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1Formulas and methods
Vectors: dot & cross product
Magnitude
|a| = √(ax² + ay² + az²)
Unit vector
û = a/|a|
Components from an angle
Fx = F·cos θ, Fy = F·sin θ
Dot product
a·b = axbx + ayby + azbz = |a||b|cos θ
Scalar projection of a onto b
a·b/|b|
Work
W = F·d
Cross product
a × b = (aybz − azby, azbx − axbz, axby − aybx)
Areas
parallelogram |a × b|, triangle ½|a × b|
Moment about O
MO = r × F, r = A − O
Volume of a parallelepiped
V = |a·(b × c)|
Method
Write every vector in components; for a force given by a magnitude and an angle, use F·cos θ and F·sin θ.
For lengths and directions use the magnitude √(x² + y² + z²) and the unit vector a/|a|.
Use the dot product for angles, projections, perpendicularity and work: a·b = |a||b|cos θ.
Use the cross product for perpendicular vectors, areas and moments: write the three components carefully, keeping the order a × b.
Beam support reactions
Equilibrium
ΣFx = 0, ΣFy = 0, ΣM = 0
Uniform load
resultant w·L at the middle of the load
Moment
M = F·d
Method
Draw a free-body diagram and replace each support by its reactions.
Replace every distributed load by a single force at its centroid.
Take moments about one support to find the reaction at the other.
Use ΣFy = 0 to find the remaining reaction, and check with a second moment equation.
Truss: method of joints
Method of joints
ΣFx = 0, ΣFy = 0 at each joint
Method of sections
ΣM = 0 about a cut point
Member force
positive = tension, negative = compression
Method
Find the support reactions for the whole truss.
Method of joints: start at a joint with at most two unknown member forces and use ΣFx = 0 and ΣFy = 0.
Method of sections: cut through the member you need and take moments about a point where the other unknown members meet.
Assume tension; a negative answer means compression.
Frames & machines
Equilibrium of each member
ΣFx = 0, ΣFy = 0, ΣM = 0
Two-force member
force along the line through its two pins
Pin force
equal and opposite on the two members it joins
Magnitude of a pin force
F = √(Fx² + Fy²)
Lever, pliers, bell crank
Fin·a = Fout·b
Toggle
Flink = F/(2 sin θ), Q = F/(2 tan θ)
Uniform load
resultant w·L at the middle of the load
Method
Draw the whole structure and find the support reactions if there are no more than three unknowns.
Spot the two-force members: loaded only at two pins, their force acts along the line between the pins.
Take the frame apart and draw every member on its own, with equal and opposite pin forces where members meet.
For each member, take moments about a pin with unknown forces, then use ΣFx = 0 and ΣFy = 0; combine components into a magnitude at the end.
Shear force & bending moment
Shear and moment
V = ΣF on one side, M = ΣF·d on one side
Bending stress
σ = M·y/I
Rectangle
I = bh³/12
Section modulus
W = I/ymax
Simply supported, uniform load
Mmax = wL²/8
Method
Find the support reactions from equilibrium of the whole beam.
Cut the beam at the section you need and keep one side.
Shear force: sum of the vertical forces on that side. Bending moment: sum of their moments about the cut.
For the maximum moment, find where the shear force is zero.
Centroid & second moment of area
Centroid
ȳ = ΣAiyi/ΣAi
Rectangle
I = bh³/12
Parallel-axis theorem
I = Ic + A·d²
Composite section
I = Σ(Ic,i + Aidi²)
2Practice problems
1.Vectors: dot & cross productMedium
Find the scalar projection compb a of a = (−7, −2, 5) onto b = (−5, 1, 0), that is, the component of a in the direction of b.
a = (−7, −2, 5)b = (−5, 1, 0)
compb a =
2.Vectors: dot & cross productHard
For a = (1, 1, −6) and b = (−5, −1, 4), find the z-component (a × b)z of the cross product a × b.
a = (1, 1, −6)b = (−5, −1, 4)
(a × b)z =
3.Beam support reactionsMedium
The simply supported beam (pin at A, roller at B) carries a linearly varying load that rises from zero at A to 20 kN/m at B. A point load P also acts at 2 m from A. Find the vertical support reaction RA.
