Structure Flashcards
2 Design Methodologies: ASD vs LRFD
Allowable Strength Design (ASD): apply service loads (D+L). Calculated stress must be less than allowable stress.
Load and Resistance Factor (LRFD): apply factor loads. These are DESIGN strengths. Apply resistance factor, phi, in most cases
Typical Factored Load Equation
Mu = 1.2Md + 1.6Ml (also true for loads)
How many reactions from roller, pin, and fixed ends?
Rolled: 1 vertical reaction
Pin: 1 horizontal and 1 vertical reaction
Fixed: 1 horizontal and 1 vertical reaction and 1 moment
Where on a truss is in compression and where in tension?
Top of a truss is in compression, whereas bottom of truss is in tension
Shear Diagrams - when constant, when linear, etc
Shear is constant under unloaded portions. Point load - shear reacts at constant rate. Uniform load - shear is linear
Slope of the shear diagram = load at that point
Moment Diagrams
Clockwise moments are +
In positive moment (smiley face) - top is in compression
For constant shear - moment is linear
For linear shear - moment is parabolic
Change in moment = area under shear diagram
0 shear = max moment
Is tension positive or negative in Truss Analysis?
Tension is POSITIVE
4 rules to identify Zero Force truss members
- If there are ONLY two non-collinear members at joint with no external load
- If there are only 3 members at joint and 2 are collinear and no external force, 3rd is 0
3: Once zero force is identified - you can assume it doesn’t exist and “delete it”
4: if there are only 1 horizontal member connected to roller with NO DIAGONALS, horizontal must be zero-force
Moment of Inertia Definition
Moment of Inertia is a measure of section resistance to bending and deflection –> bigger inertia = more resistance
Bending Stress in Beams - where is max stress?
Center of beam has 0 bending stress. Max stress is furthest from center of beam
E of steel?
E of steel = 29,000 kips
Beam Design - ASD or LRFD?
Beam design uses ONLY LRFD!
Ultimate strain for concrete? (once this is hit the concrete has lost all strength)
ultimate strain = 0.003
What is the neutral axis in a beam?
Where tension and compression forces are equal and opposite
What is local buckling?
When member yields before you reach plastic moment
What is design of steel beams controlled by?
Deflections (Total Deflection limited by L / 240 in
Controlled by bending moment requirements (deflection) - shear checked second as typically not a problem with steel
One-way vs Two-way Slab
One Way Slab –> L is greater than 2B
where B = 12 in
Net allowable pressure = ?
Gross - weight of soil - weight of column on footing
Net = Total load / Area of footing
Area of footing designed using Service or factored?
Thickness/reinforcement design?
Area of footing designed using Service loads
Thickness/reinforcement designed using Factored loads
What is a determinate structure?
Implies there are enough equations of equilibrium to determine support reactions (Fy, Fx, and Moment)
Truss Analysis - Equation to Determine if Structure is determinate, unstable, or indeterminate
m + r = 2j
where m = # members
r = # reaction components
J = # joints
if less than 2j –> unstable
if more than 2j –> indeterminate
*Note - if truss bay is not triangular –> unstable
Wide Flanged Members - Stress Distribution
0 shear stress in flange. Web has all shear stress
Max shear stress occurs at mid depth
avg shear stress = Total Shear stress/ (web thickness * web length)
How to reduce buckling potential?
Increase width of beam
Lateral braces
What is stable equilibrium
Ability of structure to return to initial position
How to calculate Moment of inertia (T-beam)
sum of Critical moment of inertia + sum of A*d^2
where d is the distance centroid of total structure to centroid of sub-area
