STEEL BRIDGE PEDESTRIAN

[Waqar Saleem]

SALAM

I have been working on a pedestrian bridge design steel structure, i have attached two snaps from sap2000 for the concept. All four towers and arches will have cladding in round shape. I have assumed bolted connections. if i assign moment release to both ends of the members i get my sections passed within .70-.80 dc ratio but when there are no moment releases assigned dc ration reaches 3.5. Pu abruptly decreases in case of moment releases. i described the member results for the lowest members of the main tower.results comparison attached excel sheet. some valuable input is requested.

 

Regards  

result comparison.xlsx

[UmarMakhzumi]

Did you model this in SAP? If yes, can you upload the file.

The top and bottom chrod of your truss would be "continuous", or else your model will be unstable. How have you modelled them? I need to know this before I answer your question.

Thanks.

[Waqar Saleem]

Thanx Umar!!! i modelled in SAP .It means main edge members of the tower will be continuous. Earlier i had the problem of difference in reactions when i provide the releases but it is solved now. i decoded this problem as i had applied wind load at the center crossing of two braces which was not transferred when i assign this joint moment release so there was a difference. Have a look at the model i wait for your valuable input.

bridge-loads-corrected.$2k

bridge-loads-corrected.LOG

bridge-loads-corrected.sdb

[UmarMakhzumi]

Hi Waqar,

Sorry for the late reply. Have been very busy for last week. I quickly ran your model and noticed that it is unstable. Please see the log below.

Thanks.

 SAP2000 v17.1.1 Advanced (Analysis Build 9267/64)
 File: C:\Users\uma75058\Desktop\SAP2K\bridge-loads-corrected.LOG




 B E G I N   A N A L Y S I S                                2016/05/19  11:41:35


 RUNNING ANALYSIS AS A SEPARATE PROCESS
 USING THE ADVANCED SOLVER (PROVIDES LIMITED INSTABILITY INFORMATION)

 NUMBER OF JOINTS                          =        1580
    WITH RESTRAINTS                        =          32
 NUMBER OF FRAME/CABLE/TENDON ELEMENTS     =        4629
 NUMBER OF SHELL ELEMENTS                  =          18
 NUMBER OF LOAD PATTERNS                   =          12
 NUMBER OF ACCELERATION LOADS              =           6
 NUMBER OF LOAD CASES                      =          12


 E L E M E N T   F O R M A T I O N                                      11:41:36



 L I N E A R   E Q U A T I O N   S O L U T I O N                        11:41:36

 FORMING STIFFNESS AT ZERO (UNSTRESSED) INITIAL CONDITIONS

 TOTAL NUMBER OF EQUILIBRIUM EQUATIONS     =        9378
 NUMBER OF NON-ZERO STIFFNESS TERMS        =      198177

 * * * W A R N I N G * * *
 THE STRUCTURE IS UNSTABLE OR ILL-CONDITIONED !!
 CHECK THE STRUCTURE CAREFULLY FOR:
 - INADEQUATE SUPPORT CONDITIONS, OR
 - ONE OR MORE INTERNAL MECHANISMS, OR
 - ZERO OR NEGATIVE STIFFNESS PROPERTIES, OR
 - EXTREMELY LARGE STIFFNESS PROPERTIES, OR
 - BUCKLING DUE TO P-DELTA OR GEOMETRIC NONLINEARITY, OR
 - A FREQUENCY SHIFT (IF ANY) ONTO A NATURAL FREQUENCY

 TO OBTAIN FURTHER INFORMATION:
 - USE THE STANDARD SOLVER, OR
 - RUN AN EIGEN ANALYSIS USING AUTO FREQUENCY SHIFTING (WITH
   ADDITIONAL MASS IF NEEDED) AND INVESTIGATE THE MODE SHAPES


 NUMBER OF EIGENVALUES BELOW SHIFT         =           0


 L I N E A R   S T A T I C   C A S E S                                  11:41:37

 USING STIFFNESS AT ZERO (UNSTRESSED) INITIAL CONDITIONS

 TOTAL NUMBER OF CASES TO SOLVE            =          12
 NUMBER OF CASES TO SOLVE PER BLOCK        =          12

 LINEAR STATIC CASES TO BE SOLVED:

 CASE: DEAD
 CASE: LIVE
 CASE: EQX
 CASE: EQY
 CASE: EQY-
 CASE: EQX-
 CASE: EQX+
 CASE: EQY+
 CASE: WX
 CASE: WY
 CASE: WX-
 CASE: WY-


 A N A L Y S I S   C O M P L E T E                          2016/05/19  11:41:37


 SAP2000 v17.1.1 Advanced (Analysis Build 9267/64)
 File: C:\Users\uma75058\Desktop\SAP2K\bridge-loads-corrected.LOG




 B E G I N   A N A L Y S I S                                2016/05/19  11:41:43


 RUNNING ANALYSIS AS A SEPARATE PROCESS
 USING THE ADVANCED SOLVER (PROVIDES LIMITED INSTABILITY INFORMATION)

