How to 'lock' truss members?

Do not specify the bottom chord member as a TRUSS member. Define it as a normal member with MY MZ releases applied at the two ends of the bottom chord. Similarly the top chord also should not be specified as TRUSS. You may assign the TRUSS specification only to the members connecting the top and bottom chords.

Everything else seems reasonable except the displacement of that upper cantilever section, where Node 12 is resulting in over 4 1/2 inches

Here is the input file:

STAAD TRUSS
START JOB INFORMATION
JOB NAME Cherry Avenue Bridge
JOB PART Truss System (East shown)
ENGINEER NAME CEK
ENGINEER DATE 17-Jan-20
END JOB INFORMATION
INPUT WIDTH 79
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 22.42 25 0; 3 44.83 26.86 0; 4 67.25 28.72 0; 5 89.67 30.58 0;
6 112.08 32.44 0; 7 143 35 0; 8 168.17 39.46 0; 9 188.19 43 0;
10 202.13 39.5 0; 11 216.06 36 0; 12 230 23 0; 13 216.06 23 0; 14 202.13 23 0;
15 188.19 23 0; 16 171.13 23 0; 17 171.13 0 0; 18 151.5 0 0; 19 134.5 0 0;
20 112.08 0 0; 21 89.67 0 0; 22 67.25 0 0; 23 44.83 0 0; 24 22.42 0 0;
MEMBER INCIDENCES
1 1 2; 2 1 24; 3 24 2; 4 2 3; 5 23 24; 6 3 24; 7 2 23; 8 3 23; 9 3 4; 10 22 23;
11 4 23; 12 3 22; 13 4 22; 14 4 5; 15 21 22; 16 5 22; 17 4 21; 18 5 21; 19 5 6;
20 20 21; 21 6 21; 22 6 20; 23 19 20; 24 6 7; 25 7 20; 26 7 19; 27 18 19;
28 7 18; 29 7 8; 30 7 16; 31 17 18; 32 16 18; 33 8 16; 34 16 17; 35 8 9;
36 15 16; 37 9 16; 38 9 15; 39 9 10; 40 14 15; 41 9 14; 42 10 14; 43 10 11;
44 13 14; 45 10 13; 46 11 13; 47 11 12; 48 12 13;
DEFINE PMEMBER
48 44 40 36 PMEMBER 1

