How does staad design columns? Why does it chooses low values of pu, muz and mux?

I have one question with regards with staad. Staad takes into account series of combination load, takes the most critical and uses it in design. What makes my model so bugling is that its taking into account less design loads then comes up with over designed columns. Perhaps if you try considering beam 335, staad uses design load 1, yet is recommending me the use of 20mm dia RSB. WTF?!?! Can someone pls help me thx.

Heres my staad file.

Comments regarding my model are welcomed. feel free to correct me.

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 11-Oct-12
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 0 0; 2 0 0 2; 3 0 0 5; 4 4 0 0; 5 4 0 2; 6 4 0 5; 7 4 0 -2; 8 2 0 -2;
10 4 0 3.5; 11 2 0 2; 12 2 0 5; 13 2 0 3.5; 15 6 0 2; 16 6 0 5; 17 6 0 -2;
18 6 0 3.5; 19 8 0 0; 20 8 0 2; 21 8 0 5; 22 0 -2.7 0; 23 0 -2.7 2;
24 0 -2.7 5; 25 4 -2.7 2; 26 4 -2.7 5; 30 8 -2.7 0; 31 8 -2.7 2; 32 8 -2.7 5;
33 2 -2.7 0; 34 6 -2.7 0; 39 0 -5.33 0; 40 0 -5.33 2; 41 0 -5.33 5;
43 4 -4.52 5; 45 8 -3.7 0; 46 8 -3.7 2; 47 8 -3.7 5; 139 4 -2.7 0;
149 4 -4.52 0; 150 2 -2.7 -2; 151 6 -2.7 -2; 152 4 -2.7 -2; 153 2 -4.52 -2;
154 6 -4.52 -2; 155 4 -4.52 -2; 156 0 2.7 0; 157 0 2.7 5; 158 4 2.7 0;
159 4 2.7 5; 160 4 2.7 -2; 161 2 2.7 -2; 162 2 2.7 0; 163 6 2.7 -2;
164 6 2.7 0; 165 8 2.7 0; 166 8 2.7 5; 167 8 -0.85 0; 168 8 -0.85 2;
169 8 -0.85 5; 170 8 1.85 0; 171 8 1.85 2; 172 8 1.85 5; 173 10 -0.85 0;
176 10 -0.85 -2; 177 12 -0.85 0; 178 12 -0.85 2; 179 12 -0.85 5;
180 12 -0.85 -2; 181 14 -0.85 0; 184 14 -0.85 -2; 185 16 -0.85 0;
186 16 -0.85 2; 187 16 -0.85 5; 188 12 -2.13 5; 189 16 -1.85 5;
191 10 -2.13 -2; 192 12 -2.13 0; 193 12 -2.13 -2; 195 14 -2.13 -2;
196 16 -1.85 0; 197 16 -1.85 2; 199 2 -1.35 -2; 200 6 -1.35 -2; 201 4 -1.35 -2;
202 4 -4.52 2; 203 2 0 0; 204 6 0 0; 205 10 1.85 0; 206 10 1.85 2;
207 10 1.85 5; 208 12 1.85 0; 209 12 1.85 2; 210 12 1.85 5; 211 14 1.85 0;
212 14 1.85 2; 213 14 1.85 5; 214 16 1.85 0; 215 16 1.85 2; 216 16 1.85 5;
217 10 1.85 -2; 218 12 1.85 -2; 219 14 1.85 -2; 220 10 0.5 -2; 221 12 0.5 -2;
222 14 0.5 -2; 223 12 -2.13 2; 224 8 4.55 0; 226 8 4.55 5; 227 12 4.55 0;
228 12 4.55 5; 229 10 4.55 0; 230 12 4.55 -2; 231 10 4.55 -2; 232 14 4.55 0;
233 14 4.55 -2; 234 16 4.55 0; 235 16 4.55 5; 236 12 1.85 3.5; 237 10 1.85 3.5;
238 14 1.85 3.5;
MEMBER INCIDENCES
1 8 7; 2 7 4; 3 4 5; 4 5 10; 5 10 6; 10 1 2; 11 2 3; 12 12 3; 14 2 11; 15 11 5;
17 6 12; 18 10 13; 19 13 11; 20 13 12; 21 16 6; 22 16 18; 23 18 10; 24 18 15;
25 5 15; 28 17 7; 30 21 20; 31 20 19; 33 20 15; 34 21 16; 50 24 23; 51 23 22;
54 22 33; 56 34 30; 57 30 31; 58 31 32; 66 39 22; 67 40 23; 68 41 24; 73 43 26;
