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Re: Basement Wall Cracking

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Is this wall, one of several walls of an enclosure? Do all the walls have nearly identical conditions of backfill? Is there any record of the history of construction? Are surface conditions outside of this wall, particularly drainage, any different compared to remaining walls? What reinforcement exists in the wall and what is the cover to rebar?

The crack pattern is similar to the yielding cracks on slab supported on four edges, when the slab is gradually loaded to failure.   In a horizontal slab, the crack parallel to the long edge would be closeto the center of the slab.  In basement slab, the horizontal crack will be shifted towards the bottom because bending moment is lower than the mid-height of the wall.  On the face of it, it seems to me that the wall has experienced larger horizontal forces than assumed in the design.  In some cases where walls are relatively more rigid to deflect, usage of active earth pressure coefficient would underestimate the forces.  Personally, I use "at rest" earth pressure coefficient when I design basement walls and other walls that have a solid slab on top of the walls.  One should also be cognizant of the probability of development of water table behind the wall, if surface drainage is not properly designed.


vicpeng wrote:

 We have a basement wall in a building that has been in service since 1977.  The problem is that it has cracked along a 55 ft section close to one corner. Wall clear height     8'-0"Wall thickness        8"Main 2.5mm crack at 5' above floor slab.Length of main crack    about 14'Crack width maximum 2.5mm Bifurcating cracks run off at about 30 degrees to contact floor and ceiling slab respectively about 20' away.  Crack width decreases from the 2.5mm to 0 over about 20' each end of main crack.  There are other secondary cracks but the main pattern is so much like the tributary area load distribution for a two-way slab. ____________________________________________   \                                        /     \   1mm                          /0.5mm       \                               /         \                           /           \         2.5mm    /                          1mm             \-------------------/- - - - - - - - - - - - - - -- - - - - - -          /                      \       \       / 1mm                   \        \     /                                \         \   /                                    \          \ /                                         \          \________________________________\________  The scales are grossly out of proportion but the pattern is as shown.  5'-0" dimension on bottom half. My numbers suggest that if the wall rebar was in the centre of the wall instead of at the inside face then I can expect about 1.5mm crack width?  This, from elementary geometry means an out of plane deformation of about 7mm, which is not much different from tolerances for waviness etc in concrete.  Would appreciate any comments or ideas on what could be causing this in an otherwise sound wall for the remaining perimeter. 
Thor Tandy  P.Eng  MCSCE
Victoria BC