Bridge Constructor in Concrete
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Premises
Concrete has density of 145 lbft3, compressive strength of 4 000 lbin2 (576 000 lbft2), elastic modulus of 115 ⋅ 106 lbin2 (16.6 ⋅ 109 lbft2), and tensile strength of zero. (See AASHTO LRFD Bridge Design Specifications 2012 §§ 3.5.1, 5.4.2.1, 5.4.2.4, and 5.4.2.7.)
The bridge is 12 ft wide and has a rectangular cross-section. Traffic is represented by a uniform load of 640 lbft in conjunction with a point load of 72 000 lb. (See AASHTO LRFD Bridge Design Specifications 2012 §§ 3.6.1.1.1, 3.6.1.2.1, 3.6.1.2.2, and 3.6.1.2.4)
Safety margins, and loads other than concrete and traffic, are ignored. This is just for fun!
Each square in Bridge Constructor's default grid is 8 ft wide. (The in-game value is 2.5 meters.)
The optimal bridge is the bridge that has the minimum volume.
(Whoops! I forgot about buckling. I'll have to do some recalculation in the future…)
Region 1
Bridge 1
The bridge must extend from (0 ft, 0 ft) to (64 ft, 0 ft).
Iteration 1
The bridge has thickness t and comprises pin 0 at (0 ft, 0 ft), member 01, pin 1 at (32 ft, y1), member 12, and pin 2 at (64 ft, 0 ft).
The forces acting on the bridge (in addition to those listed in Premises) are F0x pushing rightward at point 0, F0y pushing upward at point 0, F2x pushing leftward at point 2, and F2y pushing upward at point 2.
Rotational equilibrium of bridge about point 0
F2y ⋅ 64 ft = ∫0 ft64 ft145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y121232 ftxdx + 72 000 lb ⋅ xtruck
F2y ⋅ 64 ft = 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y121232 ft64 ft2 ÷ 2 + 72 000 lb ⋅ xtruck
F2y = 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y1212 + 72 000 lb ⋅ xtruck64 ft
Vertical equilibrium of bridge
F0y = 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y121232 ft ⋅ 64 ft + 72 000 lb − F2y
F0y = 2145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y1212 + 72 000 lb − F2y
The forces acting on member 01 (in addition to those listed above) are F1x pushing leftward at pin 1 and F1y pushing downward at pin 1.
Vertical equilibrium of member 01
F1y = F0y − 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y121232 ft ⋅ 32 ft − 72 000 lb
F1y = F0y − 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y1212 − 72 000 lb
Rotational equilibrium of member 01 about point 0
F1xy1 = F1y ⋅ 32 ft + ∫0 ft32 ft145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y121232 ftxdx + 72 000 lb ⋅ xtruck
F1xy1 = F1y ⋅ 32 ft + 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y121232 ft32 ft2 ÷ 2 + 72 000 lb ⋅ xtruck
F1x = F1y ⋅ 32 fty1 + 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y12122y1 ⋅ 32 ft + 72 000 lb ⋅ xtrucky1
Horizontal equilibrium of member 01
F0x = F1x
Horizontal equilibrium of bridge
F2x = F0x
Segment 0e extends from pin 0 along the bridge to arbitrary solid joint e at (xe, ye). The forces acting on segment 0e (in addition to those listed above) are Fex pushing leftward at point e, Fey pushing downward at point e, and Me pushing counterclockwise at point e.
Vertical equilibrium of segment 0e
Fey = F0y − 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y121232 ft ⋅ xe − 72 000 lb
Horizontal equilibrium of segment 0e
Fex = F0x
Rotational equilibrium of segment 0e about point 0
Me = Feyxe − Fexye + ∫0 ftxe145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y121232 ftxdx + 72 000 lb ⋅ xtruck
Me = Feyxe − Fexye + 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft32 ft2 + y121232 ftxe2 ÷ 2 + 72 000 lb ⋅ xtruck
Minimum/maximum compressive pressure at point e if xe < 32 ft
Pmin/max = FeyFex ⋅ y132 ft32 ft2 + y121/2 ÷ 12 ft ⋅ t ± 6Met2 ⋅ 12 ft ∈ 0 lbft2, 576 000 lbft2
Compressive force at point 0
F0 = F0x2 + F0y21/2 ≤ π2 ⋅ 16.6 ⋅ 109 lbft2 ⋅ 12 ft ⋅ t31232 ft2 + y12
Spreadsheet analysis suggests that the optimal values for y1 and t are approximately 0.153 ft and 2.24 ft, respectively, yielding a volume of 1 720 ft3.