75 45 30; 76 46 31; 77 47 32; 145 25 26; 234 33 139; 235 139 25; 236 139 34;
247 139 149; 249 150 33; 252 151 34; 253 152 139; 254 150 153; 256 151 154;
259 152 155; 260 22 1; 261 23 2; 262 24 3; 263 26 6; 264 32 169; 265 31 168;
267 139 4; 268 152 201; 269 151 200; 271 150 199; 273 1 156; 274 3 157;
275 4 158; 276 6 159; 277 7 160; 278 8 161; 280 161 160; 281 160 158;
282 159 158; 283 159 157; 284 156 157; 285 162 156; 286 162 161; 287 162 158;
288 160 163; 289 163 164; 290 158 164; 291 164 165; 292 166 165; 293 166 159;
294 165 170; 296 166 172; 297 30 167; 298 17 163; 299 167 19; 300 168 20;
301 169 21; 304 20 171; 305 169 168; 306 167 168; 311 187 186; 317 173 167;
318 173 177; 320 177 180; 321 176 180; 322 180 184; 323 184 181; 325 186 185;
326 181 185; 328 181 177; 329 173 176; 332 170 171; 333 172 171; 334 179 188;
335 187 189; 337 191 176; 338 192 177; 339 193 180; 341 195 184; 342 196 185;
343 197 186; 347 201 7; 348 200 201; 349 201 199; 350 150 152; 351 152 151;
352 17 200; 353 8 199; 354 25 202; 355 5 25; 356 4 204; 357 4 203; 358 203 1;
359 204 19; 360 17 204; 361 8 203; 362 203 11; 363 204 15; 364 172 207;
365 210 207; 369 207 237; 370 206 171; 371 170 205; 372 205 217; 373 205 206;
374 210 236; 375 209 206; 376 209 208; 377 217 218; 378 218 219; 379 205 208;
380 208 218; 381 208 211; 382 211 219; 383 211 214; 384 214 215; 385 215 212;
386 212 211; 387 212 209; 388 212 238; 389 213 210; 390 213 216; 391 216 215;
392 220 217; 393 221 218; 394 222 219; 395 220 221; 396 221 222; 397 220 176;
398 221 180; 399 222 184; 400 210 179; 401 216 187; 402 215 186; 403 214 185;
404 208 177; 409 226 166; 410 224 226; 411 224 165; 412 226 228; 413 228 227;
414 227 229; 415 229 224; 416 229 231; 417 230 231; 418 230 227; 419 228 235;
420 234 235; 421 234 232; 422 232 227; 423 232 233; 424 233 230; 425 234 214;
426 235 216; 427 231 217; 428 230 218; 429 227 208; 430 233 219; 431 236 209;
432 237 206; 433 238 213; 434 237 236; 435 236 238; 436 177 178; 437 178 179;
438 24 26; 439 32 26; 440 169 179; 441 179 187; 442 223 178; 443 178 209;
468 228 210; 518 19 170; 519 21 172;
ELEMENT INCIDENCES SHELL
494 8 7 4 203; 495 7 17 204 4; 496 4 204 15 5; 497 203 4 5 11; 498 1 203 11 2;
499 2 11 12 3; 500 11 5 10 13; 501 5 15 18 10; 502 15 20 21 16;
503 204 19 20 15; 504 10 18 16 6; 505 13 10 6 12; 506 217 218 208 205;
507 218 219 211 208; 508 208 211 212 209; 509 205 208 209 206;
510 209 236 237 206; 511 209 212 238 236; 512 211 214 215 212;
513 212 215 216 213; 514 236 238 213 210; 515 237 236 210 207;
516 171 206 207 172; 517 170 205 206 171;
ELEMENT PROPERTY
494 TO 517 THICKNESS 0.1
SUPPORTS
