Statics: cram pack

Formula sheets, the key method steps for every topic and 12 practice problems, with full solutions at the back.

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1Formulas and methods

Vectors: dot & cross product

Magnitude: |a| = √(a_x² + a_y² + a_z²)
Unit vector: û = a/|a|
Components from an angle: F_x = F·cos θ, F_y = F·sin θ
Dot product: a·b = a_xb_x + a_yb_y + a_zb_z = |a||b|cos θ
Scalar projection of a onto b: a·b/|b|
Work: W = F·d
Cross product: a × b = (a_yb_z − a_zb_y, a_zb_x − a_xb_z, a_xb_y − a_yb_x)
Areas: parallelogram |a × b|, triangle ½|a × b|
Moment about O: M_O = 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: ΣF_x = 0, ΣF_y = 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 ΣF_y = 0 to find the remaining reaction, and check with a second moment equation.

Truss: method of joints

Method of joints: ΣF_x = 0, ΣF_y = 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 ΣF_x = 0 and ΣF_y = 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: ΣF_x = 0, ΣF_y = 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 = √(F_x² + F_y²)
Lever, pliers, bell crank: F_in·a = F_out·b
Toggle: F_link = 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 ΣF_x = 0 and ΣF_y = 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/y_max
Simply supported, uniform load: M_max = 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 = ΣA_iy_i/ΣA_i
Rectangle: I = bh³/12
Parallel-axis theorem: I = I_c + A·d²
Composite section: I = Σ(I_c,i + A_id_i²)

2Practice problems

1.Vectors: dot & cross productMedium

Find the scalar projection comp_b a of a = (−4, −4, −5) onto b = (5, 6, 2), that is, the component of a in the direction of b.

a = (−4, −4, −5)b = (5, 6, 2)

comp_b a =

2.Vectors: dot & cross productHard

The unit vector n̂ is perpendicular to both a = (−5, −2, 1) and b = (−3, −1, −4) and has a positive z-component. Find its x-component n̂_x.

a = (−5, −2, 1)b = (−3, −1, −4)n̂_z > 0

n̂_x =

3.Beam support reactionsMedium

The cantilever beam is fixed to the wall at A. It carries a uniformly distributed load w along its whole length and a point load P at 4 m from the wall. Find the magnitude of the reaction moment M_A at the wall.

L = 8 mP = 30 kN at 4 mw = 4 kN/m from 0 to 8 m

M_A = kN·m

4.Beam support reactionsHard

The cantilever beam is fixed to the wall at A. It carries a linearly varying load that is 6 kN/m at the wall and falls to zero at the free end B, plus a point load P at 4 m from the wall. Find the magnitude of the reaction moment M_A at the wall.

L = 4 mw₀ = 6 kN/mP = 70 kN at 4 m

M_A = kN·m

5.Truss: method of jointsMedium

Use the method of joints to find the force F_BC in member BC. Give tension as positive and compression as negative.

span = 6 mheight = 3 mB is 5 m from Avertical load = 20 kN

F_BC = kN

6.Truss: method of jointsHard

Use the method of joints to find the force F_AB in member AB. Give tension as positive and compression as negative.

span = 8 mheight = 3 mB is 1 m from Avertical load = 20 kNhorizontal load = 15 kN →

F_AB = kN

7.Frames & machinesMedium

The frame shown has pins at A, B and C; A and C are fixed to the ground 3 m apart, and B is 2 m above the ground and 0.45 m to the right of A. A vertical load of 6 kN acts on member AB at K, 0.18 m (horizontally) from A. Find the force F_BC in member BC.

P = 6 kNL = 3 mH = 2 mhorizontal distance A to B = 0.45 mhorizontal distance A to K = 0.18 m

F_BC = kN

8.Frames & machinesHard

The beam AB of the wall bracket shown is pinned at A and propped by the strut CD (C is 0.5 m below A). The beam carries a uniform load of 2 kN/m over its full length and a point load of 4 kN at its free end B. Find the force F_CD in the strut.

w = 2 kN/mP = 4 kNAD = 1.5 mAB = 4 mAC = 0.5 m

F_CD = kN

9.Shear force & bending momentMedium

Find the bending moment M_x at the section x = 1 m from support A. Use sagging moments positive and upward shear on the left face positive.

