Continuous Prestressed Post-tensioned Segmental Concrete Box Girders

Last week we inspected four ramps in Boston and Cambridge Mass near Leverett Circle and the Boston Sand and Gravel.

The ramps take traffic from the Tobin Bridge to I-93, Zakim BridgeLeonard P. Zakim Bunker Hill Memorial Bridge, Leverett Circle or Storrow Drive. The ramps consist of continuous prestressed post-tensioned segmental concrete box girders.

To inspect the exterior of the box girders an 80' man lift was used since most of the structures were no accessible with smaller lifts due to on going construction and a park. The exterior inspection consisted of visually inspecting the box girders, piers and measuring the bearings for movement.

The interior of the box girders were inspected with three man teams since they are considered a confined space. When inside the box girder the inspection team needs an air meter, rescue air tank and radio, luckily we have only had to carry these once down the interior since there is a hatch at both ends. The interior inspection consisted of visually inspecting (even though we were always within an arms reach)the box girders even though we were always within an arms reach, utilities and measuring the pintle slots for movement. 

Tunnel Overhead Inspection

The other week I was out in East Boston to inspect the Callahan Tunnel exhaust plenum as part of the Overhead inspection.

The Callahan Tunnel was constructed in 1960, 60 feet below Boston Harbor. The tunnel has an inside diameter of 28 ' and carries two travel lanes with an ADT of 20,000. We inspected the exhaust plenum from station 8+77.8 to 57+72.1 which comes out to be almost a mile each way hunched over.

The exhaust plenum carries exhaust from the tunnel out at the vent buildings located at each end of the tunnel. The ceiling panels are held up by hangers and supplementary hangers labeled L2, L1, M, R1, R2. The hangers which have gaps require a supplementary hanger that cannot be loose. Many of the hangers have  supplementary and very few had gaps without a  supplementary . The concrete liner had numerous hollow areas and a few areas of cracking with efflorescence.  

Moving around in the plenum is difficult and depending where in the tunnel the air can be come very still and hot. By the end of each day your clothes, mask and exposed skin are covered in soot. This is pretty common when inspecting an exhaust plenum though.

Natural Bridge in Virginia

Last week on vacation I took a road trip down to Kentucky and Tennessee. On the way back I stopped at the Natural Bridge in Virginia.

The natural arch spans 90 feet across Cedar Creek and is up to 215 feet over the creek floor and consists of a limestone strata. The bridge carries US highway 11 and when cross the bridge has fencing so you can't see the view from the top (believe me I tried).

When I was at the natural bridge my first thoughts were on how they inspect a land bridge and what type on monitoring they is performed on it. I thought it would be a simple google search but so far I haven't found any information on it.

SPRAT Level I Rope Access Training

Two weeks ago I attended SPRAT Level I rope access training in Chicago. Going into the class I thought it was be easy and at most be fighting off boredom while hearing about the dangers and procedure of rope access for hours before actually getting on the ropes.

It only took 60 minutes before I knew this class was unlike any I had ever taken before. After going over the fundamentals we strapped on our climbing harness and headed out to the "hazard zone".

Day 1:

First we were shown what equipment is used for ascending and descending, we also went over our back-up devices (fall protection). Next our instructors went over how to ascend and then switch over to descending. Then we were shown how to descend. It sounds very easy but seeing it for the first time looked and was more difficult than they made it look.

After practicing for a while we took a break and went over some of our equipment (helmet, harness and carabiner).

Next we were shown how to switch from one rope set up to another. First ascending one rope setup, switching to descending and finally ascending on the second rope setup. After reaching a stopping point or your area of work/inspection you descend from your first rope setup which makes you plum over your second anchorage. 

Day 2:

Our second day we started off learning how to do a long re-belay which is close to switching from one rope to another but accentually doing it one four rope setups. This will be used a lot for inspections over areas which can not have hanging ropes or do not have anchor points directly above your point of inspection.

Next we were shown how to pass a mid-line knot. Not much to explain here but like everything else it was a little tricky at first.

Next we took a break and learned more about our back up devices (Kong and ASAP) and our I'D (industrial descender).

After we went over deviations which is where your ropes are pulled away from the anchor point to avoid an object such as a cantilevered overhang.

With a nice lunch break we gained back some strength and learned how to perform a rescue with a person in descent. This was one of the more difficult processes with lots for steps. Our first attempts took a while but were all successful.

Day 3:

Today we started off with a long commute to class due to flash flooding in the Chicago area. After passing through roads with up to two feet of standing water we finally made it to class.

Once in class we reviewed our up coming test questions and practiced some of the techniques we had learned in previous days.

Next we were shown how to do edge negotiations, which is the process of ascending up and over an end or descending down from an end. For most of our inspections we will be using this multiple times and it uses up a lot of energy.

After lunch we started off with learning how to set up anchorages and set-ups.