39 TO 41 43 45 TO 47 149 153 TO 155 188 189 191 TO 193 195 TO 197 202 223 FIXED
MEMBER RELEASE
18 435 END MZ
1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 56 TO 58 145 -
234 TO 236 249 252 253 280 TO 293 305 306 311 317 318 320 TO 323 325 326 -
328 329 332 333 348 TO 351 356 TO 365 369 TO 391 395 396 410 412 TO 424 431 -
432 TO 441 START MX
DEFINE MATERIAL START
ISOTROPIC CONCRETE
E 2.17185e+007
POISSON 0.17
DENSITY 23.5616
ALPHA 1e-005
DAMP 0.05
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
66 TO 68 73 75 TO 77 247 254 256 259 TO 265 267 TO 269 271 273 TO 278 294 -
296 TO 301 304 334 335 337 TO 339 341 TO 343 347 352 TO 355 392 TO 394 397 -
398 TO 404 409 411 425 TO 430 442 443 468 518 519 PRIS YD 0.2 ZD 0.3
MEMBER PROPERTY AMERICAN
1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 56 TO 58 145 -
234 TO 236 249 252 253 292 305 306 311 317 318 320 TO 323 325 326 328 329 -
332 333 348 TO 351 356 TO 365 369 TO 391 395 396 431 TO 440 -
441 PRIS YD 0.3 ZD 0.2
MEMBER PROPERTY
280 TO 291 293 410 412 TO 424 PRIS YD 0.2 ZD 0.2
CONSTANTS
MATERIAL CONCRETE ALL
DEFINE UBC LOAD
ZONE 0.4 I 1 RWX 12 RWZ 12 CT 0.075 S 1
SELFWEIGHT
MEMBER WEIGHT
281 282 284 286 289 410 413 416 418 420 423 UNI 3.38
12 14 18 TO 20 22 TO 24 33 34 54 56 234 236 249 252 317 318 323 326 328 329 -
358 TO 364 369 TO 373 382 383 385 386 388 390 432 TO 435 438 TO 440 -
441 UNI 5.69
MEMBER WEIGHT
348 349 395 396 UNI 1.92
1 TO 5 10 11 28 30 31 50 51 57 58 145 235 253 305 306 311 320 TO 322 325 332 -
333 350 351 374 376 TO 378 380 384 391 431 436 437 UNI 7.37
348 349 395 396 UNI 3.69
348 349 395 396 UNI 3.6
FLOOR WEIGHT
YRANGE 0 0 FLOAD 1.92
YRANGE 1.85 1.85 FLOAD 1.92
FLOOR WEIGHT
YRANGE 4.55 4.55 FLOAD 0.37
YRANGE 2.7 2.7 FLOAD 0.37
LOAD 1 LOADTYPE Seismic TITLE EQ+X
UBC LOAD X 1
LOAD 2 LOADTYPE Seismic TITLE EQ-X
UBC LOAD X -1
LOAD 3 LOADTYPE Seismic TITLE EQ+Z
UBC LOAD Z 1
LOAD 4 LOADTYPE Seismic TITLE EQ-Z
UBC LOAD Z -1
LOAD 5 LOADTYPE Dead TITLE DL
SELFWEIGHT Y -1
MEMBER LOAD
281 282 284 286 289 410 413 416 418 420 423 UNI GY -3.38
12 14 18 TO 20 22 TO 24 33 34 54 56 234 236 249 252 317 318 323 326 328 329 -
358 TO 364 369 TO 373 382 383 385 386 388 390 432 TO 435 438 TO 440 -
441 UNI GY -5.69
1 TO 5 10 11 28 50 51 57 58 145 235 253 305 306 311 320 TO 322 325 332 333 -
350 351 374 376 TO 378 380 384 391 431 436 437 UNI GY -7.37
348 349 395 396 UNI GY -3.69
348 349 395 396 UNI GY -3.6
LOAD 6 LOADTYPE Live TITLE LL
FLOOR LOAD
YRANGE 0 0 FLOAD -1.92 GY
YRANGE 1.85 1.85 FLOAD -1.92 GY
MEMBER LOAD
348 349 395 396 UNI GY -1.92
LOAD 7 LOADTYPE None TITLE 0.37 ROOFLOADS COMPLETE LOAD 1.4 AND 1.7
FLOOR LOAD
YRANGE 4.55 4.55 FLOAD -0.37 GY
YRANGE 2.7 2.7 FLOAD -0.37 GY
