Grade beam of structure adjacent to existing structure

[conradlovejoy]

I am attempting to design a safe room for a school. The safe room is a new structure that abuts the existing structure at one corner. Due to the building "overlap" a drilled pier cannot be installed at the corner of the safe room that abuts the existing structure. Imagine a rectangular building and the southeast corner is where the new and existing buildings meet and where the would-be southeast pier had to be moved north along the east wall to provide clearance from the existing building. For analysis, I have designed the east grade beam to have a cantilevered end since the would-be southeast grade beam center line is now 3 feet from the southeast corner. The south grade beam, having no pier to support the easternmost end, is now supported by the cantilevered end of the east grade beam. The roof framing spans north-south so the south grade beam carries a much larger load than the east-west grade beams. Therefor, the assumed concentrated load that the east beam must support at its cantilevered end is rather large (about 53 kips DL and 22 kips LL). The east grade beam cannot handle such a large concentrated force at that small cantilevered end. I am struggling to come up with a solution that doesn't involve changing the foundation type. I have thought about adding a helical pier at that corner but that would be another added cost to an already exorbitantly expensive structure. Any advice would be appreciated.

 

[JAE]

Can't you deepen the grade beams as required for the load? And can't you use twin piers off the cantilever support with a crossing pile cap to catch the main cantilevered grade beam?



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[KootK]

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

JAE's first option would also be my first choice. 3' isn't much of a cantilever and could likely be made to work using strut and tie analysis. Rationally, if the supported beam works in shear, the cantilever beam should work too as it is essentially just a shear transfer element with no meaningful flexure. The important bit will be the detailing of the rebar. You'll want some quickly developed top steel and a gaggle of hanger stirrups behind the supported beam.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

This would be my last choice but you could embed steel beams into the two corner grade beams. As I said above, the cantilever is really just about shear transfer. and you could definitely get that done in steel if you had to.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[conradlovejoy]

Thank you both for your ideas. In response, I understand the need for deepening the grade beam and adding more reinforcing steel but my initial attempts to do so proved to be of little help due to the large concentrated load. The grade beam through the remainder of the structure is only 30" deep so I wanted to avoid having a step down and lowering of the top of piers that was as drastic as it was seeming it would need to be from initial analysis. I was looking for a more unique solution and these ideas definitely fit the mold. Thank you.