Bridge 2
The bridge must extend from (0 ft, 0 ft) to (96 ft, 0 ft).
Iteration 1
The bridge has thickness t and comprises pin 0 at (0 ft, 0 ft), member 01, pin 1 at (48 ft, y1), member 12, and pin 2 at (96 ft, 0 ft).
The forces acting on the bridge (in addition to those listed in Premises) are F0x pushing rightward at point 0, F0y pushing upward at point 0, F2x pushing leftward at point 2, and F2y pushing upward at point 2.
Rotational equilibrium of bridge about point 0
F2y ⋅ 96 ft = ∫0 ft96 ft145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y121248 ftxdx + 72 000 lb ⋅ xtruck
F2y ⋅ 96 ft = 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y121248 ft96 ft2 ÷ 2 + 72 000 lb ⋅ xtruck
F2y = 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y1212 + 72 000 lb ⋅ xtruck96 ft
Vertical equilibrium of bridge
F0y = 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y121248 ft ⋅ 96 ft + 72 000 lb − F2y
F0y = 2145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y1212 + 72 000 lb − F2y
The forces acting on member 01 (in addition to those listed above) are F1x pushing leftward at pin 1 and F1y pushing downward at pin 1.
Vertical equilibrium of member 01
F1y = F0y − 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y121248 ft ⋅ 48 ft − 72 000 lb
F1y = F0y − 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y1212 − 72 000 lb
Rotational equilibrium of member 01 about point 0
F1xy1 = F1y ⋅ 48 ft + ∫0 ft48 ft145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y121248 ftxdx + 72 000 lb ⋅ xtruck
F1xy1 = F1y ⋅ 48 ft + 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y121248 ft48 ft2 ÷ 2 + 72 000 lb ⋅ xtruck
F1x = F1y ⋅ 48 fty1 + 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y12122y1 ⋅ 48 ft + 72 000 lb ⋅ xtrucky1
Horizontal equilibrium of member 01
F0x = F1x
Horizontal equilibrium of bridge
F2x = F0x
Segment 0e extends from pin 0 along the bridge to arbitrary solid joint e at (xe, ye). The forces acting on segment 0e (in addition to those listed above) are Fex pushing leftward at point e, Fey pushing downward at point e, and Me pushing counterclockwise at point e.
Vertical equilibrium of segment 0e
Fey = F0y − 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y121248 ft ⋅ xe − 72 000 lb
Horizontal equilibrium of segment 0e
Fex = F0x
Rotational equilibrium of segment 0e about point 0
Me = Feyxe − Fexye + ∫0 ftxe145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y121248 ftxdx + 72 000 lb ⋅ xtruck
Me = Feyxe − Fexye + 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft48 ft2 + y121248 ftxe2 ÷ 2 + 72 000 lb ⋅ xtruck
Minimum/maximum compression at point e if xe < 48 ft
Pmin/max = FeyFex ⋅ y148 ft48 ft2 + y121/2 ÷ 12 ft ⋅ t ± 6Met2 ⋅ 12 ft ∈ 0 lbft2, 576 000 lbft2
Spreadsheet analysis suggests that the optimal values for y1 and t are approximately 0.460 ft and 2.30 ft, respectively, yielding a volume of 2 650 ft3.
Bridge 3
The bridge must extend from (0 ft, 0 ft) to (160 ft, 0 ft). Piers are available at (48 ft, −24 ft) and (112 ft, −24 ft).
Iteration 1
The bridge comprises pin 0 at (0 ft, 0 ft), solid joint 1 at (x1, y1), pin 2 at (x2, y2), pin 3 at (80 ft, y3), pin 4 at (160 ft − x2, y2), solid joint 5 at (160 ft − x1, y1), pin 6 at (160 ft, 0 ft), solid joint 7 at (48 ft, −24 ft), solid joint 8 at (112 ft, −24 ft), members 01 and 56 of thickness t01, members 12 and 45 of thickness t12, members 23 and 34 of thickness t23, and members 72 and 84 of thickness t72.
