There was a discussion in Eng-Tips recently about supporting a 30’ height of unbalanced earth pressure for a proposed building to be built into a hillside. A couple responders mentioned the use of tieback anchors to keep the unbalanced earth pressure from loading the building frame. I mentioned that it is preferred to not have a rigid connection between the building and the anchored wall. This Eng-Tips (ET) thread has found its way to SimpliEngineering (SE). The following may be of interest to anyone following this topic in ET or SE.
Experience has shown that having a rigid connection can cause damage to the building and/or wall because the building and wall will have differing reactions and movements from the unbalanced earth load. As a reference, I post the attached PDF of an excerpt from a 1984 meeting reprint written by Schnabel Foundation Company, now renamed as Schnabel Geostructural Design & Construction. It briefly describes the problem of rigidly attaching a building to an anchored retaining wall. This is an old document but it is still applicable today. Schnabel has had concern for this problem from the late 1970’s and I have had to consider this problem in several of my wall designs since then.
We really need to know more about the geometry of the building in question. Here is what we do know.
Roof level is about equal to high grade level.
Length of building normal to retaining wall is 80’.
Length of retaining wall is about 30’.
The retaining wall can have a shear wall at each end which may or may not be adequate to resist earth pressure.
The property uphill from the building is owned by the same owner as the building which means that tiebacks are a possibility.
The building can be designed to accommodate pressure from the earth. This would necessitate a rigid connection between wall and building, in fact the wall would be part of the building. Structural details would need to be determined if this option were to proceed. There is no reason why the retaining wall could not incorporate tiebacks to reduce pressure on the structure if this was deemed advisable.
If the retaining wall is separate from the structure, it means building two walls instead of one. And between the two walls would be fill, probably granular fill. If the retaining wall moves toward the building, the fill would be compressed, applying pressure to the structure.
Before deciding on a course of action, a floor plan is needed and architectural requirements need to be considered, none of which are currently available.
The tiedback retaining wall should be independent of the building but close enough that no backfill is required between the building and the tiedback wall. In fact, there would not be a “building wall” in front of the tiedback wall. The framing for the building and the floor and roof slabs abut or sit on the tiedback wall’s permanent concrete facing with expansion joints or sliding joints which will allow independent and differential movements for the wall and building. The unbalanced lateral earth pressure and surcharge pressure should not get applied to the building framing or slabs.
See PDF details from a recent project. TBW .pdf (1.4 MB)
I’m wondering about the approximate magnitude of movement to be expected in the tieback wall. I suppose that would depend on many variables, but what would be a ballpark figure?
Deflection depends on the actual anchored wall design. However, the soldier beam deflection is usually finished well before you ever install the finished permanent wall facing. Soldier beams are often designed for a deflection of 1 inch or less. Once the permanent facing is attached to the soldier beam wall, there should be little or no additional deflection. There probably is a greater chance of the building moving more than the soldier beam wall.