START USER TABLE
TABLE 1 C:\USERS\CKING\DOCUMENTS\1.UPT
END
DEFINE MATERIAL START
ISOTROPIC STEEL_36_KSI
E 4.176e+06
POISSON 0.3
DENSITY 0.489024
ALPHA 6.5e-06
DAMP 0.03
G 1.60615e+06
TYPE STEEL
STRENGTH FY 5184 FU 8352 RY 1.5 RT 1.2
ISOTROPIC CONCRETE
E 453600
POISSON 0.17
DENSITY 0.14999
ALPHA 5.5e-06
DAMP 0.05
TYPE CONCRETE
STRENGTH FCU 576
ISOTROPIC STEEL
E 4.176e+06
POISSON 0.3
DENSITY 0.489024
ALPHA 6.5e-06
DAMP 0.03
TYPE STEEL
STRENGTH RY 1.5 RT 1.2
ISOTROPIC STEEL_50_KSI
E 4.176e+06
POISSON 0.3
DENSITY 0.489024
ALPHA 6.5e-06
DAMP 0.03
TYPE STEEL
STRENGTH FY 7200 FU 8928 RY 1.5 RT 1.2
END DEFINE MATERIAL
CONSTANTS
MATERIAL STEEL_36_KSI ALL
MEMBER PROPERTY AMERICAN
2 5 10 15 20 23 27 31 UPTABLE 1 C15X50BTB
3 8 13 UPTABLE 1 DBLC12X30FT
1 4 9 14 19 24 UPTABLE 1 TOPCHORD
6 17 UPTABLE 1 EYEBAR4X7L8
11 UPTABLE 1 EYEBAR5X13L
12 UPTABLE 1 EYEBAR5X7L8
16 UPTABLE 1 EYEBAR6X15
21 UPTABLE 1 4EYEBARS6X1
18 22 UPTABLE 1 DBLC15X50FT
34 UPTABLE 1 2DBLL35
MEMBER PROPERTY AMERICAN
7 UPTABLE 1 EYEBAR6X175
MEMBER PROPERTY AMERICAN
39 43 UPTABLE 1 2C10X11
37 UPTABLE 1 C15XX41
41 UPTABLE 1 EYE10
42 46 UPTABLE 1 DL4312
45 UPTABLE 1 C10BTB176
47 UPTABLE 1 C10BTB146
36 40 44 48 UPTABLE 1 C15X50S14
33 38 UPTABLE 1 2DBLL3
25 29 35 UPTABLE 1 4EYEBARS613
32 UPTABLE 1 BUILTUP33
26 28 30 UPTABLE 1 DBLC15X50FT
SUPPORTS
19 PINNED
1 18 FIXED BUT FX
MEMBER TENSION
6 7 11 12 16 17 21 25 29 35 41
LOAD 1 LOADTYPE Dead TITLE SELF WEIGHT
SELFWEIGHT Y -1
LOAD 2 LOADTYPE Dead TITLE DEAD LOAD 2
JOINT LOAD
18 TO 24 FY -21.84
2 FY -0.57
3 FY -1
4 FY -1.04
5 FY -1.08
6 FY -1.2
7 FY -1.01
8 FY -0.53
9 FY -0.84
10 FY -0.21
11 FY -0.1
14 FY -0.9
15 FY -0.9
1 17 FY -10.92
16 FY -0.57
12 FY -115
13 FY -116
LOAD 3 LOADTYPE None TITLE BUS & WS LOAD 3
JOINT LOAD
1 FY -31.1
24 FY -25.9
23 FY -35.9
22 FY -26.6
21 FY -35.3
20 FY -27.2
19 FY -32.3
18 FY -19.9
17 FY -20.6
LOAD COMB 4 COMBINATION LOAD CASE 4
1 1.0 3 1.0 2 1.0
PERFORM ANALYSIS PRINT STATICS CHECK
PRINT JOINT DISPLACEMENTS ALL
PRINT MEMBER FORCES ALL
PRINT SUPPORT REACTION
PRINT ANALYSIS RESULTS
LOAD LIST ALL
PRINT MEMBER STRESSES ALL
FINISH

Please attach 1.UPT file used in your model.

