, IS 1893)
• Overturning moment: M_ot = F · (height of resultant)

Worked example: Wind force on a building face

Given: Face 20 m wide × 30 m tall; V = 40 m/s (uniform for simplicity), C_f = 1.3

Find: Force and overturning moment.

• q = ½ × 1.225 × 40² = 980 Pa
• F = 1.3 × 0.98 × 600 = 764 kN
• M_ot = 764 × 15 = 11 470 kN·m

Answer: ≈ 764 kN, ≈ 11 500 kN·m

Worked example: Drift check

Given: Storey height 3.5 m; Computed wind drift 6 mm

Find: Check against h/500.

• Limit = 3500/500 = 7 mm
• 6 ≤ 7 ✓

Answer: OK (86 % utilised)

Common mistakes

• Wrong: Placing stiff cores asymmetrically. — Right: Keep lateral systems symmetric to avoid torsional response.
• Wrong: Designing for gravity first and 'adding' lateral later. — Right: Choose the lateral system at concept stage.

Applications

• Residential & offices: RC frames with shear walls or cores; flat-slab floors.
• Tall & supertall: Core–outrigger, tube and mega-frame systems with dampers.
• Industrial & large-span: Steel portal frames, space frames and long-span trusses.

Key takeaways

• Pick the lateral system first: frame, wall, core, tube, outrigger.
• Wind q = ½ρV²; force = C_f q A.
• Limit drift (≈ h/500) and accelerations.
• Tall buildings need special analysis and testing.

Going further

• Tuned mass dampers: Large pendulum or sloshing dampers reduce wind-induced accelerations at the top.
• Wind-tunnel testing: Scale models measure pressures and responses, often reducing design loads.
• Differential shortening: Creep and shrinkage of columns vs core must be compensated during construction.