Rule of Thumb for Sizing Steel Girders

[foreng]

I'm in the process of trying to estimate a girder depth for a feasibility level study. Looking for some quick rules of thumb for estimating girder depth based on length and girder spacing for a steel girder/composite deck panel. The structure is double lane (10.8 m deck with 0.4 m curb or 10 m running surface)and approx. 90 m in length. I'm initally using 4 girders @ 3.0 m spacing, 0.9 m deck overhang each side and a 250 mm deck width.

Thanks

 

[crossframe]

I've been told, but have not confirmed, that the most econmical simple span bridge girder (for welded plate girders) is about L/27. I think this came from the old AASHTO L/30 requirement. I almost always use this as my initial web plate depth when I'm at the "back-of-the-napkin" stage.

You could probably reduce the deck thickness a little, unless that includes a wearing surface or girder haunch.

These are rough guidelines, and your mileage may vary depending on live load, local code requirements, etc.

 

[foreng]

Thanks for the advice

This is, as you pointed out, a back of the napkin estimate.
The 250 mm does include 25mm of surfacing, the actual design deck is 225 mm.

 

[Qshake]

So to understand this you are proposing to have a total length of bridge equal to 90m or a single span of 90m?

If this is total length equal to 90m, then you can divide the spans to meet a interior span length to exterior span length ratio of 1.10 as most economical. Of course then you have see if you can actually fit the piers at those locations. If you can use a three span arrangement, this will result in the cheapest bridge that can use concrete prestressed beams. Probably around 30" to 36" or 0.760m to 1m.

If the 90m is a single span or two span you will wind up using steel. For the case of the two span bridge with equal spans a depth of girder will likely be 2m to 3m. For a single span, the depth will likely be in the range of 3m to 4m.

As was mentioned before there are some other factors not the least of which is the pedestrian access. If pedestrians are allowed across the bridge, the span to depth ratio gets higher, around L/1000 and that could control your depth.




Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[foreng]

The design is a single span 90 m, looking to clear the channel for environmental purposes. The current 40-year-old structure is a 3 span arrangement, and the department of transportation would like to replace it with another 3 span structure. I’m trying to give them a ball park idea of what a proposed clear span structure is going to cost and how it will effect the current alignment, at 3 to 4 m it is significant current structure is about 1 m.

Thanks again for the advice.

 

[cvg]

dont forget that if you are increasing your superstructure depth by 3 meters, you may lose freeboard and flow capacity in the river below. This generally means you will have to raise the roadway / bridge profile higher over the river, requiring long approaches and a lot of fill. This is not only expensive, but also causes other environmental impacts that may outweigh the ones caused by piers in the river.

 

[foreng]

Exactly, the point I'll be trying to make at the meeting is just that, the current structure is a the minimum recommended freeboard and all 3m of additional height will be in the approaches. The clear span is being pushed by the department of environment as the river has very high fisheries values. I think I have a good case for the piers.

 

[Qshake]

Foreng - Another matter to bring up in support of piers is constructing/erecting the long single span. It is going to be difficult to lift a single girder of that size without a robust crane, if not 2, on site.

To do this properly the contractor will either have to use falsework in the stream and splice. Or if lifting at once, there will need to be a laydown area constructed over the stream to splice the girder, lift it and walk it in with cranes on each side.

Not only is this expensive, it will limit the number of contractors that can bid the job thereby driving cost up due to lack of competition. If your in an area where there is no available steel fabrication shop, such a large girder may prohibit a small yet qualified fabricator from bidding due to lack of handling or laydown area in the shop for such a girder.

Good Luck.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[foreng]

Thanks Qshake,

Another compelling argument for a couple of piers.

The funny thing is the current structure comprises timber trestles on each end about 30 m in length (approx. 35 ,1960 vintage timber piles, on each end). Any modification, even two narrow piers, will be an improvement.

 

[MichSt]

I will typically use Table 2.5.2.6.3-1 in the AASHTO LRFD code for getting a ball park min structure depth. The table has values for girder only or girder and composite slab.

If you have a layout in mind, why can't you just do a quick run in your steel deign software to get a prelim girder size?? That's what I typically do for structure studies to get my number girders and rough plate sizes, weight of steel, etc. I can't see that taking more than 1/2 hour of your time.