Unit in
General
C15X40
23.594101 15.000000 0.786472 28.539400 0.413361 4314.040039 698.000000 2.920000 302.315002 93.066498 0.000000 0.000000 314.576996 114.236000 4526.009766 13.427100
C15x50BTB
29.392700 15.000000 0.979757 28.929100 0.508012 5512.980000 808.000000 5.340000 381.099000 107.733000 0.000000 0.000000 398.405000 135.649000 8277.020000 13.040500
DBLC15x50BT
29.392700 15.000000 0.979757 28.925200 0.508081 3941.700000 808.000000 5.340000 272.480000 107.733000 0.000000 0.000000 336.304000 135.427000 8277.020000 13.040500
DBLC12x30FT
17.635600 12.000000 0.734818 10.838600 0.813558 407.515000 324.000000 1.740000 75.187100 53.999900 0.000000 0.000000 82.793600 66.950700 2697.010000 10.530400
TOPCHORD
37.746400 15.500000 1.217630 23.996100 0.786512 2672.110000 1294.960000 2.734080 222.676000 131.661000 0.000000 0.000000 304.379000 199.439000 4237.840000 13.064700
EYEBAR4x7l8
7.000000 4.000000 0.875000 17.748000 0.197205 498.786000 9.333330 1.533550 56.201300 4.666670 0.000000 0.000000 58.985700 7.007870 2377.000000 2.250000
EYEBAR6x175
21.000000 5.996060 1.751150 19.500000 0.538462 1659.440000 63.000000 17.410300 170.199000 21.000000 0.000000 0.000000 186.442000 31.499900 26986.000000 2.493760
EYEBAR5X7l8
8.750000 5.000000 0.875000 17.748000 0.246506 623.483000 18.229200 1.985840 70.251600 7.291670 0.000000 0.000000 73.732200 10.925200 3078.060000 3.250000
EYEBAR5x13l
13.750000 5.000000 1.375000 18.748000 0.366705 1039.920000 28.645800 7.134750 110.924000 11.458300 0.000000 0.000000 119.347000 17.175200 11058.900000 2.250000
EYEBAR6x15
18.000000 5.996060 1.500980 16.003900 0.562362 949.500000 54.000000 11.380500 118.687000 18.000000 0.000000 0.000000 130.414000 27.000000 17639.800000 2.994090
4EYEBARS6x1
24.000000 5.996060 2.001310 20.000000 0.600000 1328.000000 72.000000 7.176000 132.800000 24.000000 0.000000 0.000000 167.843000 36.070800 11122.800000 1.993440
DBLC15x50FT
29.392700 15.000000 0.979757 12.429100 1.182410 884.817000 808.000000 5.340000 142.345000 107.733000 0.000000 0.000000 157.530000 135.649000 8277.020000 13.040500
11DBLC15x50
29.392700 15.000000 0.979757 12.929100 1.136690 966.279000 808.000000 5.340000 149.440000 107.733000 0.000000 0.000000 164.829000 135.649000 8277.020000 13.040500
4EYEBARS613
33.000000 5.996060 2.751810 21.496100 0.767582 2038.440000 99.000000 17.736000 189.622000 33.000000 0.000000 0.000000 242.976000 49.464500 27490.800000 0.492452
BUILTUP33
56.889500 15.000000 1.896320 21.925200 1.297350 2691.830000 855.539000 11.028200 245.471000 114.072000 0.000000 0.000000 369.461000 159.660000 17093.700000 11.207400
2DBLL35
12.996800 7.500000 0.866452 7.500000 0.866452 108.373000 36.983300 1.148330 28.899500 9.862190 0.000000 0.000000 35.423300 17.108400 1779.920000 5.767100
2DBLL3
9.717590 6.500000 0.747507 7.622050 0.637466 89.535100 21.196800 0.655833 23.484600 6.522090 0.000000 0.000000 28.508300 11.343300 1016.540000 5.004990
DBLC10X153F
8.977770 10.000000 0.448889 19.696900 0.227899 767.085000 134.800000 0.420000 77.876600 26.959900 0.000000 0.000000 81.879900 31.635600 651.001000 9.102220
C15X50S14
29.392700 15.000000 0.979757 21.429100 0.685812 2913.270000 808.000000 5.340000 271.861000 107.733000 0.000000 0.000000 288.916000 135.649000 8277.020000 13.040500
c10BTB146
14.696400 10.000000 0.734818 20.271700 0.362485 916.273000 182.400000 1.380000 90.397800 36.479900 0.000000 0.000000 114.941000 45.583600 2139.000000 8.530360
c10btb176
17.635600 10.000000 0.881781 20.563000 0.428820 1108.240000 206.000000 2.460000 107.774000 41.199900 0.000000 0.000000 137.904000 52.629000 3813.010000 8.236440
dl4312
12.996800 8.500000 0.764516 13.500000 0.481362 282.881000 52.919600 1.170110 41.908200 12.451700 0.000000 0.000000 60.241900 20.645200 1813.680000 6.970970
eye10
10.000000 5.000000 1.000000 16.000000 0.312500 563.333000 20.833300 2.900000 70.416700 8.333330 0.000000 0.000000 75.000000 12.500000 4495.010000 3.000000
c15xx41
41.392700 15.000000 1.379760 21.925200 0.943953 2522.370000 1033.000000 5.942030 230.017000 137.733000 0.000000 0.000000 319.469000 180.383000 9210.160000 12.240500
2C10x11
19.235600 10.000000 0.961781 20.724400 0.464081 1215.670000 219.333000 2.463230 117.309000 43.866600 0.000000 0.000000 150.794000 56.563900 3818.010000 8.076440

First of all, you should not be using STAAD TRUSS as a first line in the input file. Instead, STAAD SPACE should be used. Then, Truss specification should be assigned to the web members of the truss only, while top and bottom chords should be a regular non-Truss members. However, this does not make a large difference in the displacements. Even if we do not use Truss specification at all, still the maximum vertical displacement of node 12 is close to 4.5". So it seems that this result is related to something else(differences in member sizes, support conditions, applied loading, etc.).

First of all, you should not be using STAAD TRUSS as a first line in the input file. Instead, STAAD SPACE should be used. Then, Truss specification should be assigned to the web members of the truss only, while top and bottom chords should be a regular non-Truss members. However, this does not make a large difference in the displacements. Even if we do not use Truss specification at all, still the maximum vertical displacement of node 12 is close to 4.5". So it seems that this result is related to something else(differences in member sizes, support conditions, applied loading, etc.).