Temporary works II: bracing, cranes and construction loads

The previous lesson covered the temporary works that hold up concrete. This lesson covers the other loads and temporary structures that constructors put on and around a building while it is going up: temporary bracing and hoarding exposed to wind, excavation support, cranes and their foundations and outrigger pads, hoists and scaffolds tied to the structure, fall protection anchors, and the stockpiles and equipment placed on partly finished floors. These are often treated as routine site logistics, but each of them is a structural load or a structure in its own right, and each has caused serious failures when nobody checked it.

Construction loads are design loads too

A permanent building is designed for occupancy loads that are usually much smaller than the loads that occur during construction. An office floor is designed for a live load of about 2.4 kPa (50 psf), yet during construction it may carry pallets of drywall, rebar bundles, a scissor lift, a concrete buggy, or a mobile crane outrigger. Concentrated construction loads can be many times the design load over a small area, and they are often placed on young concrete, on steel deck that has not been filled, or on slabs that are still reshored.

In the United States, ASCE 37 (Design Loads on Structures During Construction) defines construction loads, including material, personnel and equipment loads, and OSHA 1926.20 and related rules require that structures not be loaded beyond their safe capacity. Many specifications require the contractor to submit construction loading plans for engineer review.

Wind on temporary structures

Temporary structures catch wind just as permanent ones do, and often more, because they are light, tall, and covered with sheeting or panels. Hoarding, site fencing with privacy screens, scaffolds wrapped in debris netting or shrink-wrap, temporary walls, formwork panels, and stacked materials can all be blown over or lifted. The wind load is:

where q is the design wind pressure, Cf is a force or shape coefficient and A is the exposed area. Codes allow reduced design wind pressures for short construction periods, but only within limits, and the temporary works designer chooses the value.

In the United States, ASCE 37 allows reduced wind speeds for construction periods based on the exposure duration, and OSHA Subpart L covers scaffolds, which must be tied to the structure and designed for the loads applied to them, including wind on enclosures.

Adding wrap or netting to a scaffold greatly increases its wind load. A scaffold designed and tied for an open frame may need many more ties once it is sheeted. Any change in enclosure, such as winter hoarding, must go back to the scaffold designer.

Excavation support

CON-130 covered excavation and trench safety. From a structural point of view, excavation support systems such as soldier piles and lagging, sheet piles, secant or contiguous pile walls, and trench boxes are retaining structures that carry earth pressure, groundwater pressure and surcharges, the same loads described in Lesson 9. They are temporary works, usually designed by the contractor's engineer, and they often support neighboring buildings, streets and utilities.

In the United States, OSHA 29 CFR 1926 Subpart P requires a protective system (sloping, benching, shoring or shielding) for excavations 5 ft or deeper unless in stable rock, and requires protective systems for excavations deeper than 20 ft to be designed by a registered professional engineer.

Key structural controls include keeping surcharges such as spoil piles, equipment and cranes back from the edge or including them in the design, monitoring walls and neighboring structures for movement, installing tiebacks or struts in the designed sequence without over-excavating, and managing groundwater.

Cranes and their foundations

Cranes are among the largest temporary loads on a site. Mobile cranes transfer their weight plus the load and its moment to the ground through outriggers or crawler tracks. Under the outrigger nearest the load, the reaction can approach the total weight of the crane and load combined. Tower cranes rest on engineered foundations, sit on the building structure, or climb within it, and impose large vertical, horizontal and overturning forces.

In the United States, OSHA 29 CFR 1926 Subpart CC (Cranes and Derricks in Construction) requires ground conditions to be firm, drained and graded sufficiently to support the crane, with the controlling entity responsible for ground conditions (1926.1402). ASME B30.5 covers mobile cranes and ASME B30.3 tower cranes.

Cranes on suspended slabs, podium decks or near basement walls need an engineer's check of the structure below, and often temporary shoring through several floors. Tower crane foundations are engineered for the specific crane, height and site; the foundation concrete must reach its specified strength before the crane is erected, and climbing cranes impose loads on the building that the engineer of record must accept.

Hoists, scaffolds and anchors attached to the structure

Material and personnel hoists, mast climbing work platforms and tall scaffolds are tied to the building at intervals. The ties push and pull on the structure, often on slab edges or columns that were never designed for them. Tie locations and forces should be given by the hoist or scaffold designer and accepted by the engineer of record. Ties must not be removed by other trades, for example to install windows or cladding, without a planned replacement.

Fall protection anchors must withstand the forces generated in a fall, which are large: a person falling a short distance onto a lanyard can generate several kilonewtons even with an energy absorber.

In the United States, OSHA 1926.502(d)(15) requires anchorages for personal fall arrest systems to support at least 5,000 lb per employee attached, or to be designed, installed and used as part of a complete system with a safety factor of at least two under the supervision of a qualified person.

Typical problems include anchoring to guardrails, conduit, roof drains, ductwork or light framing; anchors drilled into young concrete; and post-installed anchors not installed to the manufacturer's instructions. Use engineered or rated anchors only.

Construction loading plans

A construction loading plan brings these issues together. For a building, it shows each floor's allowable construction loads by zone, where materials may be stored (usually near columns and beams, never at mid-span or over openings), where equipment may travel, crane and hoist locations, scaffold and hoist ties, and any temporary shoring needed under heavy loads. It is prepared by the contractor with the temporary works engineer and reviewed by the engineer of record, then posted and enforced on site.

Site supervisors make loading decisions every day, often under pressure from deliveries. A posted plan, signage on each floor and a simple rule, 'no stockpiles without approval', prevent most overloads.

Loading decisions also depend on timing. A slab that can carry a pallet of block at 28 days may not carry it at 7 days, and a slab still supported by reshores may already be carrying load from the floors above. When in doubt, the question to ask is not only how heavy the load is, but what the slab is already carrying and how strong it is today.