Can You Design: Traffic Bridge (On road)

Bending, compression, tension, erosion, vibration, etc., These are the some of the stresses that the bridge materials must withstand. Combination of materials are used here. I am not sure in that. So, that combination of materials must ensure this withstanding capability. Now suggest the materials names CEans😉

hmm with a big budget? concrete with carbon nanotube rebars *grin*

Anyone seen a documentary about the construction of the Millau Viaduct? Awesome crazy engineering there.

1) Soil testing of the area
2) Type of bridge eg suspension, truss, beam, cantilever etc
3) Approximate average load on the bridge at a particular moment.
4) Using suitable factor of safety and calculating the various forces on the bridge i.e. bending, compression, torsion, shear and tension.
5) Seismic activity of the area, wind effect and water currents(if necessary)
6) Other natural activities that may cause erosion and damage the structure.
7) Take into account vibration.
8) Type of deck, pre-cast concrete or re-enforced with steel girders.
9) Number of support sections and type.
10) A solid foundation of re-enforced steel beneath every support section.
11) Type of cement to be used with its allowable stress bearing capacity. The values of different concretes can be obtained from a table.
12) Similar for the steel to be used (girders and ropes).

The bridge is then made in parts or blocks. A 3-d frame made of steel pipes is used to support the structure. The different blocks are then joined together.

I have a vague idea on this .

1. I think we need to consider the type of the land /soil we are erecting a bridge , since while erecting a bridge on road we need to big a deeper holes which gives stress to the land and building beside may collapse , so we need to measure proper distance from the locality.

2 . We need to consider the elevation depending upon road structure i mean if we have many electric poles we need to take a proper elevation such that no electric shock for the employees who build the road happen.

3. We need to consider the bending , banking , tension and suspension of the bridge.

4 . We need to consider the load on the bridge i mean if heavy vechiles are also allowed we need to build a strong bridge.

5 . We need to think of Resonance.. ( i dont know what to think of it , but that is a important factor to be considered i think )

6 . Amounts of cement and sand required , steel required ,no of supporting pillars ,places for those pillars considering point 1 here.

PS. These are my ideas may be considered or not i dont know .. but i wanted to know about resonance factor , asin my childhood i read "Soldiers are asked to break step when they are crossing a bridge because if the resonanceof their steps equals the natural resonance of the bridge it collapses "

1.We also need to consider the strength of the bridge to withstand hurricanes like wind ,earthquakes etc etc.
2. As patty said number of segments also can be considered , if we are building a bridge with many turnings then we need to make different segments , and for joining them we need some machinery etc.
3. We need to consider the vertical and horizontal movement of the bridges with air pressures , measures have to be taken to keep this movement in limits.


PS. I dont know the Engineering convention so please correct me .
Thanks

I think what will be fun is having a hypothetical situation, then apply the steps mentioned (to a certain extent) by Patty and ES. A particular expanse we want the bridge to cover perhaps? 😀 What do you think, CB?

English-Scared

PS. These are my ideas may be considered or not i dont know .. but i wanted to know about resonance factor , asin my childhood i read "Soldiers are asked to break step when they are crossing a bridge because if the resonanceof their steps equals the natural resonance of the bridge it collapses "

I second that. Because some bridges have been known to fail in the past when marching troops tried to cross it.

Amazing response guyz, Thanks a lot for your invaluable thoughts.

I am outstation and do not have much time to update this thread and give inputs to each and every post. I will be back by Monday and will update each post with my inputs. Again thanks a lot to everybody.

Also, Apologies for not updating my own created thread in a proper manner.

In between no one mentioned about the post - construction Load testing or What about if we want to use pre-fabricated blocks, forces at entry and exit points. No one bothered about curvatures? what about crossing peak traffic points (below bridge) or a water stream below bridge.

@Patty - Good coverage man!!! still I think lots of point are still untouched

@ES - We do consider resonance/Vibration/sound energy.

