OBC Large Buildings common mistakes
OBC Large Buildings · Understand the traps and how to avoid them.
Assigning an occupancy classification from the building's overall use or the owner's label (for example calling a mixed-use tower simply 'residential') instead of classifying each space, suite, and storey separately and then applying the major occupancy and mixed-occupancy rules.
Candidates memorize the broad building type from the project description and never break the building into its component occupancies, so they skip the Group A/B/C/D/E/F decision and the resulting fire-separation and construction requirements.
What to do instead
Before answering any classification item, list every distinct use in the building, assign a Group and division to each using the definitions, identify the major occupancies, then determine whether the building is treated as a single occupancy or as a mixed occupancy with required separations. Practice by taking a real floor plan and labeling each room's Group.
Treating allowable building area and building height as fixed numbers looked up once, rather than as values that change with construction type, sprinkler protection, frontage, and occupancy, and forgetting to apply the increases and limits in the correct order.
The area and height tables are memorized as isolated figures, so candidates miss that sprinklering, open frontage, and street width modify the base value, and they apply increases to the wrong base or stack them incorrectly.
What to do instead
Work every size problem as a sequence: base allowable area from the table for the occupancy and construction type, apply the sprinkler increase, apply the frontage increase using the actual perimeter and street widths, then compare to proposed area. Do the same for height, checking storeys and metres separately. Show each step so a wrong step is visible.
Confusing the roles and timing of the various submissions: treating a building permit application as if it were a development permit, or assuming a professional's sealed drawings substitute for inspections, or answering questions about inspections without distinguishing the stages (footing, framing, insulation, final).
Candidates learn the permit and inspection process as a single blurred 'approvals' step and never map which document is filed at which stage or who is responsible for notifying whom.
What to do instead
Build a one-page timeline: pre-application, development permit, building permit with required drawings and schedules, then inspections in construction order, then occupancy and final. For each stage note the applicant's action and the authority's action. Answer administration questions by locating the stage first, then the responsible party.
Answering means-of-egress questions by counting exits only, while ignoring travel distance, dead-end limits, common path, exit width, door swing, panic hardware, and the requirement that egress be continuous and unobstructed to a public way.
Exit count is the most memorable egress rule, so candidates stop there and overlook the geometric and hardware conditions that the question is actually testing.
What to do instead
For each egress scenario, check in order: number of exits required by occupant load, remoteness of exits, travel distance from the most remote point, dead-end and common-path limits, width based on occupant load, door swing and hardware, and continuity of the path. Write these as a checklist and run it on every egress item.
Calculating occupant load from the gross floor area of the whole building or the whole storey instead of applying the correct area-per-person factor to each use area and summing, and then using that load for the wrong purpose (egress width versus plumbing fixtures versus exits).
Candidates treat occupant load as one number for the building and do not realize different occupancies and areas use different factors, nor that the same load feeds several separate code calculations.
What to do instead
Compute occupant load room by room or use-area by use-area using the factor for that use, sum per storey, and note where the code allows or requires a different basis (for example fixed seating or business/mercantile assumptions). Then explicitly state which downstream calculation each load feeds before using it.
Mixing up the fire-resistance rating of a structural element with the fire-protection rating of a closure, or assuming that a rated wall automatically means a rated door, damper, or penetration seal with the same number.
The terms sound interchangeable and the tables are dense, so candidates carry one rating across to every assembly in the compartment boundary.
What to do instead
Learn the distinction: an assembly has a fire-resistance rating expressed in hours; a closure in an opening in that assembly has its own fire-protection rating, which is usually lower and depends on the wall's rating and the opening's size and location. When a question involves a rated separation, identify the assembly rating first, then look up the closure rating separately, and check that penetrations and joints are also protected.
Applying accessibility requirements to the building entrance and a token washroom but not to the full route: parking, exterior path, door hardware and clearances, turning spaces, grab bars, signage, and vertical circulation between levels.
Candidates answer from a mental image of a wheelchair user at a door and forget that accessibility is a continuous path requirement with dimensional detail at each element.
What to do instead
Trace an accessible path from the site boundary to the furthest point of each type of use, checking each element in sequence: exterior route and slope, entrance, door width and hardware, interior route and turning space, vertical circulation, then the fixtures and fittings in each room. Memorize the key dimensions as a short list and verify each one against the question's numbers.
Answering energy efficiency and environmental separation items by naming a general principle (for example 'insulate the envelope') instead of applying the specific requirement: thermal performance of the assembly, continuity of the air and vapour barrier, prevention of condensation, and the correct treatment of windows, doors, and junctions.
These provisions are less familiar than egress and occupancy rules, so candidates fall back on general construction knowledge and miss the code-specific performance values and continuity requirements.
What to do instead
Study this domain as a set of physical mechanisms: heat flow, air leakage, vapour diffusion, and moisture accumulation. For each question identify which mechanism is at issue, then state the requirement in terms of the assembly's performance, the continuity of each barrier, and the consequence of a break at a junction. Sketch the wall or roof section and mark where each barrier runs.
Reading a structural or load question as a materials question and answering with a general statement about concrete or steel instead of identifying the load type (dead, live, snow, wind, earthquake, occupancy-specific) and the load combination or design basis being tested.
Candidates have more exposure to construction materials than to load determination, so they default to material talk and never reach the actual design question.
What to do instead
Classify the load first: is it permanent, variable, environmental, or induced by use? Then recall the applicable design basis and the relevant combination or importance factor. Practice by taking a structural drawing note and naming the load type and the check it implies before considering materials.
Treating plumbing, mechanical, and electrical provisions as separate trades to be guessed at, rather than as code requirements tied to occupancy, occupant load, and life safety: fixture counts, ventilation rates, fire alarm and detection coverage, and emergency power for life safety systems.
Candidates with a general construction background know how systems are installed but not how the code sizes and triggers them, and they avoid the numerical content.
What to do instead
Link each system back to its trigger: fixture ratios to occupant load and occupancy, ventilation to occupancy and use, alarm and detection to occupancy and building height or area, emergency power to the life safety systems that depend on it. Build a table of trigger-to-requirement and rehearse it until the numbers are automatic.