bracing in steel structure -reg

Sir,

I am trying to qualify a existing steel structure for MCE condition with site spectra (model attached)

Analysis results showed a maximum displacement at the free end of the member 4.

To reduce displacement , provided bracing member (no.6) 

Clarification/doubt 

1. First case , the bracing member is supported rigidly (i.e.) it takes all forces & moments.
Analysis results showed a reduction in the displacement.


2. Second case, the bracing member is made to take only axial force, and My & Mz are released.
Analysis results showed not much reduction in displacement


Doubt is
1. Whether my first case model is correct to go ahead with modification.
Or whether bracing should take only axial loads.


2. It is observed that with the moment release of the bracing member, the natural frequency is also changing, and it is close to the model without bracing.
The model (without bracing model ) is 5.492 Hz
model (with bracing and moment release model) is ~5Hz
model (with bracing and no moment release model) is ~12Hz

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 22-Jul-21
END JOB INFORMATION
INPUT WIDTH 79
UNIT MMS NEWTON
JOINT COORDINATES
1 0 0 0; 2 0 600 0; 3 0 800 0; 4 300 600 0; 5 545 600 0; 6 0 100 0;
MEMBER INCIDENCES
1 1 6; 2 2 3; 3 2 4; 4 4 5; 5 6 2; 6 6 5;
DEFINE MATERIAL START
ISOTROPIC STEEL
E 205000
POISSON 0.3
DENSITY 7.68195e-005
ALPHA 1.2e-005
DAMP 0.03
TYPE STEEL
STRENGTH FY 253.2 FU 407.8 RY 1.5 RT 1.2
END DEFINE MATERIAL
MEMBER PROPERTY INDIAN
1 TO 5 TABLE FR ISMC150
6 TABLE FR ISMC100
CONSTANTS
MATERIAL STEEL ALL
SUPPORTS
1 FIXED
MEMBER RELEASE
6 START MY MZ
6 END MY MZ
LOAD 1 LOADTYPE Dead  TITLE DL
SELFWEIGHT Y -1 
JOINT LOAD
4 FY -60
LOAD 3 LOADTYPE Live  TITLE SL(SSE)
JOINT LOAD
4 FZ 1013
LOAD 5 LOADTYPE Live  TITLE SL(SSE)WITH -VE
JOINT LOAD
4 FZ -1013
LOAD 9 LOADTYPE Seismic  TITLE EQX(SSE)
SELFWEIGHT X 1 
SELFWEIGHT Y 1 
SELFWEIGHT Z 1 
JOINT LOAD
4 FX 1013
4 FY 1013
4 FZ 1013
4 FX 60
4 FY 60
4 FZ 60
SPECTRUM SRSS X 1 ACC DAMP 0 LIN MIS 0
0.030303 2692.2; 0.0322061 2805.3; 0.0358423 2805.3; 0.0433839 3537.6;
0.0444444 3743.9; 0.0452489 3991.3; 0.0465116 4291.6; 0.0481928 4353.6;
0.0486618 4523.3; 0.0496278 4896; 0.0502513 4946.7; 0.0506329 5087.3;
0.0508906 5215.6; 0.0544959 5442.9; 0.0549451 5818; 0.0550964 5998.6;
0.0554017 6420.1; 0.0560224 6636.1; 0.0571429 6992.9; 0.058309 7313.8;
0.0584795 7419; 0.058651 7554.2; 0.0588235 7787.1; 0.0591716 8350.6;
0.08 8350.6; 0.0851064 8238.9; 0.0858369 8181.6; 0.0877193 8278.5; 0.0913242 8664.3;
0.0925926 8820.5; 0.125 8820.5; 0.125786 8805.1; 0.127389 8559.9; 0.128205 8494.1;
