any good book for modeling bridge?
any good book for modeling bridge?
(OP)
This site has been very helpful. I searched past posts for modeling methods, but couldn't find much details.
I just started learning modeling bridge in SAP or Staad, mostly by trial and error. Are there some good books/manuals describing computer modeling of deck and bridges?
I know there's a book named Bridge Modeling by Microcomputer, but it's a 1989 ver. and out of print. SAP's demo is too simple and with no deck.
Any input? How did you pick it up?
I just started learning modeling bridge in SAP or Staad, mostly by trial and error. Are there some good books/manuals describing computer modeling of deck and bridges?
I know there's a book named Bridge Modeling by Microcomputer, but it's a 1989 ver. and out of print. SAP's demo is too simple and with no deck.
Any input? How did you pick it up?





RE: any good book for modeling bridge?
When I model a bridge, typically I do a grillage. This is a 2-d model of the bridge which excludes the cross-frames. The accuracy of this type of analysis has been shown, time and time again, to be the most useful for its simplicity. That being said there are things one needs to know in order to model the grillage correctly. Firstly the aspect ratio of the rectangles within the grillage, as well as the section properties of the combined deck and girders. For example the torsional properties of the deck within this combined section should be halfed to account for the two way nature of bridge decks.
Modelling by FE is, for most cases, beyond normal requirements unless you have a very unusual bridge. Typical short and medium span highway bridges don't warrant this kind of analysis.
HTH
Regards
VOD
RE: any good book for modeling bridge?
I've seen people mentioning this grillage method many times here, but couldn't find a detailed procedure other than concepts. I can imagine tranforming the composite deck into a grid of frame members that simulate the composite section properties, but when it comes to details, I am not sure how.
For example, you have a 40' simple bridge with 4 W21 girders spaced 6' apart, 8" conc. deck. Longitudinally, one can use the AASHTO defined effective deck width and transform the composite deck into 4 frame members; but transversely, how would you transform the deck into frame members? what effective width to use? I guess this is related to the aspect ratio you mentioned, right? say use 4' deck section properties in transverse direction(that would be 9 transverse grid lines) versus 5'? What aspect ratio you think is good?
RE: any good book for modeling bridge?
Aspect ratios tend to reflect the span:deck width ratio. Typically for a long bridge, one uses a ratio of transverse grillage beams to the spacing of longitudinal grillage beams of 2:1. For wider, more square 1:1. There is no increase in accuracy below 1. Many bridges come in around 1.5:1. It is recommended to have an odd number of total transverse grillage beams.
In your case, you can use 10' transverse concrete sections. The end sections will have concrete section properties for a tributary width of 5'.
Regards
VOD
RE: any good book for modeling bridge?
Do you place the transverse grids in the same plane as the longitudinal girders? I mean, the N.A. of the deck is actually above the girders. How would this affect the accuracy?
2ndly, does it make a deference if you define the transverse grid members to be concrete sections or transform them into equiv. steel sections?
3rdly, when applying the moving load, do you apply it only along the girder lines or it can also on top of the transverse grids?
Many thanks.
RE: any good book for modeling bridge?
1. Yes, the accuracy is more than sufficient.
2. I would not know as I have not done so, you may want to try both and see. However, when checking global deck moments, you would want results based on the torsional stiffness of the concrete deck.
3. This question is going to lead to deck moments. For deck moments investigate Westergaard analysis for local deck moments and combine with the global effects. For the grillage, the moving loads are applied to the girders only.
Regards
VOD
RE: any good book for modeling bridge?
very interesting. so when applying the wheel load, do you multiply it with impact and distribution factors? (I would think yes to both.)
Would you load multiple girders simultaniously and use the multi-lane reduction factors to the results, or load only one lane at a time?
Do you know any literature that deals with computer modeling in details?
VOD
RE: any good book for modeling bridge?
When doing a grillage or FE analysis you will not use the simplified AASHTO distribution factors. The factors were derived parametrically in order to simplify a number of girders and lanes down to one single "effective" girder.
In order to distribute your wheel loads for a grillage, you will need to locate the worst-case transverse wheel locations on the transverse cross-section, this will become apparent, and calculate the girder reactions using an additive process for each deck span considered as simply supported. This simplification to simple supports has been proven accurate for the overall bridge compared to using a continuous span deck over the girders.
Yes, you will include impact.
