Fix the pumps

Wednesday, August 25, 2010

The gates

This is part 2 of a multipart series on the lakefront gates in New Orleans. In the first part, I looked at the trench into which the gate segments, called "needles," sit when lowered. This post will look at the needles themselves.

Just as the gate structures have differences due to their differing designs, so do the needles. This is because the 17th Street gates were designed by a different firm than the Orleans and London gates. I'll get into the differences a little later, but first, the commonalities.

Framing: Vertical vs. Horizontal

The needles are very large vertically framed sections, each over 25 feet tall and about 10 feet wide. Here's one of them being placed at the Orleans Avenue site during construction of the structure in 2006 (via SCPR Flickr):

And here's the westernmost three needles at the London Avenue site this year:

The Corps own standards for these types of gates, a subset of vertical lift gates called "outlet gates," specifically say the framing should be horizontal, not vertical:
"As with navigation lock gates, vertical framing systems are not structurally efficient and require special framing to accommodate roller guides for hoisting operations. Hence, vertical framing is not recommended for new vertical lift gates, except for replacement in kind."
-USACE Engineering Manual 1110-2-2701, "Vertical Lift Gates"

From that Corps document, here's what they mean by horizontal framing:

The main continuous, load-bearing members of the framework are oriented horizontally.

Here's a detail from the drawing for the needle gates at 17th Street:

and here's a detail from the drawing for the needles at London Ave (the Orleans Ave ones are nearly identical):

As you can see, the main continuous steel members run vertically. Were these vertically framed gates more robustly designed, there would probably be continuous horizontal members at the top and bottom to which force would be transferred. However, these gates are simply transferring force along the vertical framework to wherever it comes in contact with the main structure, in this case the points here:

The force of the water at 17th St is transferred to the main structure and foundation at three points - two horizontal bearing beams directly connected to the structure and the trench.

And here's the London/Orleans design, where surge and wave forces are transferred to just two points, a horizontal bearing beam on the structure and the trench:

So the framing of the lakefront gates is apparently not in accordance with Corps engineering standards. That doesn't necessarily make it wrong - they might be quite sturdy. But it is worth noting.

The guide columns

When the gates travel up and down, they do so within vertical guide columns. Those guide columns are just wide flange beams attached to the main structure. They are supported by the main structure, but they do not act as anything but guides for the needles, because they stand unreinforced (photo via SCPR Flickr):

Since there's nothing backing them up, they are not meant to resist storm surge force. This is also in opposition to the Corps Vertical Lift Gate engineering manual, which calls for guide columns with concrete behind and around them. An example of such gates can be found just a couple of miles away from the 17th Street structure, at the south end of the 17th Street canal.

The New Orleans Sewerage and Water Board's (S and WB) Drainage Pumping Station #6 is the biggest in the area. A while back, before Katrina, so-called "fronting protection" was installed by the Corps to allow the pump station to operate during storm surge (a function now ironically meant to be provided by the Corps' structure). One can see the fronting protection in this picture taken from the cover of the Corps' Enviromental Assessment for their post-Katrina stormproofing projects on all the S and WB drainage pump stations:

Here's a detail of the DPS#6 sluice gates from a Corps public presentation given March 10, 2010:

Note the concrete around the guide rails, as well as the horizontal framing on the gate sections. These gates appear to have been designed more robustly, as well as in accordance with the Corps' own standards.

More than likely, such a scheme was not considered due to time restrictions during the design and construction of the gates in early 2006. So we get the situation we've got - vertically framed gates, unreinforced guide columns, and point concentration of the surge forces into a limited set of locations. The Corps' own consultants, Black and Veatch also tried to warn the Corps about the inadequacy of this design.

In 2006, B and V wrote their Alternatives Considerations Report. It was intended to evaluate various cost reduction schemes for the permanent pumping stations. One of those schemes was reuse of components from the Interim Closure Structures. So B and V took a look at the ICS's in some detail, with an eye to whether they would hold up long term. Here's their opinion of the guide beams:
"The structure side of the end supports consists of the flanges of vertical steel members. For many permanent installations, the gate guides are fabricated of concrete with metal embeds for bearing and sealing surfaces. Concrete is more durable for this purpose and would provide a longer term solution."

Next up are the seals. The entire concept of the gates is to keep water out of the outfall canals, so we would hope they are sealed. As it turns out - not so much.


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