0.129032 8389.6; 0.12987 8337.3; 0.130719 8221.7; 0.13245 8183.9; 0.148148 8183.9;
0.150376 8170.3; 0.151515 8070.9; 0.152672 8021.2; 0.153846 7876.2; 0.188679 7811.7;
0.190476 7663.7; 0.192308 7367.6; 0.196078 7269.1; 0.2 7235.2; 0.20202 7235.2;
0.206186 7017.4; 0.208333 6818; 0.212766 6698.6; 0.224719 6698.6; 0.232558 6643.3;
0.238095 6643.3; 0.243902 6523.3; 0.28169 6676.4; 0.285714 6862.7; 0.294118 7401.4;
0.4 7401.4; 0.416667 6965.6; 0.425532 6553.6; 0.444444 6165.4; 0.465116 5979.1;
0.487805 5979.1; 0.540541 4771; 0.555556 4571.5; 0.588235 4371.9; 0.625 4367.4;
0.645161 4218.3; 0.666667 3913.8; 0.714286 3800.2; 0.740741 3800.2; 0.8 3469.5;
0.869565 3354.2; 1.05263 3354.2; 1.17647 3192.9; 1.25 2951.3; 1.42857 2812.5;
1.53846 2514.9; 1.66667 2357.4; 2 2177.5; 2.22222 1942.9; 2.5 1616.8;
2.85714 1422.1; 4 1194.2; 20 1194.2;
LOAD 10 LOADTYPE Seismic  TITLE EQY(SSE)
SPECTRUM SRSS Y 1 ACC DAMP 0 LIN MIS 0
0.030303 2627.3; 0.0458716 3311.8; 0.0460829 3409.9; 0.0462963 3458.9;
0.0465116 3557; 0.0483092 3686.4; 0.0486618 3806.2; 0.0492611 3889.3;
0.0496278 4039.6; 0.0508906 4216.8; 0.0549451 4216.8; 0.0552486 4261.5;
0.0554017 4339.6; 0.0568182 4476.1; 0.0571429 4559.5; 0.0574713 4601.1;
0.0584795 4773.2; 0.0588235 5000.9; 0.0591716 5456.2; 0.0634921 5473;
0.0657895 5473; 0.0680272 5626.9; 0.0682594 5771; 0.0687285 6159.2; 0.0701754 6311.6;
0.0704225 6520.8; 0.0711744 6798.7; 0.0719424 6844.9; 0.0729927 6861.8;
0.0735294 6979.9; 0.0740741 7225.2; 0.0757576 7576.9; 0.0763359 7811.1;
0.0772201 8325.2; 0.0775194 8399.6; 0.0784314 8544.2; 0.0793651 8810.5;
0.08 8925.7; 0.0803213 9054.7; 0.0809717 9377.2; 0.0823045 9531.5; 0.0990099 9531.5;
0.1 9543.6; 0.102564 10037.4; 0.103093 10209.5; 0.106952 11684.5; 0.107527 11975.8;
0.108696 12599.2; 0.10929 12770.9; 0.110497 12928.8; 0.150376 12928.8;
0.151515 12780.8; 0.152672 12706.8; 0.15625 12235.5; 0.15748 11673.8;
0.15873 11393; 0.16129 11376.9; 0.165289 11376.9; 0.168067 11332.1; 0.17094 10913;
0.172414 10774.7; 0.173913 10499.3; 0.175439 10362.2; 0.178571 9989.1;
0.18018 9689.7; 0.181818 9266.8; 0.183486 9143.3; 0.186916 9083.8; 0.19802 9083.8;
0.20202 8942.8; 0.204082 8700.4; 0.206186 8528.5; 0.208333 8294; 0.212766 8280;
0.224719 8280; 0.227273 8216.1; 0.229885 7927.9; 0.232558 7703.7; 0.238095 7427.5;
0.25 7186.8; 0.253165 7186.8; 0.263158 7184.8; 0.266667 7064.4; 0.273973 7149;
0.277778 7214.9; 0.28169 7346.7; 0.294118 7500.5; 0.4 7500.5; 0.416667 6110.8;
0.425532 5056.3; 0.454545 3977.4; 0.47619 3821.5; 0.487805 3665.6; 0.540541 3247.8;
0.555556 3026.6; 0.571429 2916; 0.606061 2807.2; 0.625 2807.2; 0.645161 2730.8;