Yes, you will load multiple girders simultaneously for 1, 2, 3, etc. lanes with appropriate multi-lane reduction factors, to find which one governs.
I do not quite fully understand your last question.
Regards
VOD
RE: any good book for modeling bridge?
My previous response,
"Typically for a long bridge, one uses a ratio of transverse grillage beams to the spacing of longitudinal grillage beams of 2:1. "
Should have stated
"Typically for a long bridge, one uses a ratio of transverse grillage beam spacing to the longitudinal grillage beams spacing of 2:1. "
RE: any good book for modeling bridge?
thanks for your elaboration on the load distribution. It helps a lot.
My last question asked if you know any books/manuals that have instructions of the grillage modeling. I don't know how people, like you, learnt these nuances (e.g., aspect ratios, redueced torsional constant, etc.), from books, passed down by generations, or trial and error? I just want to find a good resource of modeling techniques.
Regards,
Koo
RE: any good book for modeling bridge?
regarding a skewed bridge, at about what skew angle, you would make the grillage perpendicular to the girders, 30deg?
If doing so, how do you model the 4 portions of deck at the corners, since they are not rectangulars?
if grid is parallel to abutment (not perpendicular to girders), the deck portions are parallelograms, should the deck width be corrected to the width that is perpendicular to the deck grid?
Koo
RE: any good book for modeling bridge?
Do not model the transverse beams parallel to the abutment. In the areas of skew extend lines perpendicular to the girder from each of the bearings to the far girder. Do not draw transverse lines from bearing to bearing. Generally for angles less than 35 degrees, these perpendicular lines from every other bearing will do. This will define your transverse slab section size. End portions of the transverse slab fronting on the skew should be averaged.
To answer your question of how I learned this analysis, these were passed down to me like many other bridge engineers who were confronted with bridge design.
I would recommend you consult with an experienced bridge engineer in your office or a sub-consultant with regards to you designing a bridge.
Regards
VOD
RE: any good book for modeling bridge?
If would be nice to have somebody as knowledgeable as you in the office.
I found 2 books called Grillage Analogy in Bridge Deck Analysis and Bridge Deck Analysis. I'll try to get them. What do you thing the book called "Bridge modeling by microcomputer"? Is it a good one to have? It's out of print.
Many thanks.
Koo
RE: any good book for modeling bridge?
You need an experienced bridge engineer in your office. Personally I do not think this is the best way to go about bridge design. You need to solve this problem first. A company should not attempt this from a liability perspective. I understand this must be hard for you.
Anyway, to explain about the book "Bridge Analysis by Microcomputer". This book is from Canadian authors, who have adopted a different approach for load distribution factors. The approach is somewhat pioneering and it is continually improving its flexibility while maintaining its simplicity. This is the reason the book, which is the basis of our Canadian Bridge Code, is now out of print. New research and understanding has prompted revisions to various parameters within the text. If you need to understand the beginnings of the Canadian approach to understanding bridge structural response, give it a read from a library.
I doubt it will help you from a grillage perspective since the reaserchers' intent was to approach bridge design from the "simplified method" I alluded to earlier. AASHTO's and Canadian distribution factors were developed independently, approach parameters differently and thus cannot be used together.
Again, do your analysis, but have an experienced bridge engineer review your work!
Hope this helps.
Regards
VOD
RE: any good book for modeling bridge?
Is it normal for a highly skewed (60deg) bridge to give high (12kips) lateral load in a FE or grid model? Where does the lateral load come from, torsion? Thanks
Koo
RE: any good book for modeling bridge?
sorry, not sure it's 12k or not,but large. The lateral load is on the bearings. Besides breaking force could have a lateral component, what else could cause it? torsion, vertical load?
Koo
RE: any good book for modeling bridge?
Are you modelling it as flat or placing it from bearing elevation to bearing elevation?
VOD
RE: any good book for modeling bridge?
I'm not sure. It's not my project. seems flat, because they couldn't figure out where's the huge lateral load come from...could be from a software bug. I asked just because I'm curious to see if there are things about skewed bridges that I'm not aware of. When I did skewed bridges, I used AASHTO load dist. guideline to get the more accurate load distribution factors for increased shear and reduced moment. But that is based on a line girder analysis. This one I am asking is a MDX FE analysis. I'm not aware of any lateral adjustment in AASHTO for skewed bridge.
Koo
RE: any good book for modeling bridge?