L = 8 mw = 10 kN/m from 0 to 8 mx = 1 m

M_x = kN·m

10.Shear force & bending momentHard

The marks A to G split the beam into six equal parts. At which mark is the bending moment largest in size, sagging or hogging? Click it in the diagram.

10 kN @ 6 m4 kN/m, 0–4 m

1A (x = 0 m)
2B (x = 1 m)
3C (x = 2 m)
4D (x = 3 m)
5E (x = 4 m)
6F (x = 5 m)
7G (x = 6 m)

11.Centroid & second moment of areaMedium

Find the second moment of area I_x of the T-section about the horizontal axis x through its centroid.

b_f = 100 mmt_f = 30 mmt_w = 20 mmh_w = 100 mm

I_x = × 10⁶ mm⁴

12.Centroid & second moment of areaHard

The T-section is used as a beam with the flange on top and carries a sagging bending moment of 50 kN·m. Find the size of the bending stress σ_top at the top edge.

b_f = 200 mmt_f = 10 mmt_w = 16 mmh_w = 140 mmM = 50 kN·m

σ_top = MPa

3Solutions

1.Vectors: dot & cross product

Answer: comp_b a = −6.698

Dot product: a·b = a_xb_x + a_yb_y + a_zb_z = |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 = (−4)·5 + (−4)·6 + (−5)·2 = −54

Length of b

|b| = √(5² + 6² + 2²) = 8.062

Scalar projection

comp_b a = a·b/|b| = −54/8.062 = −6.698

Answer

comp_b a = −6.698

Negative: a has a component pointing against b.

2.Vectors: dot & cross product

Answer: n̂_x = −0.3641

Cross product: a × b = (a_yb_z − a_zb_y, a_zb_x − a_xb_z, a_xb_y − a_yb_x) 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 M_O = r × F, with r = A − O.

Cross product

(a × b)_x = a_yb_z − a_zb_y = (−2)·(−4) − 1·(−1) = 9

(a × b)_y = a_zb_x − a_xb_z = 1·(−3) − (−5)·(−4) = −23

(a × b)_z = a_xb_y − a_yb_x = (−5)·(−1) − (−2)·(−3) = −1

Normalise

The z-component of a × b is negative, so use −(a × b).

|a × b| = √(9² + (−23)² + (−1)²) = 24.72

n̂ = −(9, −23, −1)/24.72 = (−0.3641, 0.9305, 0.04046)

n̂_x = −0.3641

Answer

n̂_x = −0.3641

3.Beam support reactions

Answer: M_A = 248 kN·m

Cantilever equilibrium: a fixed support can push up and also resist rotation, so it gives a vertical reaction R_A and a reaction moment M_A. ΣF_y = 0 gives R_A, and moments about the wall give M_A = Σ(load × distance from the wall). A distributed load acts as its resultant at the centroid of the load diagram.

Resultant of the distributed load

W = 4·8 = 32 kN, x̄ = 4 m

Moment about the wall

M_A = P·a + W·x̄ = 30·4 + 32·4 = 248 kN·m

Answer

M_A = 248 kN·m

4.Beam support reactions

Answer: M_A = 296 kN·m

Resultant of the triangular load

W = ½·6·4 = 12 kN

x̄ = L/3 = 1.333 m

Moment about the wall

M_A = W·x̄ + P·a = 12·1.333 + 70·4 = 296 kN·m

Answer

M_A = 296 kN·m

5.Truss: method of joints

Answer: F_BC = −17.57 kN

Method of joints: every pin of a truss is in equilibrium, so ΣF_x = 0 and ΣF_y = 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.

ΣM_A = 0: C_y·6 = 20·5 → C_y = 16.67 kN

ΣF_y = 0: A_y = 20 − 16.67 = 3.333 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 = 3/3.162 = 0.9487, cos γ = 1/3.162 = 0.3162

ΣF_y = 0: C_y + F_BC·sin γ = 0 → F_BC = −16.67/0.9487 = −17.57 kN

Answer

F_BC = −17.57 kN (compression)

6.Truss: method of joints

Answer: F_AB = −12.52 kN

Support reactions

Equilibrium of the whole truss. Moments about A, counter-clockwise positive.