 NUMBER OF JOINTS                          =        1580
    WITH RESTRAINTS                        =          32
 NUMBER OF FRAME/CABLE/TENDON ELEMENTS     =        4629
 NUMBER OF SHELL ELEMENTS                  =          18
 NUMBER OF LOAD PATTERNS                   =          12
 NUMBER OF ACCELERATION LOADS              =           6
 NUMBER OF LOAD CASES                      =          12


 E L E M E N T   F O R M A T I O N                                      11:41:43



 L I N E A R   E Q U A T I O N   S O L U T I O N                        11:41:43

 FORMING STIFFNESS AT ZERO (UNSTRESSED) INITIAL CONDITIONS

 TOTAL NUMBER OF EQUILIBRIUM EQUATIONS     =        9378
 NUMBER OF NON-ZERO STIFFNESS TERMS        =      198177

 * * * W A R N I N G * * *
 THE STRUCTURE IS UNSTABLE OR ILL-CONDITIONED !!
 CHECK THE STRUCTURE CAREFULLY FOR:
 - INADEQUATE SUPPORT CONDITIONS, OR
 - ONE OR MORE INTERNAL MECHANISMS, OR
 - ZERO OR NEGATIVE STIFFNESS PROPERTIES, OR
 - EXTREMELY LARGE STIFFNESS PROPERTIES, OR
 - BUCKLING DUE TO P-DELTA OR GEOMETRIC NONLINEARITY, OR
 - A FREQUENCY SHIFT (IF ANY) ONTO A NATURAL FREQUENCY

 TO OBTAIN FURTHER INFORMATION:
 - USE THE STANDARD SOLVER, OR
 - RUN AN EIGEN ANALYSIS USING AUTO FREQUENCY SHIFTING (WITH
   ADDITIONAL MASS IF NEEDED) AND INVESTIGATE THE MODE SHAPES


 NUMBER OF EIGENVALUES BELOW SHIFT         =           0


 L I N E A R   S T A T I C   C A S E S                                  11:41:43

 USING STIFFNESS AT ZERO (UNSTRESSED) INITIAL CONDITIONS

 TOTAL NUMBER OF CASES TO SOLVE            =           6
 NUMBER OF CASES TO SOLVE PER BLOCK        =           6

 LINEAR STATIC CASES TO BE SOLVED:

 CASE: EQX
 CASE: EQY
 CASE: EQY-
 CASE: EQX-
 CASE: EQX+
 CASE: EQY+


 A N A L Y S I S   C O M P L E T E                          2016/05/19  11:41:44

 

[Waqar Saleem]

Salam 

Thanx very much for this valuable review, Umar. i will check the model thoroughly and will upload again for the review. how to improve it further guidance is requested from seniors and fellows. i have not worked in structural steel earlier so need help.

 

Regards 

[UmarMakhzumi]

For any structure we need load path for gravity and lateral. So lets discuss both one by one.

Your structure is a lattice truss structure. My first observation is that this type of structure is an overkill for pedestrian bridge. Reasons being that a lattice structure involves numerous connections and that would increase fabrication cost in addition to over-design.

Lets talk about gravity load path. The vertical load that you need to transfer is the weight of pedestrians standing on the bridge and the weight on the structure itself. My first observation is on your framing. You have got two arches (I would call the concave and convex trusses as arches for simplicity). Lets call them the inverted U and V shaped arch. Why do you need 2 arches? Shouldn't the inverted U shaped arch be good along with cables suspending down supporting the deck? In that case you can even eliminate the columns. If you want to retain the columns, you are even better off providing a truss below the deck to transfer the load/ weight of pedestrians to the columns. And you don't need a lattice arch.  Also for pedestrian walkway is not straight. I wonder why? The structural concept is not robust.

Now for lateral load path. Your columns are stabilized by 4 braces which are in fact lattice trusses which in my opinion are totally unnecessary. The simplest thing to do would be to create a 3D frame and have a truss bridge connected to it and not have these braces at all. For a sketch of how the 3D frame should look like, have a look at: 

Once you do that, you should be fine. Using standard members and least connections, you can make this happen.

In addition to above, there is a bridge replacement project going on in Edmonton these days. Have a look at the website to get some design ideas:

http://www.edmonton.ca/transportation/road_projects/walterdale-bridge-features.aspx

Thanks.

[Waqar Saleem]

Salam 

Thanx Umar bhai, these arches are U and V are architectural requirements, it is more of an ornamental thing than a pedestrian bridge. All four vertical members and arches will be rapped in cladding with circular shape.  

Regards 

[Waqar Saleem]

instability issue has been resolved it was due to the diagonal bracing center point. if i break the braces at center this instability message showed up and when in do not break them at center it resolves.

[UmarMakhzumi]

On May 20, 2016 at 11:44 PM, waqar saleem said:

hese arches are U and V are architectural requirements, it is more of an ornamental thing than a pedestrian bridge. All four vertical members and arches will be rapped in cladding with circular shape.

You can provide truss below deck to transfer load and keep the arches but instead of making them lattice, you can just provide minimum angles/ channels and keep it non structural. That would keep cost low. The structure load requirements aren't that great and you can play a lot with load path. Also, be careful when doing connection design. While designing connections, do make sure your design matches analytical model. 

Thanks.