Modeling Pre-stressed Sections of Bridge

Are we engineers now being replaced by design softwares? I hope not.

Hello willyap06,

We may be partially replaced by design software, but, is that so bad? Do you really want the world to wait for us to crank out solutions on our slide rules? Design software has its place...it saves us considerable calculation time and does it without error (providing the input is correct).

In the end, the output of the program has to be interpreted by a knowledgeable practitioner...that's us!

True!

But seems like some, not all, are relying on design software for design work, not even considering that they need to learn, at least, the basics in engineering design and analysis.

For me, design softwares are just tools/aids to facilitate the process of calculation and recalculation eliminating waste of time repeating the solution manually.

We still need to provide the correct information and design criteria for the software to interpret and to produce the correct results.

Good day fellow contributors, Yes indeed, all of the above discussions and point of views are correct, i think everyone agrees to that, the bottom line here is, let us not forget the basic knowledge in design and calculations, computer softwares are here to stay and serves as vehicles to maximize output and save time, I myself also use said softwares and sometimes I resort to manual calculations to sheck and verify the integrity and output just to be sure..

The discussion was meant to tell me about the use of Staad-Pro for prestressed design...but here eveyone seems to discuss on the moral issues of using Softwares.

If anyone of you know HOW TO TAKE PRESTRESSING into Account for prestressed elements please tell me. It is my B.Tech Project to model the prestressed bridge....if anyone can help...please...

Good morning vitin,

I don't know what your difficulty is. Why don't you describe the bridge structure you are trying to model. Better still, give us a sketch. Are you using pretensioning, post-tensioning or a combination of the two? Are you attempting to model elastic deformations, ultimate strength or both?

You have asked for help but you seem reluctant to provide any useful information which might help someone respond to your dilemma.

One Picture tells a thousand stories--post it VITIN,

If you can State your problem correctly-you have half solved it.

If you cannot-- then go back to your Project guide Professor and the textbooks

Hello MUKULMAHANT:

It would be interesting to see what the exact wording of vitin's assignment was. For a student not conversant with prestressed concrete design and probably not familiar with the techniques of computer modeling, this project seems to be a bit onerous.

I agree with you that, if he expects CR4 to help him, he has to be able to state his problem clearly. To date, he has not done so.

hello guys...

I am copying a staad file for you. please run in staad.. I want to know how can I take effect of prestressing on plate elements. In the following model I am trying to take the effect of prestressing on plate elemnts but I am getting zero stress due to load 2 ie prestressing force which I have applied on the beam 3 and 10. this is a small model of my original bridge for which I was seeking help.

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 17-Feb-08
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 0 0; 2 1 0 0; 3 2 0 0; 4 0 0 1; 5 0 0 2; 6 1 0 1; 7 1 0 2; 8 2 0 1;
9 2 0 2; 10 0 -1.25 1; 11 1 -1.25 1; 12 2 -1.25 1;
MEMBER INCIDENCES
1 1 2; 2 2 6; 3 6 4; 4 4 1; 5 6 7; 6 7 5; 7 5 4; 8 2 3; 9 3 8; 10 8 6; 11 8 9;
12 9 7; 17 4 10; 18 10 11; 19 11 6; 20 11 12; 21 12 8;
ELEMENT INCIDENCES SHELL
13 1 2 6 4; 14 2 3 8 6; 15 4 6 7 5; 16 6 8 9 7;
ELEMENT PROPERTY
13 TO 16 THICKNESS 0.25
DEFINE MATERIAL START
ISOTROPIC CONCRETE
E 2.17185e+007
POISSON 0.17
DENSITY 23.5616
ALPHA 1e-005
DAMP 0.05
ISOTROPIC CONCRETE2
E 2.17185e+007
POISSON 0.17
ALPHA 1e-005
DAMP 0.05
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
18 20 PRIS YD 2.5 ZD 1.1
1 TO 12 17 19 21 PRIS YD 0.1 ZD 0.1
CONSTANTS
MATERIAL CONCRETE MEMB 13 TO 16 18 20
MATERIAL CONCRETE2 MEMB 1 TO 12 17 19 21
SUPPORTS
10 12 FIXED
LOAD 1
SELFWEIGHT Y -1
LOAD 2
MEMBER POSTSTRESS
3 FORCE 20.0 ES .5 EM .5 EE .5
10 FORCE 20.0 ES .5 EM .5 EE .5
PERFORM ANALYSIS
FINISH

Please tell me am i doing something wrong. I mean what should i do so that when I apply prestress on the I girder , its effect is seen on the deck..

Hello vitin,

The sketch to the right shows my interpretation of your data. The black numbers indicate the joints. The direction of the X, Y and Z axes are shown. The red lettering indicates four of the beams I wish to discuss.

I think that the pre-stressing force in B3 and B10 is likely being interpreted by your software as a pre-tensioning force as opposed to a post-tensioning force. Normally a bridge girder is stressed in the plant, then delivered to the site. In that situation, you would not expect to see the effects of pre-stress on the slab. Other than concrete creep, the pre-stress force cannot get into the slab.

If you want to see the effect of a post-tensioned force on the composite section, you must add it as two external forces, one at Joint 4, the other at Joint 8, and be careful with your signs (one is positive, the other is negative). That is, in fact the way a post-tensioning strand works. It is jacked on site, then anchored at each end. If the strand is draped, you must take that into account also by applying forces in the appropriate locations within the span.

Also, you have fixed Joints 10 and 12. That prevents any axial force from developing in B18 and B20. A pre-stress force would simply be absorbed by the fixed joints. You must allow a degree of freedom parallel to the X axis.

And finally, was it your intention to add shell elements 4-10-11-6 and 6-11-12-8? I don't see those on your input sheet.

Good luck.

So why are you leaving us in the dark? How did your project turn out? If we had not been interested, we would not have offered you any assistance in the first place. Don't you think you owe us some sort of closure? Or perhaps you are so self absorbed that you don't give a damn about anyone else. Well?

Hey pal, I am also an Engineering field and about to start what you have already done, could you please send me details or tutorial for modelling the bridge in staad? My e-mail address is logistic_nimish@yahoo.com