Bridge Survey 1

[Bert2]

Hi all first time ive posted in this part of the forum.

Could someone please point me in the direction for a general rule of thumb for surveying bridges.

Basics is all im after at the moment.In areas like;

:Max load capcity
:General condition
:Structural steel assessment. etc...

thanks!

 

[kieran1]

Sorry don't think there is any general rule of thumb. If you can't work out the loading and condition, you need to hire a Civil/Structural Engineer

Kieran

 

[Qshake]

Across the US and even more so Internationally there is a huge variety of bridge types and that doesn't even include historic bridges which often have unique load paths and capacities.

The governing document here in the US is the Manual for Condition Evaluation of Bridges. The 2011 version is over 500 pages. If you don't grasp the material then, as noted previously, you need to hire a civil/structural engineer.

Regards,
Qshake

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

 

[Bert2]

ok thanks i understand its a vague area for question.

But the the specific bridges in question have no historical data.

is there a quick way to asses the max capcity (as a rough idea) taken the general usage of the bridge ie normal traffic etc...

thanks.

 

[Qshake]

Bert2 - Having no historical data is quite common especially with bridges owned by smaller municipalities and that were built long ago, say pre 1900s up to 1960s. It's entirely possible that plans were lost or fell victim to fire or some other such problem.

The Manual as previously noted has historical data to help the engineer establish the likely allowable stresses based on the material of that era. Even more difficult is when encountering steel shapes, what are the section properties of those shapes. Until about 1950 or so, many steel mills produced their own steel shapes thus making them proprietary. Thankfully AISC has a reference that helps from 1870 or so to 1930 - I can't remember the exact years. If the dates or years can't be ascertained then testing is necessary. Coupons need to be taken of the steel and cores taken of the concrete. Measurements will need to be taken of both so to understand the section properties as well as provide the correct weight for dead load. Moreover, these measurements must take into account any deterioration or deformation that may compromise the capacity of the members to carry their once prescribed load.

Then there is the analysis as mentioned before. If this is a simple slab on beam bridge that could be easy to determine the capacity but will still require engineering analysis. If the system is less common than the slab on girder, well, the effort increases.

As you can see there many factors that play into this capacity analysis. unfortunately there are no guidelines or rules of thumbs to show that a certain bridge of a certain era have 3 tons or 23 tons or 36 tons of capacity.

Capacity may also vary on speed of the cars on the structure and whether or not multiple lanes are used or available. For speeds generally less than say 15 mph, impact can be ignored and for a bridge that carries only a single lane of traffic will have a higher capacity than bridge expected to carry mutiple lanes at a time.

We're not trying to be difficult. This sort of problem requires engineering analysis. As I mentioned there are no rules of thumb no nomographs, no charts. I've seen many a small subdivision group hire firms to analyze old bridges to determine if they can carry fire trucks and or ambulances. It's important and a common matter for a qualified firm to do.

Regards,
Qshake

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

 

[Bert2]

Qshake ive downloaded the Manual, and shall read through it, do you have a link to the 2011 version?

Many thanks for point me in the direction of the manual looks very interesting, and a very good base for understanding the subject.!