LOAD COMB 8 COMBINATION LOAD CASE 8
5 1.2 6 0.5 7 1.0 1 1.0
LOAD COMB 9 COMBINATION LOAD CASE 9
5 1.2 6 0.5 7 1.0 2 1.0
LOAD COMB 10 COMBINATION LOAD CASE 10
5 1.2 6 0.5 7 1.0 3 1.0
LOAD COMB 11 COMBINATION LOAD CASE 11
5 1.2 6 0.5 7 1.0 4 1.0
LOAD COMB 12 COMBINATION LOAD CASE 12
5 1.2 6 1.6 7 1.0
LOAD COMB 13 COMBINATION LOAD CASE 13
5 1.4
PERFORM ANALYSIS
START CONCRETE DESIGN
CODE ACI
CONCRETE TAKE
CLB 0.04 MEMB 1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 -
56 TO 58 66 TO 68 73 75 TO 77 145 234 TO 236 247 249 252 TO 254 256 -
259 TO 265 267 TO 269 271 273 TO 278 280 TO 294 296 TO 301 305 306 311 317 -
318 320 TO 323 325 326 328 329 332 TO 335 337 TO 339 341 TO 343 347 TO 365 -
369 TO 404 409 TO 443 468 518 519
CLS 0.04 MEMB 1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 -
56 TO 58 66 TO 68 73 75 TO 77 145 234 TO 236 247 249 252 TO 254 256 -
259 TO 265 267 TO 269 271 273 TO 278 280 TO 294 296 TO 301 305 306 311 317 -
318 320 TO 323 325 326 328 329 332 TO 335 337 TO 339 341 TO 343 347 TO 365 -
369 TO 404 409 TO 443 468 518 519
CLT 0.04 MEMB 1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 -
56 TO 58 66 TO 68 73 75 TO 77 145 234 TO 236 247 249 252 TO 254 256 -
259 TO 265 267 TO 269 271 273 TO 278 280 TO 294 296 TO 301 305 306 311 317 -
318 320 TO 323 325 326 328 329 332 TO 335 337 TO 339 341 TO 343 347 TO 365 -
369 TO 404 409 TO 443 468 518 519
REINF 0 MEMB 1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 -
56 TO 58 66 TO 68 73 75 TO 77 145 234 TO 236 247 249 252 TO 254 256 -
259 TO 265 267 TO 269 271 273 TO 278 280 TO 294 296 TO 301 305 306 311 317 -
318 320 TO 323 325 326 328 329 332 TO 335 337 TO 339 341 TO 343 347 TO 365 -
369 TO 404 409 TO 443 468 518 519
FYMAIN 413688 MEMB 1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 -
56 TO 58 66 TO 68 73 75 TO 77 145 234 TO 236 247 249 252 TO 254 256 -
259 TO 265 267 TO 269 271 273 TO 278 280 TO 294 296 TO 301 305 306 311 317 -
318 320 TO 323 325 326 328 329 332 TO 335 337 TO 339 341 TO 343 347 TO 365 -
369 TO 404 409 TO 443 468 518 519
MINMAIN 16 MEMB 66 TO 68 73 75 TO 77 247 254 256 259 TO 265 267 TO 269 271 -
273 TO 278 294 296 TO 301 304 334 335 337 TO 339 341 TO 343 347 352 TO 355 -
392 TO 394 397 TO 404 409 411 425 TO 430 442 443 468 518 519
FC 20700 MEMB 1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 -
56 TO 58 66 TO 68 73 75 TO 77 145 234 TO 236 247 249 252 TO 254 256 -
259 TO 265 267 TO 269 271 273 TO 278 280 TO 294 296 TO 301 305 306 311 317 -
318 320 TO 323 325 326 328 329 332 TO 335 337 TO 339 341 TO 343 347 TO 365 -
369 TO 404 409 TO 443 468 518 519
FYSEC 248000 MEMB 1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 56 -
