I am designing a Tapred section as per IS 800:2007 LSD, By considering LTB. When I do it manually, the Moment capacity of beam is 772.8 kNm as per Cl 8.2.2

We have kept some verification example in the help file. You may refer those for clarification.

file:///C:/Program%20Files/Bentley/Engineering/STAAD.Pro%20CONNECT%20Edition/Help/GUID-E6604057-DD21-4779-8428-A08817A6ECA2.html

If you still have doubts, please upload the STAAD model along with your hand calculation for investigation at our end.

Please find the Manual Calculation for the beam considered for design. I had used IS 800: 2007  

formula for Mcr calculation. Eventhough with formula mentioned in STAAD Example, there is a meagre variation in Mcr value. 

Please find the STAAD Editor attached below. Beam ID Considered for design is 9 

I run the file in below mentioned version. 

STAAD.Pro CONNECT Edition *
* Version 22.06.00.138

**********

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 30-Jul-21
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
5 0 10 0; 6 25 10 0; 7 0 24 0; 8 25 24 0; 9 50 10 0; 10 50 24 0; 11 75 10 0;
12 75 24 0; 13 100 10 0; 14 100 24 0;
MEMBER INCIDENCES
7 5 7; 8 6 8; 9 7 8; 10 9 10; 11 8 10; 12 11 12; 13 10 12; 14 13 14; 15 12 14;
DEFINE MATERIAL START
ISOTROPIC STEEL
E 2.1e+08
POISSON 0.3
DENSITY 76.8195
ALPHA 1.2e-05
DAMP 0.03
G 7.88462e+07
TYPE STEEL
STRENGTH RY 1.5 RT 1.2
END DEFINE MATERIAL
MEMBER PROPERTY
9 11 13 15 TAPERED 1.016 0.01 1.016 0.175 0.008
7 8 10 12 14 TAPERED 0.95 0.012 0.95 0.5 0.025
CONSTANTS
BETA 90 MEMB 7 8 10 12 14
MATERIAL STEEL ALL
SUPPORTS
5 6 9 11 13 FIXED BUT MZ
LOAD 1 LOADTYPE None TITLE DL
SELFWEIGHT Y -1
MEMBER LOAD
9 11 13 15 UNI GY -1.6
LOAD 2 LOADTYPE None TITLE COLL
MEMBER LOAD
9 11 13 15 UNI GY -5.6
9 11 13 15 UNI GY -8
LOAD 3 LOADTYPE None TITLE LIVE
MEMBER LOAD
9 11 13 15 UNI GY -6
LOAD COMB 4 COMBINATION LOAD CASE 4
1 1.5 2 1.5 3 1.5
PERFORM ANALYSIS PRINT ALL
LOAD LIST 4
PARAMETER 1
CODE IS800 LSD
BEAM 1 ALL
FYLD 345000 ALL
FU 450000 ALL
LAT 0 MEMB 9 11 13 15
LX 1.5 MEMB 9 11 13 15
LY 1.5 MEMB 9 11 13 15
MAIN 180 ALL
STP 2 ALL
TRACK 2 ALL
CHECK CODE ALL
FINISH

Please calculate Mcr with this equation

Here is the calculation for major axis bending capacity (Member 15)