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RE: Basement wall restrained by wood floor[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Basement wall restrained by wood floor
- From: "Cain, William" <bcain(--nospam--at)ebmud.com>
- Date: Thu, 26 Aug 1999 08:04:00 -0700
I have investigated a number of failures of basement walls. The predominate cause has usually been related to the type of soil retained, the detailing of the wall drainage and the backfill conditions, not the structural detailing (although I have seen a FEW failures where the top of wall connection was poorly detailed. I have yet to see one with a diaphragm deflection or failure problem. Doesn't mean it can't happen, I just haven't seen one.). I have seen both cantilevered designed walls and walls supported by the 1st floor diaphragm fail when subjected to the loads imposed by clayey retained soils. If a soil has low expansion potential, the usually assumed design loads appear to be very conservative (i.e., the wall probably never sees the design loads used). However, with a material that has a high expansion characteristic, even with high assumed design loads, poor detailing of the drainage and backfill conditions can lead to a progressive movement of the wall as the soil is alternately wet and dry with seasonal moisture changes. When the wall moves out due to expansion, it doesn't move all the way back when it dries. Over the years, the effect is a net movement of the wall in the direction it is being pushed. When expansion potential is very high, consideration should be made to removal of the entire active wedge behind the wall. I have seen walls with conventional drainage (approx. 1' min width of rock behind the wall for most of the height, wrapped in geotextile fabric with a perforated drain line leading to a gravity outlet) and normal assumed design loads that have rotated beyond repair even though they were designed by the usual means because of expansive conditions. I guess what I'm trying to say is that we need to listen carefully to our geotechnical colleagues when we encounter expansive soils to be retained by a wall that is part of a structure (cantilevered or pinned-pinned). For competent, free draining soils, the problems are minimal if good drainage practices are followed, regardless which way the wall is designed. One key detail, that I have found helpful in mitigating moisture changes, that is often omitted from drainage details is to cap the wall drain with about a foot of impermeable soil (often the native soil does just fine for this) to prevent the wall drain from collecting surface waters. If there is a sidewalk or other paved area adjacent to the building, this can serve the purpose as well. Rather than blanket statements that we shouldn't design walls as pinned-pinned or we shouldn't design walls as cantilevers, we need to understand the particular site involved and deal with the issues presented by that site. Both types of walls have their place if the important factors are considered in the design and the detailing AND CONSTRUCTION is carefully executed. Another detail that I've seen inappropriately applied is to place a drainage panel, such as Mirafi, etc., against the back of the wall in lieu of a conventional drain with clayey backfill placed and compacted against it. The natural permeability of the soil in place is usually greater than the permeability of the same clayey soil after it has been compacted. When constructed in this manner, the drainage panel has been effectively cut off from the ground water it is supposed to be draining and might as well not be installed. The same principle applies to a conventional rock drain but is usually not a problem because most contractors I have encountered will simply use the drain rock as backfill. Thus there is no intervening impermeable material between the drain and the natural, in-place soil. Regards, Bill Cain, SE Oakland, CA -----Original Message----- From: James_F_Fulton(--nospam--at)RohmHaas.Com [SMTP:James_F_Fulton(--nospam--at)RohmHaas.Com] Sent: Thursday, August 26, 1999 5:15 AM To: seaint(--nospam--at)seaint.org Subject: Basement wall restrained by wood floor In 99.9% of the residential construction I've seen, the basement wall is supported by a strip footing, usually 20" wide or so. This type of wall relies on the support at the top provided by the first floor. In one direction, the direction of the floor joists, not only the floor deck but also the joists act to provide the restraint. Pretty stiff and pretty strong in this direction. In the other direction, parallel to the joists, a blocking or bridging system has to be used to transfer the transverse shear at the bottom of the rim joists, that bears on the sill plate that's anchored bolted into the top of the wall, to the level of the floor deck. In this case, the floor deck is indeed providing most of the strength and stiffness it would seem. But I don't see why it cannot be relied upon do its job. To design the wall as a cantilever against backfill loads has expensive implications for residential construction now that a structural footing is needed.
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