With regard to Carina’s comment; I assume that the R of .25 is an accepted average for soil. I think that the number is about right, on average. Of course R varies tremendously with moisture content and is not a stable number with soil. It will be nil after a soaking rain. However, the effects of thermal capacitance and latent heat transfers produce effects that behave a lot like insulation under specific circumstances.
I am attaching some scans of marked up plans (based on 10-22-09 revision). As soon as possible we would like to receive current CADD drawings that pertain to the roof (plans, details and sections). We will build our details and sections off of these.
Scan #1- First option for lines of anchors. The purple line represents the south side of the exposed beam. The red lines represent attachment to the medial walls. I understand from our conversation that attachments to the walls may not be necessary. However, I am showing the option nonetheless.
Scan #2- Second option for lines of anchors. In this case the anchors would be set into the slab. As we discussed, none of us like all these penetrations through the waterproofing. However, if this is the only option, then we will have to rely on Dave Polk to come up with a reliable way to seal them. It seems to me that these would have to be stainless steel if they are set in the slab. However, the group may have another idea.
In both of the above cases, I recommend that the anchors be 18 inches apart (east to west alignment). Cables will be attached to the anchors which will either support a cellular confinement web or a high-strength geogrid. With 12 inches of wet soil on a 20 foot long slope with pitch of 1:1, I estimate the force on each anchor as being about 1,800 pounds , with a factor of safety of 1.5. However, most of the slope lengths will be shorter, averaging 10 feet, and ranging down to zero. The force on an anchor supporting a 10 foot long slope would be about 1,200 pounds. It would be simpler to have all anchors the same, but it may be cheaper to tailor the anchor to the load requirement.
[there is another interesting possibility. A high strength geogrid could be laid out on top of the completed waterproofed slab. Each strip of geogrid would extend from the northern-most wall all the way to the bottom of the slope (I think about 50 feet). The medial walls could be poured on top of the geogrid. Now… you can see that each MD stand of the geogrid would be weighted on the flat surface by the overburden. I haven’t done the math, but I think that this stabilizing/resisting force would be sufficient to support the downhill forces on the slope. No anchors needed. This could be a simple and inexpensive idea if the architect and engineer don’t think I am crazy.]
Scan #3- shows areas where there will be no sheet drain
Scan #4- my PRELIMINARY assessment for where scuppers will be required. I am assuming that there will be a row of area roof drains, as shown on my mark-up, and the channel drain --- nothing more. The scuppers which are shown as red boxes would be essential. There are six. It would be a good idea to increase the number of scuppers. The orange circles represent tentative locations for supplemental scuppers. The scuppers represent the only places where flow can pass across a medial wall. The arrangement of scuppers, as I have shown them, would avoid one of the conditions we briefly discussed today—namely runon from uphill areas into the area subtended by the lower path. Rather water would flow through scuppers 5 and 6 and then along inside the narrow planter all the way to the southwest corner. I do not think that the channel drain will be sufficient to serve this flow. An area drain should be located immediately downgradient of scupper #1.
Note that there will be a very large rate of flow generated by water accumulating against the medial wall that I have labeled ‘W.’ This flow will be ‘piling’ up under the path. Therefore a sheet drain (e.g., eco-drain-S 900) will not be sufficient to convey this flow to the west. Perforated conduits must be installed under the pavement to carry this flow. To allow enough ‘head-room’ for this conduit, the thickness of the path profile may have to be greater--- perhaps 8 inches, instead of the minimum of 5.5 inches we discussed earlier.
Note also that there is a great concentration of flow at a single point which happens to be at the same place that scupper 1 outlets. I am certain that a large grate, not a channel drain will be needed hear. Alternatively, area drains could be installed to cut down the volume of flow.
I don’t understand the pattern of flow in the area circled in pink. Earlier plans showed the flow direction north toward drains on the south side of the ramp. I don’ t think that these exist anymore.
What is the design storm that is being used to size runoff conveyances elsewhere on the project?
In the absence of a designated storm, I think a good starting point would be to think of each scupper as 4 inches high and 24 inches wide. The inverts would be the surface of the slab. At each of the six primary scuppers a vertical shaft would provided to allow inspection (and cleaning if necessary). The shaft would have a lockable lid.
Scan #5- This is rough hand sketch of conditions at the section labeled alpha on the plan.
I assume that these sketches will spur some more discussion. Next step will be prepare proper construction details for the various conditions, using the architectural CADDs as a base.
No comments:
Post a Comment