If the FE analysis is accounting for slope along the road, then bearing elevations may be creating the lateral force. The anchor bolts at the lower end of the bridge can then be designed for the lateral force.
Make sure the end conditions of the analysis match the bearing design, ie. It is recommended to release most of the interior and exterior girder bearings for lateral movement since fixing all these bearings creates internal transverse deck forces due to temperature variations.
Regards
VOD
RE: any good book for modeling bridge?
It seems you suspect the lateral load could come from the slope. Is it possible for a skewed bridge to generate lateral load from its vertical loading? torsion?
BTW, I started modeling a small 2lane simple span bridge (38')in sap7. I got much larger reaction (1/5 more) and smaller moment (1/3 less) than the AASHTO based results. (sort like a deep beam?) I suspect it's the dist. factor made the difference, but it's a big difference. I didn't use a grillage though, I put shell eleement between girders and used the auto generated moving load in sap. I'll try model it as a grillage some time.
Koo
RE: any good book for modeling bridge?
Any slope in either direction might do this. Typically, for flat grillages, I do not get lateral loads. I don't typically analyse them on a grade, unless I feel I should.
I suspect your skew is between 30 and 35 degrees with 1/5 larger values for shear.
I also think you should look at your 1/3 less moment. It doesn't seem right. From what I can remember the simplified AASHTO method and the simplified Canadian method yielded different results for two design lane bridges.
You should check your skewed bridge grillage against a straight bridge grillage to verify for moment. If the AASHTO method gives you less moment, use the grillage moment, as this is for sure.
Regards
VOD
RE: any good book for modeling bridge?
My model is based on a straight bridge though. It's the simplest form of a composite steel bridge. So I'm quite supprised with the results. I can't find anything I did wrong in the model. I don't know how sap or other programs move its self-generated AASHTO load, only along the bridge or transversely within the lane as well. But I would think it's reasonable to have some discrepancy with the AASHTO simplfied method due to its "simplied" nature, but not this big though.
Koo
RE: any good book for modeling bridge?
Not to say your analysis is wrong, but to correctly model your bridge, do not use shell elements since you do not know how SAP is distributing the loads.
Do the grillage first, then as you get results from the grillage, compare them with your "shell between girders" model. As you get more similar results with different bridges you will then decide which one is easier based on your trust of the particular program. I have never done this "shell between girders" as some programs have problems mixing shells and members.
At the end of it all, check it against the "simplified " method in AASHTO. One should not use the AASHTO method without verifying it each time. This is why I always do a grillage and the "simplified" method together. Estimate your time for the job with this in mind.
Regards
VOD
RE: any good book for modeling bridge?
sorry that I still need to ask you several questions about grillage modeling. I started building a simple steel bridge in StaadPro. It has 4 beams at 9.67' spacing and 38 feet span, 2lanes for HS20 truck. I put 3 transverse frame members with concrete deck properties (9.5' wide-19'w-9.5'w). After load it with 2 moving HS20 trucks, I observed that 1)wheel load is distributed correctly to girders by simple beam assumption; 2) the resulting reaction and max. momemnt on the most heavily loaded interior girder is way less than a AASHTO line beam analysis would give (234 vs. 400 k-ft), but a nearby exterior girder is overloaded (200 vs. 128 k-ft). Apparently, load is redistributed by transverse members.
My questions are:
1) Is the result reasonable or correct at all?
2) How would you change the torsional propertie of the transvere member (deck) in Staad? Will it affect the outcome by much? (I tried to change the wide conc. deck property to a week steel beam, the resulting forces barely changed at all.)
3) How would you model the composite action in the girders? Use the CM section option in Staad or transform it into a steel I section?
4) Do you have the Staad.Bevea package? Will it give different results?
If you have the patience and interest, I could put my input file here for some critique.
Thanks alot.
Koo
RE: any good book for modeling bridge?
First, your inputs should be like this.
When you put in your transverse section properties. Torsional properties (J or R) should be for concrete cross section. This does make a difference.
For longitudinal beams, transform the composite section into steel. For its sectional area in Staad, use the steel girder only otherwise you will have additional area of the transformed concrete into steel as dead load that is accounted for in the transverse sections.
I would stay away from the options in Staad for now and just use the edit screen.
Finally, for the live load and superimposed dead load analysis, which we are discussing, the exterior long. girders should have all the section properties of any barrier wall and sidewalk if any, including its transformed torsional properties.
