OBC Building Structural common mistakes
OBC Building Structural · Understand the traps and how to avoid them.
Applying ultimate limit state factored loads to a serviceability check, or using service loads for a strength check, so deflection, drift, or vibration results are computed with the wrong load combination.
Candidates memorize a single load combination table and then reuse it for every check, forgetting that limit states are paired with specific load factors and that serviceability uses unfactored (or lower-factor) combinations.
What to do instead
Before computing anything, state which limit state is being checked and write the governing combination beside it; keep a two-column scratch sheet (strength vs. service) and verify every number is drawn from the correct column.
Treating a tributary area as the full span area, or splitting tributary load incorrectly between one-way and two-way spanning members, producing member forces off by a factor of roughly two.
Tributary width is often half the span to each side, and candidates forget the interior-member versus edge-member distinction or default to one-way distribution on a two-way slab.
What to do instead
Sketch the member, draw the 45-degree yield lines or half-span boundaries, shade the actual tributary region, and compute its width explicitly before multiplying by the uniform load.
Forgetting to reduce live load for large tributary areas, or applying the reduction when the occupancy or member type does not permit it.
The reduction provision has exceptions (e.g., certain roof, assembly, or storage occupancies) and a lower bound, and candidates either ignore the clause entirely or apply it indiscriminately.
What to do instead
Memorize the reduction formula, its minimum floor, and its exclusion list as a single unit; on each problem, first ask whether the occupancy is reducible, then compute and check against the minimum.
Computing snow load without the exposure, thermal, or slope factors, or using ground snow load directly as roof load.
The snow load chain has several multiplicative modifiers, and under exam time pressure candidates collapse it to a single ground-to-roof conversion.
What to do instead
Write the full snow load equation with every factor named before substituting numbers, and confirm each factor's value against the stated terrain, roof surface, and insulation conditions in the problem.
Using the wrong velocity pressure exposure category or missing the topographic factor and internal pressure coefficient when computing wind pressures on an enclosed building.
Exposure depends on upwind terrain roughness and building height, and internal pressure depends on enclosure classification; these are easy to overlook because they are stated in prose rather than in the equation.
What to do instead
Extract exposure category, enclosure classification, and any topographic feature from the problem statement first, then assemble the velocity pressure equation with each coefficient labeled before multiplying.
Selecting the seismic force-resisting system and its response modification factor without checking height limits, or mixing R, overstrength, and deflection amplification factors.
The three seismic factors look similar and are tabulated together; candidates also skip the system height-limit footnote that disqualifies the chosen system for a tall building.
What to do instead
For each seismic problem, write the system name, confirm it is permitted at the given height and seismic design category, then list R, the overstrength factor, and the deflection amplification factor separately with their distinct uses.
Assuming a fixed base or perfect pin without verifying that the detailing can develop that restraint, leading to unconservative moment or deflection results.
Idealized support conditions are drawn as triangles and circles, and candidates treat them as physical facts rather than assumptions that must be justified by connection detail.
What to do instead
For every support, state the assumed rotational restraint and name the detail that provides it; where the detail cannot develop moment, redo the analysis as pinned and compare.
Confusing strength reduction factors with load factors, or applying the wrong resistance factor for the limit state (tension-controlled versus compression-controlled flexure, shear versus flexure).
Both are decimal multipliers applied in the same design inequality, and the resistance factor changes with strain condition and failure mode.
What to do instead
Write the design inequality with the resistance factor and load effect on opposite sides, label each, and determine the strain or failure mode before choosing the resistance factor value.
Detailing a connection that the member check never verified, such as ignoring block shear, prying action, or the eccentricity of a bolted bracket.
Candidates stop once the main member passes and treat the connection as a formality, missing secondary limit states and the moment induced by eccentric load transfer.
What to do instead
After the member check, list every connection limit state explicitly (bolt shear and bearing, block shear, weld capacity, prying, eccentric moment) and verify each with its own calculation.
Ignoring required fire-resistance ratings and the cover or member-size adjustments that accompany them, or applying a fire rating to a material without the corresponding protection.
Fire resistance appears as an occupancy-driven table separate from structural design, and candidates treat it as unrelated to the section or cover they are sizing.
What to do instead
Determine the required fire-resistance rating from occupancy and building type first, then confirm the chosen assembly or protective cover achieves that rating before finalizing member size or reinforcement cover.