@Ash - Its tool early to decide for the cost. Also main idea of creating this thread was to force engineering mind to think beyond their stream. May be length of the bridge is 200 mts. Now you can assume the cost as well. By the way in engineering we had a separate subject as engineering estimation and costing.

Appeal to all other CEan's to participate in this.

-CB

This is purely a question, nothing else.

Is Bending and Curves are same for bridges or not??

And one more question , Do we consider Banking angle for bridges?

we do consider banking angle whenever we design any path way (be it a normal road network or bridge).

In general Bending and curves are never a same thing.

-CB

According to me The Factors to be considered for Designing a Bridge are as follows :


1. In the first place the purpose of the bridge must be looked at. This will be the key to decide the many factors of design in the futher process.
For instance the bridge will handle what kind of traffic, whether very heavy load, or lighter freight, or some medium traffic, what kinds of vehicles their activity, etc.

2. The DYNAMIC LOADING on the bridge is the main factor to be taken into account. It must be calcualted for the condition of maximum Heavy duty traffic with maximum traversal frequency, ie point of Heaviest Moving Load.

3. The BUCKLING OF COLUMNS ie the supporting pillars is another important factor.
The ELEVATION of the bridge structure is decisive in this, as greater the height of the columns greater is the buckling for given area of c/s.
Thus the ELEVATION of the bridge is decisive in the design procedure.

4. The Weather conditions must be considered for the effects of the Humidity in the air which cuses CORROSION of the Steel and weakens it reducing the life. This is critical for location in proximity to the sea.

5. The TEMPREATURE VARIATION of the place must be considered so as to compensate for the EXPANSION and CONTRACTION of the Steel structure and resulting THERMAL STRESSES.

6. The VIBRATIONS are a very critical factor in bridge design.
The Vibrations due to the Dynamic Moving Load on the bridge and the Natural phenomena like Earthquakes must be considered and the strength design must be in acordance to it.
The aim must be to avoid the RESONANCE frequencies.
Damping can be implemented in the foundation itself to reduce vibrations.

7. The MAXIMUM DEFLECTION caused due to the Swaying caused by WIND FORCE across the supports ie pillars must be considered. This Deflection must be kept to a permissible minimum and the design must be in accordance to it.

8. The SPAN of each section between two consecutive supports must be finalised. The DEFLECTION phenomena above plays an important role in deciding the Span.

9. The SELF WEIGHT ie the load due to the weight of the various components of the bridge ie the concrete sections, steel elements like girders, rods, beams, trusses etc. must be considered as SELF LOAD.

10. The FINAL LOADING must be calculated considering the above factors namely, Dynamic Loading, the Self Weight.

11. The proper conditions of operation must be consideed and a suitable FACTOR OF SAFETY must be decided so as to sustain the Worst Loading Conditions in any Abnormal situation.

12. The STRESSES on the structure ie the TENSILE and COMPRESSIVE STRESS ( both due to DIRECT and BENDING LOADS), SHEAR STRESS due to vertical loading, VIBRATIONAL STRESSES, THERMAL STRESSES must be calculated, and implement it in the strength design accordingly.

13. The S.F.D. and B.M.D. must be then drawn.

14. The Horizontal Load Bearing members ie the Steel members must be chosen.
These members are usually GIRDERS, TRUSSES, RODS, I-BEAMS which are basically load bearing members.
The number of members comprising the Truss will be depending on the type of loading.
* The I- BEAM is known to take the Maximum Flexural load due to the virtue of its M.I. due to its shape.

15. The number of supporting Pillars must be decided and their Dimensions, shape must be finalised so as to provide maximum strength in the design and also the Aerodynamic profile to minimize the Wind Force causing unstability.
The SPAN is also decisive here.

16. The Steel used in the members must be having Maximum Permissible Tensile Stress value sufficient enough to bear the Design Stresses.
The proper grade can be chosen from the H.T.S. grades.