57 TO 58 66 TO 68 73 75 TO 77 145 234 TO 236 247 249 252 TO 254 256 -
259 TO 265 267 TO 269 271 273 TO 278 280 TO 294 296 TO 301 305 306 311 317 -
318 320 TO 323 325 326 328 329 332 TO 335 337 TO 339 341 TO 343 347 TO 365 -
369 TO 404 409 TO 443 468 518 519
MINSEC 10 MEMB 1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 -
56 TO 58 66 TO 68 73 75 TO 77 145 234 TO 236 247 249 252 TO 254 256 -
259 TO 265 267 TO 269 271 273 TO 278 280 TO 294 296 TO 301 305 306 311 317 -
318 320 TO 323 325 326 328 329 332 TO 335 337 TO 339 341 TO 343 347 TO 365 -
369 TO 404 409 TO 443 468 518 519
TRACK 2 MEMB 66 TO 68 73 75 TO 77 247 254 256 259 TO 265 267 TO 269 271 273 -
274 TO 278 294 296 TO 301 334 335 337 TO 339 341 TO 343 347 352 TO 355 392 -
393 TO 394 397 TO 404 409 411 425 TO 430 442 443 468 518 519
DESIGN BEAM 1 TO 5 10 TO 12 14 15 17 TO 25 28 30 31 33 34 50 51 54 56 TO 58 -
145 234 TO 236 249 252 253 280 TO 293 305 306 311 317 318 320 TO 323 325 -
326 328 329 332 333 348 TO 351 356 TO 365 369 TO 391 395 396 410 412 TO 424 -
431 TO 441
DESIGN COLUMN 66 TO 68 73 75 TO 77 247 254 256 259 TO 265 267 TO 269 271 273 -
274 TO 278 294 296 TO 301 334 335 337 TO 339 341 TO 343 347 352 TO 355 392 -
393 TO 394 397 TO 404 409 411 425 TO 430 442 443 468 518 519
CONCRETE TAKE
END CONCRETE DESIGN
FINISH

That's a lot to digest.

It's been a while since I've run STAAD and don't have a current veriosn to run this model.

I'd start by plotting the output and each of the load conditions to make sure loads are being applied correctly. Then check the column reactions to see that they make sense. Lastly, look at the each of the results and confirm that the program is, in fact, doing what you think it is.

Good luck.

i'll try that

I use staid almost everyday, Looks your design input is good but I'll check in my model to verify.

I've used staad for over 17 yrs, my observation in conc design the columns are design based on the code minimum requirement such section sizes and area if steel. My suggestions your loads have several combinations. The problem is you design the columns in theory of this program it can only design the columns only as compression member and not tension your load combinations and lateral forces makes some of these columns into tension that's why your column design is in question. I suggests take of all lateral loads and design the columns purely gravity loads and combinations then make another file with lateral loads but design the Columns as Beam and check how much steel differs and use that in your final column design. Let me know if you have issues.

this is the output from STAAD...the capacity is calcuted based on the minimum Area of steel