Why construction managers need structures literacy

Every building is a structure before it is anything else. Long before the finishes go in, the frame has to carry its own weight, the people and things that will use it, and the snow, wind and earthquakes it will meet over its life. During construction it also has to carry loads that the finished building never sees: wet concrete in forms, cranes on slabs, stacked materials, unbraced walls and half-built frames that are not yet stable. Most serious construction collapses happen during this temporary stage, not after the building is finished.

This course teaches structures from the constructor's point of view. It is not a structural design course, and it will not qualify you to design a beam, a footing or a shoring system. Licensed professional engineers do that, and the law in every province and state reserves it to them. What a construction manager needs is structures literacy: enough understanding of loads, members and systems to read structural drawings with insight, to recognize when a site decision affects the structure, to ask the engineer the right question at the right time, and to plan and supervise temporary works safely.

This course builds on the materials covered in CON-110 and CON-111 and the mathematics in CON-130. Structural drawings were introduced in CON-120 (lesson 7). Here we look at why the structure is shaped the way it is and what that means for the people who build it.

This course is for awareness only. It does not replace the judgment of the engineer of record, a temporary works engineer or the requirements of the building code and occupational health and safety law in your jurisdiction. When in doubt, stop and ask an engineer.

What structures literacy looks like on site

Structures literacy shows up in everyday decisions that seem small but can change how a building carries load.

• A trade asks to core a 150 mm hole through a post-tensioned slab next to a column. Do you approve it, or send an RFI to the engineer first?
• A supplier delivers 20 pallets of drywall and wants to stack them all on one bay of a newly poured floor. Can that slab take the load at seven days old?
• The crane contractor proposes to set a mobile crane on the podium slab rather than on grade. Who checks the slab and the outrigger pads?
• A masonry wall is built to full height on a windy afternoon before the floor that will brace it is in place. Is it safe overnight?
• The formwork crew wants to strip the slab soffit forms early to keep the cycle moving. What strength does the concrete need, and who confirms it?

In each case the right answer is not to calculate the structure yourself. It is to recognize that a structural question exists, to know roughly how big the risk is, and to get the right person to answer it before the work proceeds.

Who is responsible for what

Structural responsibility on a project is divided among several parties, and confusion about who owns what is a common root cause of failures.

A key distinction is between the permanent works, which the engineer of record designs, and the temporary works, which the contractor is responsible for. In most standard contracts the contractor controls means, methods and sequences of construction, and with that control comes responsibility for the stability of the partly built structure. That is why contractors retain their own temporary works engineers.

In Canada the practice of professional engineering is regulated by provincial and territorial associations, such as Professional Engineers Ontario (PEO), APEGA in Alberta and Engineers and Geoscientists BC (EGBC). Their acts reserve structural design to licensed engineers, and provincial occupational health and safety rules often require engineered designs for particular temporary works. In Ontario, for example, the construction projects regulation (O. Reg. 213/91) requires certain formwork, falsework and reshoring to be designed by a professional engineer and checked before concrete is placed.

The building code and the structural standards

The building code sets the loads a structure must resist and points to material standards that set how each material is designed.

As in other parts of construction, adoption lags publication. A project designed under one edition may be built while the next edition is being adopted. The engineer of record works to the edition referenced in the building permit, and the construction manager should know which one applies.

The design process and where constructors fit

The structural engineer works through a sequence that mirrors the architectural design phases. In schematic design the engineer chooses a structural system and grid. In design development members are sized and the lateral system is settled. In construction documents every member, connection and detail is drawn and specified. During construction the engineer reviews shop drawings, answers requests for information and carries out field reviews.

Constructors can add a great deal of value early. Choosing a structural system affects cost, schedule, crane size, formwork reuse and site logistics. A slab that repeats from floor to floor lets formwork cycle quickly. A steel frame with consistent member sizes reduces fabrication cost. A constructor who understands structures can offer this kind of advice during preconstruction instead of discovering problems after the drawings are complete.

Units and conventions

Canadian structural drawings use SI units. Loads are given in kilonewtons (kN) for point loads, kilonewtons per metre (kN/m) for line loads and kilopascals (kPa, which is kN/m²) for area loads. Stresses are in megapascals (MPa). Concrete strength is specified in MPa, for example 30 MPa or 35 MPa. Masses are converted to forces with g = 9.81 m/s², so 1,000 kg exerts about 9.81 kN.

Engineers also use sign conventions and abbreviations you will see throughout this course and on drawings: D for dead load, L for live load, S for snow, W for wind and E for earthquake; V for shear, M for bending moment and P for axial load. Drawings mark typical details with TYP and similar conditions with SIM, as you learned in CON-120.

How structures fail during construction

Studies of construction collapses repeatedly find the same causes. The permanent structure is rarely the problem on its own; failures usually involve temporary works, sequence or an unreviewed change.

• Formwork and falsework failures during concrete placement, often from inadequate design, missing bracing, poor foundations for shores or placing concrete faster than the design assumed.
• Premature stripping or loading of concrete that has not reached the strength the design needs.
• Loss of stability of partly erected steel, precast or timber frames before the permanent bracing and diaphragms are complete.
• Unbraced masonry and tilt-up walls blown over by wind.
• Overloading of floors with stored materials or equipment, including cranes and concrete pumps.
• Excavation and trench collapses, and undermining of adjacent foundations.
• Field changes made without the engineer's review, such as cutting a beam for a duct or substituting a lighter member.

Lesson 12 returns to case studies. For now the lesson is that a structure is most vulnerable while it is being built, and the constructor is the party in control at that time.

How this course is organized

The course moves from fundamentals to systems to temporary works. Lessons 2 to 4 cover loads, forces and members. Lessons 5 to 8 look at concrete, steel, timber and masonry systems and what each means for construction. Lesson 9 covers foundations and retaining structures. Lessons 10 and 11 deal with temporary works: formwork, falsework, shoring, reshoring, bracing, excavation support and crane loads. Lesson 12 brings everything together in case studies and a capstone temporary works plan for a mid-rise concrete building.