17. The Concrete must be STEEL REINFORCED specially for the pillars.
The concrete must be having sufficient COMPPRESSIVE STRENGTH so as to withstand the design loads.

18. For SUSPENDED BRIDGES the CABLE used must be of HTS grade and the WIRE used must be having enough strands to compensate for the heavy Swaying due to wind , as it is a free to swing structure. The Spans must also be thoughtfully decided considering the mentioned factor.

19. The ground soil analysis must be properly done before work begins( I cant comment on this as Im not a Civil guy.)

20. There must be proper Anti Corrrosive treatment or coating of al the steel members used.
A proper paint with protective qualities against climatic effects must be used.

There must be many more factors which we have not thought of, so I request Crazy Boy to please elaborate on those as it will be a good learning experience.

1. Structure of the bridge. I guess arc types of bridges are best.[ Don't remember exactly but somewhere in my school I read about types of bridges.Correct me if wrong.]
2. Materials to be used so as to maximize the durability of bridges.eg. Iron , Steel. Best is CNTs. Its tensile strength is very good.
3. Appropriate height above the ground so as it doesn't cause inconvenience to big big lorries.
4. Making it water and heat resistant as much as possible - again to maximize its life.
5. The base towers at the two ends. They are the main parts of bridge that bears most of the weight age.

Stunning post Rohan !!!

Did you take reference from any site or you yourself thought about all these points. Anyways nice efforts.

@ Shalini - Nice efforts 😀
Though you told on very elementary level (may be slightly wrong at time), still for a non-civil/mech person its appreciable efforts.

I will come back with Inputs on Monday/tuesday when I will be back on CE.

-CB

@CrazyBoy :
Thanks for the compliment:smile:

I did not refer to any site for the points mentioned.

These points were purely what came to my mind as a mechanical engineer when I started to think about the basic elements that would be critical in a design from strength and weather point of view.

Waiting for some core civil aspects from you on this topic.

My analysis will very much around the analysis of Rohan, I will just modify it and add the aspects of the civil engineering. Many non-civil engineers are following this thread so I will take care of the point of not going deep down into technical terms. Anybody interested in going deep down into the technicalities if the Bridge down can ask separate questions. I will PM him/her the solution or reference links.

Decide Bridge type:
a. In the first place the purpose of the bridge must be looked at. Bridge type may be decided by how the forces of tension, compression, bending, torsion and shear are distributed through their structure
b. kind of traffic, whether very heavy load, or lighter freight, or some medium traffic, what kinds of vehicles and their activity also employs its part in deciding the type of bridge.
c. Also its must to check the location where we need to build the bridge, the shape and type of bridge must collate with the soil type/geographical condition of the relevant area.

Material: Generally two types of material are being used while constructing a Bridge:
1. Pre-fabricated blocks
2. Created at the time
a. Material of the bridge is very much related to the surrounding atmosphere and load. Generally bridges are constructed using concrete; however some popular bridges are constructed using wood or cast iron as well.
b) The Weather conditions must be considered for the effects of the Humidity in the air which causes CORROSION of the Steel and weakens it reducing the life. This is critical for location in proximity to the sea.

