Bridge weight capacity question

[KevinFromTexas]

I heard that pedestrian bridges are usually engineered for higher weight capacities than auto bridges. Is this true? In the Austin forum, we were trying to figure out how much weight a typical pedestrian bridge is engineered to carry, versus an auto bridge. There's a new bridge planned for Austin that will cross our river. It'll carry either light rail or a street car system, and possibly a pedestrian use and maybe a bus lane. There's some debate though about whether the bridge will fit in one of the proposed routes, and if maybe the street car line or light rail line will instead be planned for an existing auto bridge. Our question was could the existing bridge (which is over 70 years old) support the weight of a fully loaded train or street car, plus the other normal auto and pedestrian traffic?

Here's the thread in the Austin forum discussing the possible new bridge.
http://forum.skyscraperpage.com/show...66#post4697866

[Holden West]

I was reading about London's new Millennium footbridge over the Thames and how it started swaying. Unlike cars or rail that move steadily, a pedestrian puts one foot in front of the other, constantly shifting weight from one side of the body to the other. Multiply this by hundreds of people and your bridge starts to wobble. The bridge had to be strengthened following this discovery.

The Bottom Line seemed to be that five tons of people cause more havoc than a five ton truck (but ain't that always the case).

[Kelvin]

Pedestrian bridges do not have specifically defined loads in the way the vehicular or train bridges do. However, that being said, some bridge codes do provide guidance as to what a reasonable load might be.

For example, the Canadian Highway Bridge Design Code (note the term "Highway") would suggest a basic pedestrian loading for a sidewalk should one be required on a vehicular bridge. One could extrapolate that loading condition to be something reasonable for a "pedestrian only" structure. CHBDC suggests, if I recall correctly:

5.0 - s/L (in kPa) as the basic area load applied to a sidewalk area,

where s is the girder spacing (assuming you even have a girder structure) and L is the span length.

Alternatively, congested spaces are often defined in building codes at about 4.8 kPa (100 psf) and this is sometimes extrapolated for use or comparison in pedestrian structure loads.

With respect to structural dynamics, it is true that pedestrian activity can excite a bridge - meaning causing it to move or vibrate in a way that is not pleasing or comfortable to the users. The question becomes more about the acceleration experienced as opposed to the actual movement.

It is interesting to note that as the number of pedestrians using a structure, the liklihood of creating resonant problems diminshes. As the number of people increases they tend to cancel each other out more and more. Exceptions do exist - for example if a large number of people were to walk in unison on the structure, like a marching band for example. That could be bad!

Vehicles also produce the same effect and, additionally, we also apply "Dynamic Load" factors (aka the Impact factor) to vehicular loads to approximate the added effect of very large moving loads pounding over a structure. Adding an extra 20% to 50% of a large vehicle's total mass is not uncommon.

Truck loads are highly variable - so what bridge engineers have done to simply the analysis of structures, is to create "envelope" trucks that are intended to create the worst-cases effects in a structure using single axle & load configuration. This allows that we don't have run every known truck axle configuration known to exist in order to which which one(s) create the maximal effects. Typical trucks are between 40 and 60 tons - some even higher at 80 tons.

Lane loads, or lines of regular vehicles, are simply assigned a nominal value of perhaps 500 to 700 lb/ft across the width of the lane and a lane could be anywhere from 10' to 14' normally (SI: 9 kN/ in a 3.6m lane). Note this is approximately 50 psf or 2.5 kPa (for comaparison purposes to ped loading).

[scalziand]

If I recall correctly, the ASCE Load Manual does give a higher load for heavily trafficked pedestrian areas than structures meant to support vehicles. 80 psf for the pedestrian areas and only 50 psf for vehicles. I'll have to double check when I get a chance though.

[Kelvin]

Is that ASCE-7? One should check AASHTO LRFD as well (but I don't have current copies of either).

[Holden West]

Originally Posted by Kelvin

As the number of people increases they tend to cancel each other out more and more. Exceptions do exist - for example if a large number of people were to walk in unison on the structure, like a marching band for example. That could be bad!

Indeed it would!

[Amanita]

In 1987, the Golden Gate Bridge celebrated its 50th birthday with a re-enactment of the 1937 Pedestrian day. So many people swarmed out onto the bridge that the arc of its center span flattened under the weight. A full load of densely packed pedestrians weighs much more than a full load of vehicles.

In 1973 when the first Bospherus bridge in Turkey opened, a similar phenomenon occured. So many people mobbed their new bridge that their weight exeeded the bridge's design load by 3x! When they shifted from one side to the other, the light standards on the deck could be seen tilting, suggesting that the same thing was happening to the deck! But the bridge held firm.

In 2003, the Brooklyn Bridge was the scene of a mass evacuation from Manhattan during the blackout. Many people felt a great deal of vibration in the bridge's structure, raising concerns of damage or overloading.

[scalziand]

Originally Posted by Kelvin

Is that ASCE-7? One should check AASHTO LRFD as well (but I don't have current copies of either).

ASCE 7-05 Minimum Design Loads for Buildings and Structures to be precise. I went back and checked the numbers. Since it's focused more on buildings than for bridges, I doubt the numbers are very useful. For areas with high intensity pedestrian traffic(Corridors, bleechers,"Yards and Terraces"), the load is given as 100psf. For parking garages without trucks, the load is only 40 psf. However, for "Sidewalks, Vehicular driveways, yards subject to trucking" the load dramatically increases to 250 psf.

That's a good idea checking AASHTO. I'll have to check and see if the school library has a copy. I'm pretty sure I've seen one in there somewhere.