Next we learned more about our handled ascender, pulley and sling and reviewed the information in each pieces manual. 

Day 4:

Practice makes perfect! Today we worked hard practicing everything we learned and perfecting anything we felt needed improvement. For me I practiced rescues, edge negotiations and long re-belays.

Day 5:

Test day.

Written 40 question multiple choice. (Slipped up and got one wrong)

Oral Test I had to explain everything there is to know about helmets (there is more than you think there is)

Finally we get on the ropes and show them everything we know.

Everyone in our class passed with flying colors.

If you're looking into taking a SPRAT rope access class I highly recommend Elevated Safety they have a great group of instructors who are friendly, knowledgeable and have work experience in multiple fields. Just in our class we had 5 bridge and transportation engineers, 2 rock fall guys and a historical façade restoration. 

Transit Steel Trapezoidal Box Girder

A few weeks ago I inspected a two span continuous steel trapezoidal box girder which carries the subway over Leveret Circle in Boston. The bridge consists of curved two box girders, a composite deck and reinforced concrete substructure. The viaduct was constructed as part of the Green Line Relocation Project from Haymarket Station to Science Park Station with an original design loading of Green Line Passenger Car.

Constructed in May 2005, The bridge spans 135 feet over gravel and 162.75 feet over Leveret Circle. The bridge has a maximum width of 30 feet out to out. 

The concrete composite deck has hairline cracks in the negative moment region.

A 45' lift truck was used to inspect the span over Leveret Circle and the span over gravel was inspected by ladder. Each girder's interior was accessed by hatches near ground level over the gravel.

When we tried to access the girders we had to cut the locks off with a grinder but one lock was already removed. When then heard and saw movement inside the girder. A homeless man had been living inside the girder. We notified our police details and they helped escort the tenant who left without incident. Inside the girders were filled with an accumulation of homeless peoples debris making for a difficult and unpleasant inspection.

Rules of the hole.....

Concrete Encased Steel Beams

A few weeks ago I did a routine inspection of a concrete encased steel beam bridge. The bridge is located in Tiverton RI. The five concrete encased beams span 35.1'. To access the underside of the bridge we used a small boat which halfway through the inspection sprung a leak. The remaining time was spent in waders, which happened to also have leaks. 

The beams exhibit spalling to the encasement along the undersides exposing the steel bottom flanges. The exposed steel bottom flanges have areas of section loss up to 44% of the original thickness. We were able to find the remaining thickness with a D-meter after spending some time cleaning the surface and making it smooth enough the get a reading.

The concrete deck has several spalls in each bay with exposed and severed rebar.

Just upstream from the bridge there is a spillway with a fish ladder. The damned off pond has large schools of fish and it was a crazy site since they were probably farmed fish and were not afraid of people.

Overhead Tunnel Inspection

The Tip O'Neill Tunnel was opened 10 years ago and carries the Central Artery below Boston's downtown area.  The overhead inspection consists of the exhaust plenums, roof slab, utilities and lighting.

The tunnel's ceiling is has fireproofing throughout the underside which makes it difficult to determine the condition. Areas of the ceiling fireproofing have been removed to access utilities and allow inspectors to view the roof slab. The exposed areas of the slab show localized hairline cracks.

The lighting along the top of the walls have been connected with metal clips which are prone to failing and now have six acetal zip ties. The majority of the metal clips are in place with six ties but a few of the lighting structures have loose clips or broken ties causing displacement and cracked lenses. 

To access the tunnel the we piggybacked with MassDOT as they cleaned the tunnel. It was pretty interesting being able to see how the tunnel was cleaned. First the safety walks were cleaned of debris using some sort of leaf blower. Secondly the walls were sprayed with some type of soap solution at a high pressure. Next they were scrubbed. After that the walls were scrubbed by a dozer with a brush attachment. After the brushing a water truck with attached power washer sprayed off the wall. Finally the lanes were cleaned using street sweepers. The entire time the exhaust fans were blowing to help clear out any dust. It's great to see them maintaining the structures since we don't always get to actually witness it.

Washington bridge collapse

Yesterday a deck truss in Washington state near Mount Vernon collapsed while carrying four-lane Interstate traffic over the Skagit River.

The new mentions a truck present at the time of the collapse and may have struck an overhead member. Typically you would not think of sway bracing or other lateral members when you think of fracture critical bridges like a truss. This collapse has me thinking of a ways that might improve future designs, ratings and inspections of bridges (trusses).

Could the force of a collision cause a gusset plate to fail at a diagonal or vertical member of a truss?

Should gusset plate ratings incorporate checks for these addition collision loads?

Should we design our lateral members to shear at a desired force to not cause a collapse?

As always for inspections we need to check the vertical clearance at the portals and any lateral members over the roadway and verify that signs are in place in advance and at the bridge.

I look forward to hearing more about this collapse (that sounds bad) so that we can improve future designs, inspections and ratings.