0.666667 2557.1; 0.714286 2467.5; 0.740741 2467.5; 0.8 2290.7; 0.869565 2235;
1.05263 2235; 1.17647 2119.8; 1.25 1956.5; 1.42857 1869.1; 1.53846 1665.7;
1.66667 1575.2; 2 1449.9; 2.22222 1297; 2.5 1077.8; 2.85714 948.4; 4 798.5;
20 798.5;
LOAD 11 LOADTYPE Seismic  TITLE EQZ(SSE)
SPECTRUM SRSS Z 1 ACC DAMP 0 LIN MIS 0
0.030303 2872.6; 0.0433839 3653.8; 0.0438596 3653.8; 0.0458716 3701.5;
0.0465116 3870.9; 0.0483092 3968.8; 0.0486618 4069; 0.0492611 4145.6;
0.0496278 4265.9; 0.0503778 4304.5; 0.0508906 4408.5; 0.0522193 4428.2;
0.0526316 4677.1; 0.0546448 4790.1; 0.0547945 4857; 0.0550964 4923.9;
0.0554017 5057.7; 0.0568182 5221.2; 0.0571429 5367.7; 0.0574713 5440.9;
0.0576369 5514.1; 0.0584795 5757.7; 0.058651 5953.6; 0.0588235 6247.4;
0.0591716 6933; 0.0625 7324.2; 0.0628931 7433.8; 0.0630915 7537.3; 0.0632911 7662.3;
0.0641026 8302.2; 0.0643087 8419.5; 0.0645161 8595.5; 0.0649351 9006;
0.0657895 9477.6; 0.0680272 9539.8; 0.0687285 9690.3; 0.0699301 9704.1;
0.0704225 9784.1; 0.0722022 9784.1; 0.0727273 9816.3; 0.0738007 9953.5;
0.0760456 10289; 0.0766283 10428.6; 0.0775194 10839.6; 0.078125 10986.3;
0.0787402 11264.6; 0.0793651 11642.6; 0.0796813 11974; 0.08 12040.9;
0.0806452 12107.8; 0.0809717 12362.4; 0.0873362 12362.4; 0.0884956 12380.2;
0.0888889 12521.3; 0.0896861 12935.1; 0.0904977 13146; 0.0913242 13409.6;
0.0917431 13488.7; 0.0925926 13725.9; 0.125 13725.9; 0.125786 13711.1;
0.128205 13038.5; 0.129032 12278.3; 0.12987 11898.2; 0.131579 11072.7;
0.133333 10517.3; 0.134228 9961.8; 0.136054 9911.5; 0.147059 9911.5;
0.148148 9804; 0.150376 9499.8; 0.188679 9103.7; 0.190476 8931.1; 0.192308 8585.8;
0.19802 8539; 0.20202 8422.3; 0.204082 8305.9; 0.253165 8287; 0.25974 8219.3;
0.263158 8151.6; 0.273973 8154.3; 0.28169 8240.7; 0.377358 8240.7; 0.4 8002.3;
0.416667 7506.8; 0.425532 6962.6; 0.454545 6073.6; 0.487805 6045.4; 0.540541 4848;
0.555556 4613.3; 0.588235 4378.6; 0.625 4302; 0.645161 4197.9; 0.666667 3946.5;
0.714286 3857.2; 0.740741 3857.2; 0.8 3520.2; 0.869565 3398.9; 1.05263 3398.9;
1.17647 3202.8; 1.25 2963.7; 1.42857 2834.5; 1.53846 2512; 1.66667 2360.7;
2 2181.5; 2.22222 1945; 2.5 1620.2; 2.85714 1424; 4 1196.6; 20 1196.6;
LOAD COMB 20 LC1(SSE)
1 1.0 9 1.0 3 1.0 
LOAD COMB 21 LC2(SSE)
1 1.0 10 1.0 3 1.0 
LOAD COMB 22 LC3(SSE)
1 1.0 11 1.0 3 1.0 
LOAD COMB SRSS 23 LC4(SSE)
-1 1.0 9 1.0 10 1.0 11 1.0 -3 1.0 1
LOAD COMB 24 LC1(SSE WITH -VE SL)
1 1.0 5 1.0 9 1.0 
LOAD COMB 25 LC2(SSE WITH -VE SL)
1 1.0 5 1.0 10 1.0 
LOAD COMB 26 LC3(SSE WITH -VE SL)
1 1.0 5 1.0 11 1.0 
LOAD COMB SRSS 27 LC4(SSE WITH -VE SL)
-1 1.0 -5 1.0 9 1.0 10 1.0 11 1.0 1
PERFORM ANALYSIS PRINT STATICS CHECK
FINISH
FINISH
 
STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 22-Jul-21
END JOB INFORMATION
INPUT WIDTH 79
UNIT MMS NEWTON
JOINT COORDINATES
1 0 0 0; 2 0 600 0; 3 0 800 0; 4 300 600 0; 5 545 600 0; 6 0 100 0;
MEMBER INCIDENCES
1 1 6; 2 2 3; 3 2 4; 4 4 5; 5 6 2; 6 6 5;
DEFINE MATERIAL START
ISOTROPIC STEEL
E 205000
POISSON 0.3
DENSITY 7.68195e-005
ALPHA 1.2e-005
DAMP 0.03
TYPE STEEL
STRENGTH FY 253.2 FU 407.8 RY 1.5 RT 1.2
END DEFINE MATERIAL
MEMBER PROPERTY INDIAN
1 TO 5 TABLE FR ISMC150
6 TABLE FR ISMC100
CONSTANTS
MATERIAL STEEL ALL
SUPPORTS
1 FIXED
MEMBER RELEASE
LOAD 1 LOADTYPE Dead  TITLE DL
SELFWEIGHT Y -1 
JOINT LOAD
4 FY -60
LOAD 3 LOADTYPE Live  TITLE SL(SSE)
JOINT LOAD
4 FZ 1013
LOAD 5 LOADTYPE Live  TITLE SL(SSE)WITH -VE
JOINT LOAD
4 FZ -1013
LOAD 9 LOADTYPE Seismic  TITLE EQX(SSE)
SELFWEIGHT X 1 
SELFWEIGHT Y 1 
SELFWEIGHT Z 1 
JOINT LOAD
4 FX 1013
4 FY 1013
4 FZ 1013
4 FX 60
4 FY 60
4 FZ 60
SPECTRUM SRSS X 1 ACC DAMP 0 LIN MIS 0
0.030303 2692.2; 0.0322061 2805.3; 0.0358423 2805.3; 0.0433839 3537.6;
0.0444444 3743.9; 0.0452489 3991.3; 0.0465116 4291.6; 0.0481928 4353.6;
0.0486618 4523.3; 0.0496278 4896; 0.0502513 4946.7; 0.0506329 5087.3;
0.0508906 5215.6; 0.0544959 5442.9; 0.0549451 5818; 0.0550964 5998.6;
0.0554017 6420.1; 0.0560224 6636.1; 0.0571429 6992.9; 0.058309 7313.8;
0.0584795 7419; 0.058651 7554.2; 0.0588235 7787.1; 0.0591716 8350.6;
0.08 8350.6; 0.0851064 8238.9; 0.0858369 8181.6; 0.0877193 8278.5; 
0.0913242 8664.3;
0.0925926 8820.5; 0.125 8820.5; 0.125786 8805.1; 0.127389 8559.9; 
0.128205 8494.1;
0.129032 8389.6; 0.12987 8337.3; 0.130719 8221.7; 0.13245 8183.9; 
0.148148 8183.9;
0.150376 8170.3; 0.151515 8070.9; 0.152672 8021.2; 0.153846 7876.2;
0.188679 7811.7;
0.190476 7663.7; 0.192308 7367.6; 0.196078 7269.1; 0.2 7235.2; 0.20202 7235.2;
0.206186 7017.4; 0.208333 6818; 0.212766 6698.6; 0.224719 6698.6;
0.232558 6643.3;
0.238095 6643.3; 0.243902 6523.3; 0.28169 6676.4; 0.285714 6862.7; 
0.294118 7401.4;
0.4 7401.4; 0.416667 6965.6; 0.425532 6553.6; 0.444444 6165.4; 0.465116 5979.1;
0.487805 5979.1; 0.540541 4771; 0.555556 4571.5; 0.588235 4371.9; 0.625 4367.4;
0.645161 4218.3; 0.666667 3913.8; 0.714286 3800.2; 0.740741 3800.2; 0.8 3469.5;
0.869565 3354.2; 1.05263 3354.2; 1.17647 3192.9; 1.25 2951.3; 1.42857 2812.5;