L = 5 mw0 = 20 kN/mP = 80 kN at 2 m
RA = kN
4.Beam support reactionsHard
The simply supported beam has a pin at A and a roller at B. Find the vertical support reaction RA.
L = 6 mP = 55 kN at 2.5 mw = 4 kN/m from 0 to 4 m
RA = kN
5.Truss: method of jointsMedium
Use the method of joints to find the force FAB in member AB. Give tension as positive and compression as negative.
span = 6 mheight = 2 mB is 5 m from Avertical load = 25 kN
FAB = kN
6.Truss: method of jointsHard
Use the method of joints to find the force FAC in member AC. Give tension as positive and compression as negative.
span = 6 mheight = 2 mB is 1 m from Avertical load = 40 kNhorizontal load = 10 kN ←
FAC = kN
7.Frames & machinesMedium
The frame shown has pins at A, B and C; A and C are fixed to the ground 5 m apart, and B is 3 m above the ground and 3 m to the right of A. A vertical load of 5 kN acts on member AB at K, 1.8 m (horizontally) from A. Find the force FBC in member BC.
P = 5 kNL = 5 mH = 3 mhorizontal distance A to B = 3 mhorizontal distance A to K = 1.8 m
FBC = kN
8.Frames & machinesHard
A uniform horizontal boom AB of weight 1 kN is pinned to a wall at A and held by a cable from its end B to the point C on the wall, 3 m above A. A load of 12 kN hangs at B. Find the tension T in the cable.
W = 12 kNWb = 1 kNAB = 5 mAC = 3 m
T = kN
9.Shear force & bending momentMedium
The simply supported beam carries a clockwise couple M0 = 30 kN·m at x = 5 m from A, and no other load. Find the bending moment Mx just to the right of the couple. Use sagging moments positive and upward shear on the left face positive.
L = 6 mM0 = 30 kN·m clockwise, at 5 m
Mx = kN·m
10.Shear force & bending momentHard
The simply supported beam carries a uniformly distributed load w over its whole span and a point load P. Find the maximum bending moment Mmax in the beam.
L = 4 mP = 80 kN at 2 mw = 6 kN/m from 0 to 4 m
Mmax = kN·m
11.Centroid & second moment of areaMedium
Find the second moment of area Ix of the T-section about the horizontal axis x through its centroid.
bf = 260 mmtf = 10 mmtw = 18 mmhw = 120 mm
Ix = × 10⁶ mm⁴
12.Centroid & second moment of areaHard
A cast-iron beam has the T-section shown, with the flange on top. The allowable stresses are 60 MPa in tension and 120 MPa in compression. Find the largest sagging bending moment Mallow the beam can carry.
Dot product:a·b = axbx + ayby + azbz = |a|·|b|·cos θ, a scalar. It gives the angle between two vectors, cos θ = a·b/(|a|·|b|), and it is zero when they are perpendicular. The scalar projection of a onto b is a·b/|b|, and a constant force F moving through a displacement d does work W = F·d.
Dot product
a·b = (−7)·(−5) + (−2)·1 + 5·0 = 33
Length of b
|b| = √((−5)² + 1² + 0²) = 5.099
Scalar projection
compb a = a·b/|b| = 33/5.099 = 6.472
Answer
compb a = 6.472
2.Vectors: dot & cross product
Answer: (a × b)z = 4
Cross product:a × b = (aybz − azby, azbx − axbz, axby − aybx) is perpendicular to both vectors, and |a × b| = |a|·|b|·sin θ is the area of the parallelogram they span (the triangle has half of it). Order matters: b × a = −(a × b). The moment of a force F applied at point A about point O is MO = r × F, with r = A − O.
