OBC Building Structural flashcards

36 cards. Read the term and the definition here. The deck at the top of this page is optional.

Strength design inequality

Factored resistance must equal or exceed factored load effect: phi times nominal strength is greater than or equal to the sum of load factors times nominal loads.

Load combination 1.2D+1.6L

Basic ASCE 7 strength combination for gravity; used when live load governs and no lateral load acts. Dead load is amplified by 1.2, live by 1.6.

Importance factor

Multiplier on wind, seismic, or snow loads based on occupancy category. Typical buildings use 1.0; essential facilities use up to 1.5.

Deflection limit L/360

Common serviceability limit for floor members under live load. Span in inches divided by 360 gives allowable deflection; stiffer limits apply to sensitive finishes.

Stiffness method

Matrix analysis that assembles member stiffness into a global matrix, solves for joint displacements, then back-calculates member forces. Basis for most structural software.

Moment distribution method

Iterative hand method using fixed-end moments, joint stiffness ratios, and half carry-over to far ends. Continues until unbalanced moments become negligible.

Influence line

Diagram of a response quantity at a fixed point as a unit load moves across the structure. Used to position moving loads for maximum effect.

Second-order P-Delta effect

Additional moments caused by gravity loads acting on laterally displaced shape. Must be included when stability coefficient exceeds 0.10 per ASCE 7.

Live load reduction

Permits reducing uniform live load based on influence area using L equals L0 times (0.25 plus 15 divided by square root of KLL times AT). Not allowed above 100 psf.

Flat roof snow load

Computed as 0.7 times exposure, thermal, and importance factors times ground snow load. Minimum value of 20 times importance factor applies when ground snow exceeds 20 psf.

Snow drift surcharge

Additional trapezoidal load on lower roofs or beside parapets caused by wind-deposited snow. Drift height depends on upper roof length and ground snow load.

Concentrated live load check

Uniform floor live loads must also be checked against a concentrated load applied over a small area, such as 2000 pounds on 2.5 square feet for offices.

Velocity pressure

Wind pressure computed as 0.00256 times exposure, topographic, and directionality factors times basic wind speed squared. Increases with height above grade.

Seismic base shear

Total lateral design force equal to seismic response coefficient times effective seismic weight. Coefficient depends on spectral acceleration, response modification, and importance.

Response modification factor R

Reflects ductility and overstrength of a lateral system; higher R means more energy dissipation and lower design forces. Special moment frames use R of 8.

Site class amplification

Soft soils amplify seismic shaking, especially at long periods. Site coefficients Fa and Fv increase design spectral accelerations for classes D through F.

Terzaghi bearing capacity

Ultimate bearing pressure equals cohesion term plus surcharge term plus weight term, each multiplied by bearing capacity factors Nc, Nq, and N-gamma.

Consolidation settlement

Time-dependent compression of saturated clay under added load. Computed from compression index, initial void ratio, layer thickness, and change in effective stress.

At-rest earth pressure

Lateral pressure coefficient for soil that does not yield, equal to one minus sine of friction angle. Higher than active pressure and used for restrained walls.

Liquefaction susceptibility

Loose saturated sands below the water table can lose strength during earthquakes. Evaluated using standard penetration resistance and cyclic stress ratio.

Tension-controlled section

Concrete section where tensile strain in steel exceeds the yield strain plus 0.003 at nominal strength. Permits phi of 0.90 for flexure.

Minimum flexural reinforcement

Concrete beams require steel area at least the greater of 3 times square root of f-prime-c divided by fy, or 200 divided by fy, times width times effective depth.

Concrete development length

Embedment needed to develop bar yield force without pullout. Depends on bar diameter, concrete strength, cover, spacing, and coating or epoxy factors.

Concrete shear strength Vc

Nominal shear carried by concrete, approximately 2 times square root of f-prime-c times web width times effective depth in psi units for nonprestressed members.

Masonry modulus of elasticity

Em equals 900 times the specified masonry compressive strength per TMS 402. Used to compute stiffness and slenderness effects in masonry walls.

Mortar Type S

Medium-high strength mortar with 1800 psi compressive strength, commonly specified for structural masonry walls. Balances strength, workability, and bond.

Masonry slenderness ratio

Ratio of effective height to radius of gyration, h over r. Allowable compressive stress is reduced as this ratio increases beyond 99.

Load duration factor

NDS factor CD adjusts reference design values for duration of maximum load: 1.0 normal, 1.15 snow, 1.6 wind and seismic, 0.9 permanent only.

Beam stability factor

NDS factor CL reduces allowable bending stress for laterally unbraced beams. Depends on slenderness ratio, effective length, and depth-to-width ratio.

Wood column stability

Factor Cp reduces allowable compressive stress based on effective length to depth ratio. Computed from Euler buckling stress and reference compressive strength.

Steel lateral-torsional buckling

Unbraced beams may fail by twisting and lateral deflection before reaching plastic moment. Nominal strength depends on unbraced length relative to Lp and Lr.

Block shear rupture

Connection failure combining tensile rupture on one plane with shear rupture on a perpendicular plane. Nominal strength uses net and gross areas with a reduction factor.

Fillet weld strength

Nominal strength equals 0.6 times electrode strength times effective throat (0.707 times weld size) times length. Resistance factor is 0.75 for LRFD.

Wood char rate

Approximate rate of 1.5 inches per hour at which heavy timber burns. Used to size members for required fire resistance ratings.

Intumescent coating

Fire-protective paint that swells when heated to insulate steel. Thickness depends on section factor and required hourly rating.

Prying action

Additional bolt tension caused by flexure of connected plates at a tee or angle connection. Must be included in bolt tension design.