Once you have done these, compare your results again, it appears that your load distribution is correct but the analysis is going wrong.
Regards
VOD
RE: any good book for modeling bridge?
I specified trans. member to be concrete and gave it a demension (9" by 9.5'). Isn't J a property of the section and generated automatically by Staad based on my input geometry? I found nowhere I can edit this property.
You also mentioned earlier that "the torsional properties of the deck within this combined section should be halfed to account for the two way nature of bridge decks." where would you define it Staad? I assume this is in the long. direction and how would you bring this 1/2J of deck into a combined section?
Lastly, how would you transform a barrier into a composite section? just like a deck, shrink it in width? then the N.A. of the combined section would be brought very high. Is this composite action from barrier supposed to be accounted for?
One answer usually brings out more quetions...sorry
Koo
RE: any good book for modeling bridge?
There was an edit screen pre-windows version, it should still be there, check with REI on this.
I do not know how the input geometry interacts with section properties, but I doubt it.
J also known as R is the sum of individual R. Check Omar Blogett's "Design of Steel Structures". Typically for b/d for 20 to infinity the beta value is 0.333, but for smaller values interpolate from the provided Table 1 on pg 2.10-2
As you sum the individual values you make sure to half the slab's contribution not the haunch's or anything else concrete.
Yes, shrink the width, yes the NA rises. Yes this should be accounted for, as the stiffness of this girder increases, affecting the structural response to live load.
Regards
VOD
RE: any good book for modeling bridge?
That should be "Design of Welded Structures" by Blodgett published by James F. Lincoln Arc Welding Foundation.
This book is found in most structural engineering offices.
Regards
VOD
RE: any good book for modeling bridge?
to analyze the system as a finite element structure or you can choose the grid option. Comparing the results from MDX and STAADPRO you must you OFFSET BEAMS from the ELEMENTS which are used to model the deck. The supports are put on the ELEMENTS in this case. By using MDX you don't need to mess with these things; MDX does it automatically.
RE: any good book for modeling bridge?
glad you are here. I was looking for you. I modeled a simple steel composite bridge in both Staad and SAP with the method you described before( plate for deck, frame for beam and bolster). The results are good at supports, but midspan bending moment is much less than AASHTO values from App.A and D.F. tables (25%, 15% less for each). I know there will be some discrepency but most comments I read, including yours, said the FE model gives very good results. (I first thought DF in AASHTO is too conservative, but a more accurate LRFD fomular gives similar value.) Can you give me some comments on this?
I had Staad and Sap people checked my model, and they said it looks good. Do you compare your model results to AASHTO?
Koo
RE: any good book for modeling bridge?
RE: any good book for modeling bridge?
1. Can you clarify if it's MPZ or FPZ? Your two posts gave them differently. Are you sure it's correct command? Member release has FX(full release), KFX(spring) and MP(partial), which one are you trying to tell me? Partial momement release in Z?
2. Is 0.999 an arbitrary number? how did you discover it?
3. Among FE model, AASHTO code values, and field tests, how are they compare to each other, any percentage range?
Thanks. I've been fighting this for a while...
Koo
RE: any good book for modeling bridge?
Ed
RE: any good book for modeling bridge?
I tried MPZ, MPY, but no effect. Then I used MZ, MY, indeed, the links become a connection rod taking only compression. But the total moments at beam and slab still didn't add up to AASHTO value.
I think by using MPZ or MP, the deck and girder effectively become non-composite, right? The moments in girder and deck don't reflect the actural composite condition, then what's the point of obtaining them at all? Can you use them for anything?
Maybe it makes sense that FE model gives a reduced moment since sharing among girders is factored in vs. a single beam approach in AASHTO. Any comment?
Koo
RE: any good book for modeling bridge?
Not to get involved in the specifics of your FE model with Ed, but I don't think you can make that assumption about the extent of load sharing. Usually the single beam approach, for applicable bridges, and FE models should be within a few percent.
Regards
VOD
RE: any good book for modeling bridge?
That's what I thought first, but after checked my models and have Staad, Sap tech supports checked them. I couldn't find any problem with them. But I am not sure Ed's end release method is correct in modeling a composite section. (there should be large axial load since NA of the section is up in the deck.) Staad/sap will always give correct moment if it's a non-composite section. I think what makes the deal is to produce good result in composite action. So far I have no luck. I do hope people here like you and Ed can help me have this solved. Thanks.
Koo