This structure has 24 unknown forces.
The forces acting on the bridge (in addition to those listed in Premises) are: rightward F0x and upward F0y at point 0; leftward F6x and upward F6y at point 6; leftward F7x and upward F7y at point 7; and leftward F8x and upward F8y at point 8.
⋅
F0x |
F0y |
F1x |
F1y |
M1 |
F2x12 |
F2y12 |
F2x23 |
F2y23 |
F3x |
F3y |
F4x34 |
F4y34 |
F4x45 |
F4y45 |
F5x |
F5y |
M5 |
F6x |
F6y |
F7x |
F7y |
F8x |
F8y |
Rotational equilibrium of bridge about point 0
F6y ⋅ 160 ft − F7x ⋅ 24 ft + F7y ⋅ 48 ft − F8x ⋅ 24 ft + F8y ⋅ 112 ft = 145 lbft3 ⋅ 12 ft ⋅ t01 + 640 lbftx12 + y1212x1∫0 ftx1xdx + ∫160 ft − x1160 ftxdx + 145 lbft3 ⋅ 12 ft ⋅ t12 + 640 lbftx2 − x12 + y2 − y1212x2 − x1∫x1x2xdx + ∫160 ft − x2160 ft − x1xdx + 145 lbft3 ⋅ 12 ft ⋅ t23 + 640 lbft80 ft − x22 + y3 − y221280 ft − x2 ⋅ ∫x2160 ft − x2xdx + 72 000 lb ⋅ xtruck
F6y ⋅ 160 ft − F7x ⋅ 24 ft + F7y ⋅ 48 ft − F8x ⋅ 24 ft + F8y ⋅ 112 ft = 145 lbft3 ⋅ 12 ft ⋅ t01 + 640 lbftx12 + y1212x1160 ft2 − 160 ft − x12 + x12 ÷ 2 + 145 lbft3 ⋅ 12 ft ⋅ t12 + 640 lbftx2 − x12 + y2 − y1212x2 − x1x22 − x12 + 160 ft − x12 − 160 ft − x22 ÷ 2 + 145 lbft3 ⋅ 12 ft ⋅ t23 + 640 lbft80 ft − x22 + y3 − y221280 ft − x2160 ft − x22 − x22 ÷ 2 + 72 000 lb ⋅ xtruck
F6y ⋅ 160 ft − F7x ⋅ 24 ft + F7y ⋅ 48 ft − F8x ⋅ 24 ft + F8y ⋅ 112 ft = 145 lbft3 ⋅ 12 ft ⋅ t01 + 640 lbftx12 + y1212x1 ⋅ x1 ⋅ 160 ft + 145 lbft3 ⋅ 12 ft ⋅ t12 + 640 lbftx2 − x12 + y2 − y1212x2 − x1x2 − x1 ⋅ 160 ft + 145 lbft3 ⋅ 12 ft ⋅ t23 + 640 lbft80 ft − x22 + y3 − y221280 ft − x280 ft − x2 ⋅ 160 ft + 72 000 lb ⋅ xtruck
F6y ⋅ 160 ft − F7x ⋅ 24 ft + F7y ⋅ 48 ft − F8x ⋅ 24 ft + F8y ⋅ 112 ft = 145 lbft3 ⋅ 12 ft ⋅ t01 + 640 lbftx12 + y1212 + 145 lbft3 ⋅ 12 ft ⋅ t12 + 640 lbftx2 − x12 + y2 − y1212 + 145 lbft3 ⋅ 12 ft ⋅ t23 + 640 lbft80 ft − x22 + y3 − y2212 ⋅ 160 ft + 72 000 lb ⋅ xtruck
Vertical equilibrium of bridge
F0y + F6y + F7y + F8y = 145 lbft3 ⋅ 12 ft ⋅ t01 + 640 lbftx12 + y1212x1 ⋅ 2x1 + 145 lbft3 ⋅ 12 ft ⋅ t12 + 640 lbftx2 − x12 + y2 − y1212x2 − x1 ⋅ 2x2 − x1 + 145 lbft3 ⋅ 12 ft ⋅ t23 + 640 lbft80 ft − x22 + y3 − y221280 ft − x2 ⋅ 280 ft − x2 + 72 000 lb
F0y + F6y + F7y + F8y = 2145 lbft3 ⋅ 12 ft ⋅ t01 + 640 lbftx12 + y1212 + 145 lbft3 ⋅ 12 ft ⋅ t12 + 640 lbftx2 − x12 + y2 − y1212 + 145 lbft3 ⋅ 12 ft ⋅ t23 + 640 lbft80 ft − x22 + y3 − y2212 + 72 000 lb
Horizontal equilibrium of bridge
F0x − F6x − F7x − F8x = 0 lb
Bridge 4
The bridge must extend from (0 ft, 0 ft) to (128 ft, 0 ft). A pier is available at (88 ft, −24 ft).