Cross product, component by component
(a × b)x = aybz − azby = 1·4 − (−6)·(−1) = −2
(a × b)y = azbx − axbz = (−6)·(−5) − 1·4 = 26
(a × b)z = axby − aybx = 1·(−1) − 1·(−5) = 4
Requested component
(a × b)z = 4
Answer
(a × b)z = 4
Check: a × b is perpendicular to both vectors, so its dot product with a and with b is zero.
3.Beam support reactions
Answer: RA = 64.67 kN
Static equilibrium: a body at rest has ΣFx = 0, ΣFy = 0 and ΣM = 0 about any point. Replace each distributed load by its resultant (its area) acting at the centroid of the load diagram, and take moments about a support to remove that support’s reaction from the equation.
Resultant of the triangular load
W = ½·20·5 = 50 kN
x̄ = 2L/3 = 3.333 m
Equilibrium
Moments about A, counter-clockwise positive. Then vertical force balance.
ΣMA = 0: RB·5 = 50·3.333 + 80·2
RB = 65.33 kN
RA = 130 − 65.33 = 64.67 kN
Answer
RA = 64.67 kN
4.Beam support reactions
Answer: RA = 42.75 kN
Static equilibrium: a body at rest has ΣFx = 0, ΣFy = 0 and ΣM = 0 about any point. Taking moments about one support removes that support’s reaction from the equation.
Free-body diagram
Replace the supports with reaction forces. A roller gives one vertical reaction; with only vertical loads the pin gives one vertical reaction too.
Equilibrium
Moments about A, counter-clockwise positive. Then vertical force balance.
UDL resultant: W = w·ℓ = 4·4 = 16 kN, acting at x̄ = 2 m
ΣMA = 0: RB·6 = 55·2.5 + 4·4·2
RB = 169.5/6 = 28.25 kN
ΣFy = 0: RA = 71 − 28.25 = 42.75 kN
Answer
RA = 42.75 kN
5.Truss: method of joints
Answer: FAB = −11.22 kN
Method of joints: every pin of a truss is in equilibrium, so ΣFx = 0 and ΣFy = 0 at each joint. Start at a joint with at most two unknown member forces. A negative result means the member is in compression.
Support reactions
Equilibrium of the whole truss. Moments about A, counter-clockwise positive.
ΣMA = 0: Cy·6 = 25·5 → Cy = 20.83 kN
ΣFy = 0: Ay = 25 − 20.83 = 4.167 kN
Joint A
Assume every member is in tension (pulling away from the joint). Only AB has a vertical component at A.
sin α = 2/5.385 = 0.3714
ΣFy = 0: Ay + FAB·sin α = 0
FAB = −4.167/0.3714 = −11.22 kN
Answer
FAB = −11.22 kN (compression)
6.Truss: method of joints
Answer: FAC = 8.333 kN
Method of joints: every pin of a truss is in equilibrium, so ΣFx = 0 and ΣFy = 0 at each joint. Start at a joint with at most two unknown member forces. A negative result means the member is in compression.
Support reactions
Equilibrium of the whole truss. Moments about A, counter-clockwise positive.
ΣMA = 0: Cy·6 = 40·1 − 10·2 → Cy = 3.333 kN
ΣFy = 0: Ay = 40 − 3.333 = 36.67 kN
ΣFx = 0: Ax = 10 kN
Joint C
The roller gives only a vertical reaction, so joint C has just two unknown member forces. Assume both in tension.
sin γ = 2/5.385 = 0.3714, cos γ = 5/5.385 = 0.9285
Frames and machines: unlike a truss, a frame contains at least one member that carries forces at more than two points, or a load between its pins. First look for two-force members: a member loaded only at two pins has its force along the line through them, so it adds one unknown, not two. Then use the whole structure for the support reactions when there are no more than three unknowns. Finally take the frame apart: draw each member separately, with the pin forces equal and opposite on the two members they connect, and write ΣFx = 0, ΣFy = 0 and ΣM = 0 for it. Taking moments about a pin removes both components of that pin force from the equation.
