Wednesday, November 16, 2011

RE: Unreinforced Masonry Wall from 1965

Back in 1965, and depending on the project location, walls often were NOT designed for lateral loads.  In many areas of the country lateral loads were not designed for many buildings.  In the relative early days, a lot of architects and engineers would use the empirical method of "designing" masonry.  Many architects and engineers would count the brick as part of the wall and would use the empirical ratios in the UBC.  They would also count the air space between the 8" wall 4" brick facing.  Therefore an 8" wall with 4" brick and a 2" air space was treated as a 14" wall. 
The issue came to a head in the 1970's when we started to insulate walls.  This was a departure from the integral masonry and the building components were separated for thermal reasons.  Historically, plaster knitted a lot of components together whether structurally intentional or not. 


Per UBC 1970:

2.  Empirical requirements.  A.  Ratio of height or length to thickness. The ratio of unsupported height to thickness or the ratio of unsupported length to thickness (one or the other

but not both) for solid masonry walls or bearing partitions shall not exceed 20, and shall not exceed 18 for walls of hollow masonry or cavity walls. In computing the ratio for cavity

walls, the value for thickness shall be the sum of the nominal thicknesses of the inner and outer widths of the masonry. In walls composed of different kinds or classes of units or mortars, the ratio of height or length to thickness shall not exceed that allowed for the weakest of the combination of units and mortars of which the member is composed.


Table I in the 1970 UBC allowed a URM exterior unreinforced wall to have a h/t ratio of 20.  If you had any reinforcing you could go to 30.  If the wall was interior, the ratios were 36 and 48. 

I lost a client many years ago because I could not get the numbers to work.  The client found an engineer who said the walls were OK using the overall wall thickness of a cavity wall and allowed for the empirical method.  A wind storm blew the walls down some years ago.  No one was hurt. 
One approach is to do a test with an air bag.  Put the wind force on the wall and test it.  I doubt it will work, but it might.  Anchorage at the top is another major issue.  It is not likely to work. 
To rehab the wall, you can strong back the wall with structural steel at intervals that allow the CMU to span horizontally.  And use post installed anchors to resist the suction loads. 

Regards, Harold Sprague

On Mon, Nov 14, 2011 at 2:05 PM, Daniel Popp <> wrote:

We have a renovation project in a low seismic area (SDC B). The 27-foot-tall exterior walls are unreinforced ungrouted (hollow) 8" concrete masonry interwoven with 4" brick face shells (12" nominal thickness). The occupancy is changing from Category 2 to Category 3 (>300 people in one space), so we are obligated to check the existing walls. However, due to the height, I cannot make them work using the "unreinforced masonry" provisions in MSJC Section 2.2.

Does anyone have any idea how the original engineer may have made these walls work in 1965? It strikes me that the code provisions for unreinforced masonry have probably not changed much in 50 years (seismic regions excepted). The owner is very sensitive to budget (who isn't these days?), so I'd like to find a way around an costly retrofit.

Thanks for your input,
Dan Popp, S.E.

David Topete, SE