1.53846 2514.9; 1.66667 2357.4; 2 2177.5; 2.22222 1942.9; 2.5 1616.8;
2.85714 1422.1; 4 1194.2; 20 1194.2;
LOAD 10 LOADTYPE Seismic  TITLE EQY(SSE)
SPECTRUM SRSS Y 1 ACC DAMP 0 LIN MIS 0
0.030303 2627.3; 0.0458716 3311.8; 0.0460829 3409.9; 0.0462963 3458.9;
0.0465116 3557; 0.0483092 3686.4; 0.0486618 3806.2; 0.0492611 3889.3;
0.0496278 4039.6; 0.0508906 4216.8; 0.0549451 4216.8; 0.0552486 4261.5;
0.0554017 4339.6; 0.0568182 4476.1; 0.0571429 4559.5; 0.0574713 4601.1;
0.0584795 4773.2; 0.0588235 5000.9; 0.0591716 5456.2; 0.0634921 5473;
0.0657895 5473; 0.0680272 5626.9; 0.0682594 5771; 0.0687285 6159.2; 
0.0701754 6311.6;
0.0704225 6520.8; 0.0711744 6798.7; 0.0719424 6844.9; 0.0729927 6861.8;
0.0735294 6979.9; 0.0740741 7225.2; 0.0757576 7576.9; 0.0763359 7811.1;
0.0772201 8325.2; 0.0775194 8399.6; 0.0784314 8544.2; 0.0793651 8810.5;
0.08 8925.7; 0.0803213 9054.7; 0.0809717 9377.2; 0.0823045 9531.5; 
0.0990099 9531.5;
0.1 9543.6; 0.102564 10037.4; 0.103093 10209.5; 0.106952 11684.5; 
0.107527 11975.8;
0.108696 12599.2; 0.10929 12770.9; 0.110497 12928.8; 0.150376 12928.8;
0.151515 12780.8; 0.152672 12706.8; 0.15625 12235.5; 0.15748 11673.8;
0.15873 11393; 0.16129 11376.9; 0.165289 11376.9; 0.168067 11332.1; 
0.17094 10913;
0.172414 10774.7; 0.173913 10499.3; 0.175439 10362.2; 0.178571 9989.1;
0.18018 9689.7; 0.181818 9266.8; 0.183486 9143.3; 0.186916 9083.8; 
0.19802 9083.8;
0.20202 8942.8; 0.204082 8700.4; 0.206186 8528.5; 0.208333 8294; 0.212766 8280;
0.224719 8280; 0.227273 8216.1; 0.229885 7927.9; 0.232558 7703.7;
0.238095 7427.5;
0.25 7186.8; 0.253165 7186.8; 0.263158 7184.8; 0.266667 7064.4; 0.273973 7149;
0.277778 7214.9; 0.28169 7346.7; 0.294118 7500.5; 0.4 7500.5; 0.416667 6110.8;
0.425532 5056.3; 0.454545 3977.4; 0.47619 3821.5; 0.487805 3665.6;
0.540541 3247.8;
0.555556 3026.6; 0.571429 2916; 0.606061 2807.2; 0.625 2807.2; 0.645161 2730.8;
0.666667 2557.1; 0.714286 2467.5; 0.740741 2467.5; 0.8 2290.7; 0.869565 2235;
1.05263 2235; 1.17647 2119.8; 1.25 1956.5; 1.42857 1869.1; 1.53846 1665.7;
1.66667 1575.2; 2 1449.9; 2.22222 1297; 2.5 1077.8; 2.85714 948.4; 4 798.5;
20 798.5;
LOAD 11 LOADTYPE Seismic  TITLE EQZ(SSE)
SPECTRUM SRSS Z 1 ACC DAMP 0 LIN MIS 0
0.030303 2872.6; 0.0433839 3653.8; 0.0438596 3653.8; 0.0458716 3701.5;