Design:
We must consider all the design standards for the bridge design / load computation. A few points for computing load are as follows:
1. Load computation:
a. The STRESSES on the structure i.e. the TENSILE and COMPRESSIVE STRESS (both due to DIRECT and BENDING LOADS), SHEAR STRESS due to vertical loading, VIBRATIONAL STRESSES, THERMAL STRESSES must be calculated, and implement it in the strength design accordingly.
b. The SELF WEIGHT i.e. the load due to the weight of the various components of the bridge ie the concrete sections, steel elements like girders, rods, beams, trusses etc. must be considered as SELF LOAD.
c. The DYNAMIC LOADING on the bridge is the main factor to be taken into account. It must be calculated for the condition of maximum Heavy duty traffic with maximum traversal frequency, ie point of Heaviest Moving Load. A safety factor is considered for computing the load against which bridge is tested before opening for the traffic.
d. Vibration, resonance, sound, waves and earthquake must be considered while computing the force. Separate permissible values for forces are given for these external parameters load computation.
e. The MAXIMUM DEFLECTION caused due to the Swaying caused by WIND FORCE as well as the traffic across the supports (columns) must be considered. This deflection must be kept to a permissible minimum and the design must be in accordance to it.
f. After calculating the self/dynamic loading and other parameters we give consideration to the span between the two consecutive columns. Span and cross section of beam is decided on the loading calculated.
g. In order to analyze the proper loading the S.F.D. and B.M.D. must be then drawn.

Construction: Before construction of the Bridge it is mandatory to divert the traffic. After that it is checked that nearby proximity should not be affected because of the vibration/stress caused during the foundation construction.

Foundation:
a. Now bridge foundation mainly depends on the type of bridge. Dampening must be removed from soil and water level should be considered very much before laying the Bridge foundation.
b. If there are too many columns in the bridge, while designing the foundation we must take care that stress on the adjacent columns doesn't produce excessive stress in the foundation of other columns. For this appropriate foundation strength and distance between the columns must be calculated.
c. If bridge is over a river then pile foundation is required at times.
d. Loading - After creation of the foundation a load testing needs to be done for the settlement of the foundation. After testing the permissible settlement we proceed with the construction of the columns.

Columns:
a. The ELEVATION of the bridge structure is decisive in this, as greater the height of the columns greater is the buckling for given area of c/s. Thus the ELEVATION of the bridge is decisive in the design procedure.
b. The number of supporting Pillars must be decided on the basis of span and loading. On the basis of mentioned load/span columns Dimensions, shape must be finalized so as to provide maximum strength in the design
c. Loading - After creation of the column a load testing needs to be done for the settlement of the column. After loading on the each column and testing the permissible settlement we proceed with the construction of the pathway.

Path way:
a. Construction of path way is started after the construction of columns. Some time pre-fabricated blocks are used for the pathways.
b. The VIBRATIONS are a very critical factor in bridge design. The Vibrations due to the Dynamic Moving Load on the bridge and the Natural phenomena like Earthquakes must be considered and the strength design must be in accordance to it.

Flooring:
a. for concrete flooring the TEMPREATURE VARIATION of the place must be considered so as to compensate for the EXPANSION and CONTRACTION of the Steel structure and resulting THERMAL STRESSES. I believe many of you had observed the spaces between the concrete blocks on roads. That gap is kept to compensate for the EXPANSION and CONTRACTION of the Steel structure and resulting THERMAL STRESSES.

Load Testing:
After completion of the structure a proper load testing has to be done before opening the bridge for the traffic.

I had not mentioned the cross section of the beams, type of reinforcement, concrete grades, loading and casting. If anyone has interest in the core design part, dozens of e-books are available on the web with the specifications. Still whenever you face problems you can PM me for the help.

crazyboy

Hi All,

As promised, CrazyBoy is back with Can You Design; series again.

This thread is the part II of the "Can you Design" series. All of us extensively use bridges in our daily transportation routes. Have we ever give a thought that how much efforts they takes to design them.

In this series you all guyz need to decide on, from where you need to start, what will be the materials, which force/design you will like to see for designing the gigantic structure. Anyone of you can start another thread with the product of your interest.

A few rules:-
1. Anybody can post in this thread, Specially I would like to see more posts from Non-civil engineers. Just think what all forces/material you will compute for designing a Overhead Water Tank. I am assuring you it would be a fun and good learning exercise.
2. For all Civil engineers, pour your expert comments.
3. I will keep giving my inputs in this thread..

Lets see how near we can reach in designing a bridge.

-CB

I will give this project to my workers they will design it since i am just 17 and new to this mate 😀