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Background

On May 5th 2022, a bridge was struck in Brantford, Ontario, Canada by a commercial vehicle. This vehicle struck a reinforced concrete protection beam placed on the on-coming side for the traffic lanes passing under the bridge. The beams purpose is to protect the already damaged CPCI girders supporting the bridge. As a result of the collision the beam was knocked to the highway below:

enter image description here

Alternate view of fallen protection beam:

enter image description here

If this was a pre-stressed girder I would have expected to see some pre-stressing strands in the broken section, but there does not appear to be any. This is leading me to believe that this is a regularly reinforced concrete beam. There appears to be plenty of of C15M (Coated 15M bars) shear stirrups that the beam should have sufficient shear reinforcing (An analysis would be ultimately required to verify).

enter image description here Street View Link

Going to street view I can see that the girder has been in place prior to 2011, and was even around for a bridge rehabilitation in 2015.

Looking at the street view pictures I would like to say "what a pretty example of shear cracking throughout the beam". However, if it were a typical beam, I would have expected the diagonal cracking to become more vertical towards mid span. Instead the cracking appears to follow regular intervals and roughly the same angle until about mid span where it arches over.

The other oddity to me is the cracking at 90 degrees to the shear crack which is very noticeable to the right of mid span but also appears to be developing to the left of mid span. This cracking can be seen, to various degrees, looking at the historical photos in street view.

As an aside, knowing the design techniques for the area and generation of the bridge, it is highly probable that all the stirrups were installed vertically (as opposed to inclined).

Note:

  • There is no vertical load on this beam other than self weight
  • There appear to be no collision damage with the beam in the historical photos prior to the collision. A news report states that the beam has beam hit 2-3 times in the past year.
  • Based on seams in the concrete I would estimate this beam to span about 18.3 m.
  • cross-section dimensions of the rectangular beam are probably about 250-450 mm wide and about 1200 mm tall.

Question

Why is the concrete cracking in this pattern prior to the collision (3rd photo)?

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  • $\begingroup$ What we are looking at? is the first photo showing the beam/parapet wall falling sideway, or it is just a raised walkway? $\endgroup$
    – r13
    May 10, 2022 at 11:10
  • $\begingroup$ @r13 The first photo is the protection beam laying on the ground after the strike. Its the same beam shown in its position in the now 3rd photo which was take a year ago or so. $\endgroup$
    – Forward Ed
    May 10, 2022 at 11:53
  • $\begingroup$ I was talking with a fellow engineer this morning and we were speculating that this might be torsional cracking? Going back in street view history it appears that the bridge deck cantilever was over top of the girder with a bearing strip in between potentially. During the 2015 rehabilitation it appears the cantilever was cut back and no longer sat on top of the girder. We are speculating that eccentric loading from anchor bolts along the top edge into the spacer C-channel and then into the adjacent bridge deck. When the bridge flexes it induces torsion/twisting into protection beam. $\endgroup$
    – Forward Ed
    May 10, 2022 at 13:56
  • $\begingroup$ The green is a corrosion protection on steel rebar reinforcement. Why do you think the rebar was not prestressed ? $\endgroup$ May 10, 2022 at 15:48
  • $\begingroup$ @blacksmith37 Correct, hence the designation of C15M and the description in brackets. "If this was a pre-stressed girder I would have expected to see some pre-stressing strands in the broken section, but there does not appear to be any." $\endgroup$
    – Forward Ed
    May 10, 2022 at 16:21

2 Answers 2

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Below is my inspection. Note the cracks between the support points - from diagonal near support and gradually turns vertical towards the midspan.

enter image description here

Street View: The cracks in the circle forms the crisscross pattern, which could be the result of structural crack plus thermal crack.

enter image description here

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  • $\begingroup$ I was wondering about the cracking that occurred before the collision that you can see in street view. In street view, the cracks do no turn horizontally at mid span. $\endgroup$
    – Forward Ed
    May 10, 2022 at 13:38
  • $\begingroup$ On the street view (2nd photo), I see diagonal cracks on both ends and seemly forming a semi-circle near bottom at the midspan. The only odds I can see is the cracks tilting in both directions forming a grid at the right to the midspan. I didn't see horizontal crack though. $\endgroup$
    – r13
    May 10, 2022 at 13:49
  • $\begingroup$ Street view is the now 3rd photo. also has a link to google street view just underneath. $\endgroup$
    – Forward Ed
    May 10, 2022 at 13:57
  • $\begingroup$ While they may indeed be control joints (achitectural grooves), I thought they were more likely the seems left over from the form work which is normally 4'X8' sheets. Part of my thinking for this is the last panel on the right is shorter than all the others. If they were control joints, or architectural grooves, they would either be symmetric, or evenly space. If you look closely on the left end just right of the pier cap you can also see the opposite cracking pattern starting show as hairline cracks. $\endgroup$
    – Forward Ed
    May 10, 2022 at 16:30
  • $\begingroup$ It could be for both architectural or structural reasons, you can walk up and have a clear look. Form panel joint will be somewhat rough, control joint usually has filler material in between. Since the form panel likely is made of metal, the size is fixed, but the bridge length isn't, so you will see a smaller panel at the ends, similar to placing tiles in the bathroom. $\endgroup$
    – r13
    May 10, 2022 at 17:55
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Cracks appear where the stresses are perpendicular to normal forces when the concrete tension exceeds material strength. Clearly, the images show that the beam was not designed to carry load bearing capacity beyond self weight and this itself was incorrectly designed,probably to save on costs.

It may be that factors of safety are not design-included which would have resulted in ULS being the maximum requirement for the beam with snow loading and thermal variation causing the visible defects

All indicators point to a change in the neutral axis vertically towards the beam's topside. I expect that this changed the intensity of internal forces causing increased tensile stress in the reinforcing beyond expected design capacity.

The other oddity to me is the cracking at 90 degrees to the shear crack which is very noticeable to the right of mid span but also appears to be developing to the left of mid span.

Stress acts perpindicular to strain. Understanding which is which will tell you what the problem is and the root cause.

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