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RE: Sec 1815 UBC (slabs subject to expansive soils)

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There are four code sections that address this issue in the 97 UBC...Sections 1803, 1804.4, 1806.2 & 1815.1.  The application of each section depends somewhat on if you have a soils/geo report and: 1) if that report has determined an EI and 2) if there are specific or general recommendations re the mitigation of expansive soils and 3) if the design engineer believes he has the authority and responsibility to design an engineered foundation system that when compared to the min. recommendations may exceed the general min. recommendations of the soils engineer.
 
The way I look at it is:
 
    Sec. 1803:  "...expansion index >20...shall require special design consideration".   The EOR should design the foundation in accordance with sec. 1815 or 16 and this should exceed any recommendations from the soils report.
 
    Sec. 1804.4:  "When expansive soils are present, the building official may require special provisions ..."  Without a soils report or engineering, (say for conventional construction) the building official may require certain min. steel in the foundation and slab.  An example of this is in the LACO Code, Sec. 1804.4 which revised the UBC to include certain steel and foundation depth, and moisture saturation.
 
    Sec. 1806.2:  "Slab-on-grade ... located on expansive soils may be designed per 1815 or 1816 or such other engineering design based on geotechnical recommendations ..."  The use of 1815 requires the EOR to design "beam spacing", "beam design" and "slab reinforcing" and to compare this the the min. soils report recommendations.  "...or other engineering design ..." I believe may be a post tensioned slab as recommended by the soils engineer or caisson system as recommended by the soils engineer.  In either case the EOR should be responsible for this.
 
    Sec. 1815.1:  "... Use of this section...limited to three stories or less in which gravity loads are transmitted to the foundation by bearing walls".  I guess this doesn't apply to tilt-ups or loads transferred by column forces.
 
In any case I believe the EOR should be the one designing the foundation system using the recommendations of the soils engineer and the engineered design should meet or exceed the recommendations.  The EOR should not defer to and consider the recommendations as constituting an engineered design.  Hope this helps...
 
Steve Widmayer, PE
 
 
-----Original Message-----
From: LEDSTRUCTURES(--nospam--at)aol.com [mailto:LEDSTRUCTURES(--nospam--at)aol.com]
Sent: Tuesday, December 11, 2001 9:00 AM
To: seaint(--nospam--at)seaint.org
Subject: Sec 1815 UBC (slabs subject to expansive soils)

Regarding the design of slabs subject to expansive soils: Section 1815 of the UBC  requires a series of beams provided in conjuction with the slab to help resist  the expansion of the soil.  I have heard two different opinions about what this design calls for. #1 ) The beams are actually control joints that allow the slab to crack at predetermined locations. "or" #2) the beams are infact concrete beams that stiffen the slab similiar to the waffle design.  I realize the code is pretty explicite on an actual beam design. But I what to make sure I'm using proper procedure. Can anyone provide asome comments?

Thanks
LSI