Two-force member BC
BC = √(2² + 3²) = 3.606 m
Whole frame: moments about A
The ground pushes on BC at C along CB; its horizontal component passes through A, only the vertical one has a moment.
FBC·(3/3.606)·5 = 5·1.8
FBC = 2.163 kN (compression)
Answer
FBC = 2.163 kN
8.Frames & machines
Answer: T = 24.3 kN
Frames and machines: unlike a truss, a frame contains at least one member that carries forces at more than two points, or a load between its pins. First look for two-force members: a member loaded only at two pins has its force along the line through them, so it adds one unknown, not two. Then use the whole structure for the support reactions when there are no more than three unknowns. Finally take the frame apart: draw each member separately, with the pin forces equal and opposite on the two members they connect, and write ΣFx = 0, ΣFy = 0 and ΣM = 0 for it. Taking moments about a pin removes both components of that pin force from the equation.
Cable angle
BC = √(5² + 3²) = 5.831 m, sin β = 0.5145
Moments about A
T·0.5145·5 = 12·5 + 1·2.5
T = 24.3 kN
Answer
T = 24.3 kN
9.Shear force & bending moment
Answer: Mx = 5 kN·m
Method of sections: cut the beam where you want the internal forces and keep one side. The shear force V balances the vertical forces on that side, and the bending moment M balances their moments about the cut. An applied couple makes the bending moment diagram jump by the size of the couple.
Support reactions
Moments about A, counter-clockwise positive: RB·L − M0 = 0. Vertical balance: RA + RB = 0.
RB = M0/L = 30/6 = 5 kN (upward)
RA = −RB = −5 kN (downward)
Cut just right of the couple
The left part now contains RA and the couple.
Mx = RA·a + M0 = −5·5 + 30 = 5 kN·m
Answer
Mx = 5 kN·m
10.Shear force & bending moment
Answer: Mmax = 92 kN·m
Method of sections: cut the beam where you want the internal forces and keep one side. The shear force V balances the vertical forces on that side, and the bending moment M balances their moments about the cut. Maximum moment: since dM/dx = V, the bending moment has its peak where the shear force passes through zero, either at a point load or inside a distributed load.
Support reactions
Equilibrium of the whole beam.
UDL resultant: W = w·ℓ = 6·4 = 24 kN, acting at x̄ = 2 m
ΣMA = 0: RB·4 = 80·2 + 6·4·2
RB = 208/4 = 52 kN
ΣFy = 0: RA = 104 − 52 = 52 kN
Where is V = 0?
V(2⁻) = RA − w·2 = 40 kN
V(2⁺) = 40 − 80 = −40 kN
The shear changes sign at the load, so the maximum is under the load.
Maximum moment
Mmax = RA·2 − w·2²/2 = 52·2 − 6·2²/2 = 92 kN·m
Answer
Mmax = 92 kN·m
It occurs at x = 2 m from A.
11.Centroid & second moment of area
Answer: Ix = 7.598 × 10⁶ mm⁴
Composite areas: split the section into rectangles. The centroid is the area-weighted average ȳ = ΣAᵢyᵢ/ΣAᵢ. The second moment of area about the centroidal axis adds each part’s own bh³/12 and its parallel-axis term A·d², where d is the distance from the part’s centroid to the overall centroid. Bending stress is σ = M·y/I.
Split into two rectangles
Measure every y from the bottom edge.
flange: Af = 260·10 = 2600 mm², yf = 120 + 10/2 = 125 mm
Composite areas: split the section into rectangles. The centroid is the area-weighted average ȳ = ΣAᵢyᵢ/ΣAᵢ. The second moment of area about the centroidal axis adds each part’s own bh³/12 and its parallel-axis term A·d², where d is the distance from the part’s centroid to the overall centroid. Bending stress is σ = M·y/I.