Masonry Column Lateral Design
Masonry Column Lateral Design
(OP)
I am having trouble in the design of a cantilevered masonry column system. These systems seem to be pretty typical for support of entry canopies or even larger canopy systems (I am picturing bank drive throughs). The basic system is a light roof deck (metal deck or plywood) with light framing (wood i-joists, cold formed studs or light channels). This type of system probably does not provide enough resistance to allow for a pinned column design so a canitlevered masonry column is assumed. The question is in the lateral seismic force calculations what R value do you use from the ASCE? There doesn't seem to be a value that corresponds for cantilevered masonry columns. I tried a small value (R=1.25) but the design of the footings seems unreasonable. The ASCE forces you to use the overstrength factor for footing design for cantilever column systems and this makes the overall overtuning force very large. Has anyone designed this type of system?






RE: Masonry Column Lateral Design
Other options if with seismic it seems unreasonable are to use HSS columns wrapped in CMU (either as cantilever-flagpoles or part of a moment frame), OR a cantilevered concrete column wrapped in CMU (4" CMU cladding looks like 8" for the most part)... I know these are not as economical.
RE: Masonry Column Lateral Design
You can use a R of 1.5. In addition, you do not have to use overstrength for OT check or ASD soil bearing with overstrength. The footing and other connections that provide resistance should be designed for overstrength.
RE: Masonry Column Lateral Design
Sandman: Thanks for that code reference. I am not sure that section allows you to use 1.5 it just says you can use the piers as lateral resistance if you use a R value of max. 1.5. Also I am not reading that the overstrength factor only applies to the footing and other connections. I am referencing ASCE 7-05 section 12.2.5.2 and it states "Foundation and other elements used to provide overturning resistance at the base of cantilever column elements shall have the strength to resist the load combinations with over strength factor of Section 12.2.3.2." I don't really understand the use of designing the reinforced concrete footing to resist the moment with overstrength factor but not designing the footing to resist overturning or soil bearing for overstrength. The amplified force will never get to the footing because the system will fail due to soil bearing or overturning before it gets to the footing.
RE: Masonry Column Lateral Design
Other suggestions and then I will leave you alone in seismic land:
You could see if the architect could live with another 8-16" wider of a column after you explain why, that can do a lot for a small CMU column.
Anything bigger than 16" x 16" and you can also use vertical rebar in the center void and solid grout everything.
RE: Masonry Column Lateral Design
RE: Masonry Column Lateral Design
There are a few reasons why overstrength is not used for soil bearing or overturning checks. Overstrength is used to help ensure that elements stay within the yield zone; it is not an increase on the seismic load to the system or line, part of the reason that overstrength is in the load combination equations not in Cs equations. So your overturning should be based on load combinations without overstrength along with soil bearing.