0.0465116 3870.9; 0.0483092 3968.8; 0.0486618 4069; 0.0492611 4145.6;
0.0496278 4265.9; 0.0503778 4304.5; 0.0508906 4408.5; 0.0522193 4428.2;
0.0526316 4677.1; 0.0546448 4790.1; 0.0547945 4857; 0.0550964 4923.9;
0.0554017 5057.7; 0.0568182 5221.2; 0.0571429 5367.7; 0.0574713 5440.9;
0.0576369 5514.1; 0.0584795 5757.7; 0.058651 5953.6; 0.0588235 6247.4;
0.0591716 6933; 0.0625 7324.2; 0.0628931 7433.8; 0.0630915 7537.3; 
0.0632911 7662.3;
0.0641026 8302.2; 0.0643087 8419.5; 0.0645161 8595.5; 0.0649351 9006;
0.0657895 9477.6; 0.0680272 9539.8; 0.0687285 9690.3; 0.0699301 9704.1;
0.0704225 9784.1; 0.0722022 9784.1; 0.0727273 9816.3; 0.0738007 9953.5;
0.0760456 10289; 0.0766283 10428.6; 0.0775194 10839.6; 0.078125 10986.3;
0.0787402 11264.6; 0.0793651 11642.6; 0.0796813 11974; 0.08 12040.9;
0.0806452 12107.8; 0.0809717 12362.4; 0.0873362 12362.4; 0.0884956 12380.2;
0.0888889 12521.3; 0.0896861 12935.1; 0.0904977 13146; 0.0913242 13409.6;
0.0917431 13488.7; 0.0925926 13725.9; 0.125 13725.9; 0.125786 13711.1;
0.128205 13038.5; 0.129032 12278.3; 0.12987 11898.2; 0.131579 11072.7;
0.133333 10517.3; 0.134228 9961.8; 0.136054 9911.5; 0.147059 9911.5;
0.148148 9804; 0.150376 9499.8; 0.188679 9103.7; 0.190476 8931.1;
0.192308 8585.8;
0.19802 8539; 0.20202 8422.3; 0.204082 8305.9; 0.253165 8287; 0.25974 8219.3;
0.263158 8151.6; 0.273973 8154.3; 0.28169 8240.7; 0.377358 8240.7; 0.4 8002.3;
0.416667 7506.8; 0.425532 6962.6; 0.454545 6073.6; 0.487805 6045.4;
0.540541 4848;
0.555556 4613.3; 0.588235 4378.6; 0.625 4302; 0.645161 4197.9; 0.666667 3946.5;
0.714286 3857.2; 0.740741 3857.2; 0.8 3520.2; 0.869565 3398.9; 1.05263 3398.9;
1.17647 3202.8; 1.25 2963.7; 1.42857 2834.5; 1.53846 2512; 1.66667 2360.7;
2 2181.5; 2.22222 1945; 2.5 1620.2; 2.85714 1424; 4 1196.6; 20 1196.6;
LOAD COMB 20 LC1(SSE)
1 1.0 9 1.0 3 1.0 
LOAD COMB 21 LC2(SSE)
1 1.0 10 1.0 3 1.0 
LOAD COMB 22 LC3(SSE)
1 1.0 11 1.0 3 1.0 
LOAD COMB SRSS 23 LC4(SSE)
-1 1.0 9 1.0 10 1.0 11 1.0 -3 1.0 1
LOAD COMB 24 LC1(SSE WITH -VE SL)
1 1.0 5 1.0 9 1.0 
LOAD COMB 25 LC2(SSE WITH -VE SL)
1 1.0 5 1.0 10 1.0 
LOAD COMB 26 LC3(SSE WITH -VE SL)
1 1.0 5 1.0 11 1.0 
LOAD COMB SRSS 27 LC4(SSE WITH -VE SL)
-1 1.0 -5 1.0 9 1.0 10 1.0 11 1.0 1
PERFORM ANALYSIS PRINT STATICS CHECK
FINISH
FINISH

Both the model is attached. kindly give your comment