PRINCIPALES FUNCIONARIOS

The performance of residential construction is typically assessed based on one of the following criteria 1) floor levelness (American Concrete Institute, ACI, 117-90), 2) angular distortion and 3) magnitude and extend of distress in vertical and horizontal construction, which is the most important criteria by which to judge the significance of structure distress due to soil movement. Various bodies developed standards for acceptable foundation movement and/or minimum construction quality criteria. Some of the more important ones are: 1) the American Concrete Institute (ACI), 2) the Building Research Advisory Board (BRAB), 3) the Department of Housing and Urban Development and the Federal Housing Administration (HUD/FHA), 4) the Post Tensioning Institute (PTI), and 5) the Uniform Building Code (UBC). Most of these standards use language quantifying the floor levelness and the structure stiffness in terms of angular distortion to preserve the structure severability within designed structure lifetime. The angular distortion is defined as the maximum floor differential divided by the horizontal distance between the measurements. Most design governing bodies limit the angular distortion at 1/240 as summarized in Table 2.1. The floor levelness is defined as the slab deviation from horizontal over the entire area of the slab and expressed with FF values (of FL for local levelness over the

area of 10’) and calculated as 12.5/∆z, where ∆z is the floor differential measured in inches (ACI 117-90); see Section 3.6 for more details. As per the ACI standard, the FL is limited at 10 while

FF at 15. Local institutions, such as the Arizona Registrar of Contractors, AROC, specify the local standard of care through publications quantifying the maximum acceptable distortions and distress of horizontal and vertical components of the structure.

Table 2.1. Angular Distortion Criteria Based on Design Manuals (summarized in Advanced Foundation Repair, 2007).

Construction Type

Governing Body ACI 318-89

(1992), BRAB, Report No. 33 (NAC, 1968) FHA 4900.1 (1982) UBC Section 2307 (1988) AROC (1997)

Wood 1/240 1/240 1/240 1/240 1/580

Masonry 1/240 1/300 1/240 1/240

2.6.1 As-built Floor Deviation from Horizontal

Extensive research has been inspired by the assumption made in litigations that newly constructed slab-on-grade deviation from the horizontal is negligible. As a consequence, the obtained manometer results are evaluated as the net post-construction movement causing structure distress. Table 2.2 presents summary of literature review of slab deviation from the horizontal for newly constructed conventional and PT slabs-on-grade. It illustrates that the initial slab deviations might be as large as 2.2”, but more commonly are about 0.5”. For newly constructed slabs the average angular distortion was found to be about 1/340. Based on the research done by Noorany et al. (2005) the local levelness FL numbers of the newly constructed

slabs in the study ranged from 7 to 175, with a mean value of 43. These results indicate that small percentage of newly constructed foundation systems do not comply with the acceptable slab distortions used in the identification of expansive soil problems. More commonly, however, the in-compliance deviation leads to overestimate of actual slab movement and potentially misdiagnosis of structure distress as expansive soil related.

Table 2.2. Newly constructed slab deviation from horizontal and angular distortion.

Ref. Location # of

Slabs

Slab deformation [in] Angular dist. min.-max. ave. min.-max. ave.

Koenig, 1991 San Antonio, TX 54 0.125-1.0 0.54

Marsh et al., 1999 S. CA 6 0.6-1.0 0.75

Walsh, et al., 2001 Phoenix, AZ 89 0.25-1.18 0.53 1/857-1/101 1/334

Noorany et al., 2005 CA 971 0.2-2.2 0.53 1/1000 - 1/71 1/346

2.6.2 Post-Construction Slab Distortion

Three studies of post-construction slab distortions correlated to the cosmetic and structural structure distress were found. The results are presented in terms of the net angular distortion and net relative deflection without the consideration for the initial state of the slab. Skempton and MacDonald (1956) performed a long term study on numerous residential structures to correlate the differential settlement of a foundation to building distress. The study focused on pier foundations, not slab-on-grade. In spite of this, these results are frequently cited in the literature concerning threshold criteria for cosmetic damage in wood frame houses supported on slab-on-ground foundations with an argument that the interior partitions in a pier- supported building are subjected to the same distress as interior walls supported by a slab-on- ground foundation. Based on the Skempton and MacDonald study (1956) the cosmetic damage occurs at angular distortion of 1/300 and maximum slab relative deflection of 1.75”.

The study of 34 slab-on-grade residences in San Diego California by Day (1990) concluded that cosmetic damage relates to 1.15” slab differential and 1/300 angular distortion. Structural damage was observed to correspond to 3.5” slab deformation and 1/100 angular distortion.

The study by Marsh and Thoney (1999) consisted of 12-year long monitoring program of 400 slab-on-grade, wood-frame residences. Different threshold criteria were developed for heave and different for settlement where distress due to heave was found to occur at smaller angular distortions. The onset of cosmetic damage was correlated with about 1.1” floor relative

deflection and 1/330 angular distortion. Structural damage was correlated with relative deflection larger than 4” and angular distortion of 1/90.

The significance of these research findings is two-fold. 1) Cosmetic damage to residential structures occurs at about 1/300 angular distortion, a value smaller then the design requirements for floor levelness of 1/240. 2) The angular distortion of newly constructed slabs is on average 1/340, a value almost equal to the value assigned to correlate with cosmetic damage. Additionally, the angular distortion of newly constructed slabs can take values as small as 1/71, a value correlated with structural damage The obvious question here is, what is the magnitude of movement that contributed to the distress? The identification of soil movement related structure distress appears to be bias and incomplete when limited to relative deflection and angular distortion. The bias is caused by unknown initial conditions. There is a need for a comprehensive protocol for residential construction distress identification due to expansive soil movement.