Bridge strike to pretensioned beams

[LittleWheels]

Several pretensioned M-beams on one of my overbridges have been struck by an over-height lorry. On the worst beam, several strands are exposed and the surrounding concrete has significant cracking (looks like shale). The visible damage to the flange soffit almost extends to the web with a spalled area exceeding a square foot. The beam supports a service bay containing large electricity, water and gas mains, amongst other things. Replacing the beam would be extremely difficult, due to a prior strengthening scheme that has tied the beam to adjacent beams.

I am unsure how to evaluate how far the cracking extends into the beam. Possibly I could map the extent of cracking with impact-echo or ultrasound?

How large an area is it practicable to repair? Possibly plate bonding or a metal sleeve splice? External post-tensioning is probably out.

 

[LeoSC]

I'm not an expert, but it seems that ultrasonic testing would be the most appropriate to determine the depth of the cracks. It should also give you a map. Since the girder is tied to adjacent ones, it would be prudent to inspect those girders as well.

 

[MikeHydroPhys]

Shear is expected to be extended back from visible to deep in the system and will weaken the existing reinforcing bind to the concrete even if repair or partial replacement could be envisaged
you might save the trouble and time by looking for a suitable solution of structural replacement rather than have to go back sorting out the mess accruing later if any searches go awry
the failure is bound to have thrown disproportionate load onto the remaining system and affected the foundation

cut their and your losses, assume the worst structural degradation for your own safety a hard hat is not going to protect in this case !

MikeHydroPhys

mdshydroplane

 

[jheidt2543]

DaveMinter,

There is a lot in the trade magazines regarding the use of fiber-reinforced wraps used to strenthen beams and columns. You might try to find a manufacturer of the material and see if they can help. It might be a good alternative to a total replacement.

 

[LittleWheels]

Thanks for the thoughts.

Replacement remains a last resort. There is a certain amount available on the web regarding repairs to pretensioned beams, particularly from Michigan, Florida and Missouri. Also, some interesting work is occurring in Cambridgeshire.

The plan at the moment is to map the cracking, to inspect connecting members and to calculate the remaining load capacity. Assuming cracking is not too extensive, it seems possible to repair the beam, either by preloading and concrete patching or by composite plate bonding.

Has anybody found a practical method of determining the tension in exposed tendons? My prelim calcs seem to show that residual capacity is fairly closely tied to the extent of cracking, with some resultant loss of tension. Determining the existing tension in the (pretensioned) tendons would help.

 

[KarlT]

I recently worked on a project in which an existing i-girder was struck by a backhoe. Several strands broke and some of the stirrups were also torn apart.

The local Hwys Dept elected to splice the existing strands with couplers and re-tension them. See link for Grab-it splices - they worked really well.

http://www.pci.org/host_sites/PSI/repair-1.html

We actually only oversaw the stressing operations, the dept. also had a very extensive preloading, and repair details and specs as well.

You may want to find out if a local precaster has the shop drawings for the original girders - that way you can find out the original grade and stress of the strands and also the original girder capacity.

 

[LittleWheels]

Luckily, there has been virtually no damage to the strands themselves (but I'll remember that solution for the next bridge).

I've got the drawings and know the specified strand tension. It still would be helpful to find the tension remaining in the exposed strands. Loss of concrete cross-section allows some shortening and loss of tension. If the actual tensions have not dropped significantly, repair seems more reasonable.