ΣM_A = 0: C_y·8 = 20·1 + 15·3 → C_y = 8.125 kN

ΣF_y = 0: A_y = 20 − 8.125 = 11.88 kN

ΣF_x = 0: A_x = −15 kN

Joint A

Assume every member is in tension (pulling away from the joint). Only AB has a vertical component at A.

sin α = 3/3.162 = 0.9487

ΣF_y = 0: A_y + F_AB·sin α = 0

F_AB = −11.88/0.9487 = −12.52 kN

Answer

F_AB = −12.52 kN (compression)

7.Frames & machines

Answer: F_BC = 0.5833 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 ΣF_x = 0, ΣF_y = 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 = √(2.55² + 2²) = 3.241 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.

F_BC·(2/3.241)·3 = 6·0.18

F_BC = 0.5833 kN (compression)

Answer

F_BC = 0.5833 kN

8.Frames & machines

Answer: F_CD = 67.46 kN

Resultant of the uniform load

w·L = 2·4 = 8 kN at 2 m

Moments about A for the beam

F_CD·0.3162·1.5 = 8·2 + 4·4 = 32 kN·m

F_CD = 67.46 kN

Answer

F_CD = 67.46 kN

9.Shear force & bending moment

Answer: M_x = 35 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.

Support reactions

Equilibrium of the whole beam.

UDL resultant: W = w·ℓ = 10·8 = 80 kN, acting at x̄ = 4 m

ΣM_A = 0: R_B·8 = 10·8·4

R_B = 320/8 = 40 kN

ΣF_y = 0: R_A = 80 − 40 = 40 kN

Cut at x = 1 m and keep the left part

Only forces left of the cut appear. A UDL portion acts as one force at the middle of that portion.

M = R_A·x − (each load left of the cut × its distance to the cut)

M_x = 40·1 − 10·1·0.5 = 35 kN·m

Answer

M_x = 35 kN·m

10.Shear force & bending moment

Answer: E (x = 4 m)

Largest bending moment: M changes slope wherever the shear force changes, so its peaks sit where the shear force crosses zero: under a point load, inside a distributed load, or over a support of an overhang. At a fixed support of a cantilever the moment is largest at the wall. Compare the size of M at those places, sagging or hogging.

Support reactions

Take moments about one support, then use the vertical force balance.

R₁ = 3 kN, R₂ = 23 kN

Bending moment at every mark

Sum the moments of all forces on one side of the cut. Sagging is positive, hogging negative.

M_A = 0, M_B = 1, M_C = −2, M_D = −9, M_E = −20 kN·m, M_F = −10, M_G = 0

Answer

Mark E: M_E = −20 kN·m

11.Centroid & second moment of area

Answer: I_x = 6.962 × 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: A_f = 100·30 = 3000 mm², y_f = 100 + 30/2 = 115 mm

web: A_w = 20·100 = 2000 mm², y_w = 100/2 = 50 mm

Centroid

ȳ = (A_fy_f + A_wy_w)/(A_f + A_w) = (3000·115 + 2000·50)/5000 = 89 mm

Parallel-axis theorem

flange: 100·30³/12 + 3000·(115 − 89)² = 2.253 × 10⁶ mm⁴

web: 20·100³/12 + 2000·(50 − 89)² = 4.709 × 10⁶ mm⁴

I_x = 2.253 × 10⁶ + 4.709 × 10⁶ = 6.962 × 10⁶ mm⁴ = 6.962 × 10⁶ mm⁴

Answer

I_x = 6.962 × 10⁶ mm⁴

12.Centroid & second moment of area

Answer: σ_top = 232 MPa

Split into two rectangles

Measure every y from the bottom edge.

flange: A_f = 200·10 = 2000 mm², y_f = 140 + 10/2 = 145 mm

web: A_w = 16·140 = 2240 mm², y_w = 140/2 = 70 mm

Centroid

ȳ = (A_fy_f + A_wy_w)/(A_f + A_w) = (2000·145 + 2240·70)/4240 = 105.4 mm

Parallel-axis theorem

flange: 200·10³/12 + 2000·(145 − 105.4)² = 3.157 × 10⁶ mm⁴

web: 16·140³/12 + 2240·(70 − 105.4)² = 6.462 × 10⁶ mm⁴

I_x = 3.157 × 10⁶ + 6.462 × 10⁶ = 9.619 × 10⁶ mm⁴

Bending stress

The top edge is h − ȳ from the axis.

y = 150 − 105.4 = 44.62 mm

σ = M·y/I = 5 × 10⁷·44.62/9.619 × 10⁶ = 232 MPa

Answer

σ_top = 232 MPa

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