6.4 Construction: Means, Methods & Safety

Temporary Structures and Construction Loads

Temporary structures support the work until the permanent structure can carry itself:

• Formwork molds and contains fresh concrete; it must resist lateral concrete pressure (which increases with pour rate and decreases as concrete sets), dead load, and worker/equipment live load.
• Shoring is vertical support that transfers loads to a stable foundation (e.g., shores under elevated slabs, or trench shoring against soil).
• Falsework is the temporary framework supporting bridge spans or formwork during construction.
• Reshoring re-supports a slab after stripping forms but before full strength.

Construction loads are often the governing case during erection: stacked materials, equipment, impact from placing concrete, wind on bare frames, and unbalanced loads before the system is complete. The American Concrete Institute (ACI) and ASCE 37 give construction load criteria. A common exam idea: the fresh-concrete lateral pressure on wall forms, p = C_w·C_c·[150 + 9000·R/T] (psf), where R is pour rate (ft/hr) and T is temperature (°F), bounded by a maximum of ρ·h (full fluid head).

Excavation, Trench Safety, and OSHA Soil Types

Excavation collapse is one of construction's deadliest hazards — a cubic yard of soil weighs roughly 3,000 lb. Under OSHA 29 CFR 1926 Subpart P, any excavation 5 ft (1.5 m) deep or greater must have a protective system unless cut entirely in stable rock; a competent person must inspect daily. The three protective systems are sloping/benching, shoring (supports like hydraulic shores or trench jacks), and shielding (trench boxes).

OSHA classifies soil by unconfined compressive strength and assigns maximum allowable slopes:

tsf = tons per square foot. Note Type A excludes fissured, previously disturbed, or vibration-exposed soil. Spoil piles must be kept at least 2 ft from the trench edge, and atmospheric testing is required in excavations deeper than 4 ft where hazardous atmospheres may exist.

Cranes, Rigging, and the Focus Four

Cranes and rigging raise the heaviest loads on site. Core safety ideas the FE may touch:

• Stay clear of and de-energize/maintain clearance from overhead power lines (electrocution risk).
• Respect the load chart / rated capacity — capacity drops as the load radius increases and the boom is lowered.
• Sling tension rises as the sling angle to horizontal decreases: T = W/(n·sin θ). At a 30° sling angle, each of two slings carries the full load W, not half.
• Inspect rigging for wear; use tag lines to control swing.

OSHA's 'Focus Four' hazards cause the majority of construction fatalities — memorize them:

1. Falls (from heights — the single leading cause; guardrails/PFAS required generally at 6 ft in construction)
2. Struck-by (vehicles, falling/flying objects, swinging loads)
3. Caught-in/between (trench cave-ins, machinery, pinch points)
4. Electrocution (power lines, unguarded wiring, lockout/tagout failures)

Productivity is output per unit input (e.g., cubic yards placed per crew-hour); learning-curve effects, weather, congestion, and overtime fatigue all reduce it. Safety is a productivity issue too: incidents stop work and raise insurance/experience-modification costs.

Scaffolding, Fall Protection, and Spacing Trade-offs

Scaffolding must be designed to support its own weight plus at least 4 times the maximum intended load without failure (OSHA), and suspension-scaffold rigging to 6 times the intended load. Supported scaffolds with a height-to-base width ratio greater than 4:1 must be restrained (guyed/tied) against tipping. Planking, guardrails, and access (ladders, not climbing cross-bracing) are routine inspection items.

Fall protection in construction is generally required at 6 ft above a lower level (29 CFR 1926.501): guardrail systems, safety-net systems, or personal fall arrest systems (PFAS) — anchorage, body harness, and connector. (Note general industry triggers fall protection at 4 ft, and scaffolds at 10 ft — depth-versus-context distinctions the FE may probe.)

Sloping vs shoring trade-off. Sloping/benching needs space and generates more spoil to re-handle, but uses no equipment; shoring or trench boxes save right-of-way and excavation volume but add cost and installation hazard. The competent person selects the system from soil type and depth.

These thresholds are recurring FE Construction questions; know the numbers, not just the concepts.

When a question gives a depth and a soil type, the safe path is mechanical: first ask whether the excavation reaches 5 ft (protective system required), then read the maximum allowable slope for that soil type (A 3/4:1, B 1:1, C 1.5:1) to size a sloped cut, or specify shoring or a trench box if right-of-way is tight. For overhead work, check the 6-ft fall-protection trigger and whether scaffold height-to-base width exceeds 4:1. Reading the wrong threshold for the context (general-industry 4 ft instead of construction 6 ft, or a Type B slope on Type C soil) is the usual trap.