Region 5
Bridge 1
The bridge must extend from (0 ft, 0 ft) to (320 ft, 0 ft).
Iteration 1
The bridge has thickness t and comprises pin 0 at (0 ft, 0 ft), member 01, pin 1 at (160 ft, y1), member 12, and pin 2 at (320 ft, 0 ft).
The forces acting on the bridge (in addition to those listed in Premises) are F0x pushing rightward at point 0, F0y pushing upward at point 0, F2x pushing leftward at point 2, and F2y pushing upward at point 2.
Rotational equilibrium of bridge about point 0
F2y ⋅ 320 ft = ∫0 ft320 ft145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212160 ftxdx + 72 000 lb ⋅ xtruck
F2y ⋅ 320 ft = 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212160 ft320 ft2 ÷ 2 + 72 000 lb ⋅ xtruck
F2y = 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212 + 72 000 lb ⋅ xtruck320 ft
Vertical equilibrium of bridge
F0y = 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212160 ft ⋅ 320 ft + 72 000 lb − F2y
F0y = 2145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212 + 72 000 lb − F2y
The forces acting on member 01 (in addition to those listed above) are F1x pushing leftward at pin 1 and F1y pushing downward at pin 1.
Vertical equilibrium of member 01
F1y = F0y − 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212160 ft ⋅ 160 ft − 72 000 lb
F1y = F0y − 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212 − 72 000 lb
Rotational equilibrium of member 01 about point 0
F1xy1 = F1y ⋅ 160 ft + ∫0 ft160 ft145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212160 ftxdx + 72 000 lb ⋅ xtruck
F1xy1 = F1y ⋅ 160 ft + 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212160 ft160 ft2 ÷ 2 + 72 000 lb ⋅ xtruck
F1x = F1y ⋅ 160 fty1 + 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y12122y1 ⋅ 160 ft + 72 000 lb ⋅ xtrucky1
Horizontal equilibrium of member 01
F0x = F1x
Horizontal equilibrium of bridge
F2x = F0x
Segment 0e extends from pin 0 along the bridge to arbitrary solid joint e at (xe, ye). The forces acting on segment 0e (in addition to those listed above) are Fex pushing leftward at point e, Fey pushing downward at point e, and Me pushing counterclockwise at point e.
Vertical equilibrium of segment 0e
Fey = F0y − 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212160 ft ⋅ xe − 72 000 lb
Horizontal equilibrium of segment 0e
Fex = F0x
Rotational equilibrium of segment 0e about point 0
Me = Feyxe − Fexye + ∫0 ftxe145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212160 ftxdx + 72 000 lb ⋅ xtruck
Me = Feyxe − Fexye + 145 lbft3 ⋅ 12 ft ⋅ t + 640 lbft160 ft2 + y1212160 ftxe2 ÷ 2 + 72 000 lb ⋅ xtruck
Minimum/maximum compression at point e if xe < 160 ft
Pmin/max = FeyFex ⋅ y1160 ft160 ft2 + y121/2 ÷ 12 ft ⋅ t ± 6Met2 ⋅ 12 ft ∈ 0 lbft2, 576 000 lbft2
Spreadsheet analysis suggests that the optimal values for y1 and t are approximately 6.53 ft and 10.9 ft, respectively, yielding a volume of 41 800 ft3.