Masonry Column Lateral Design

[bgsmith]

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?

 

[a2mfk]

I have used this type of system you describe plenty of times, unfortunately I am no help because this is in wind-only country...

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.

 

[sandman21]

ACI 530-05 section 3.1.3.1 or ACI 530-08 section 1.17.4.3.2.4

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.

 

[bgsmith]

a2mfk: Using a HSS column wrapped with CMU was an alternate I thought of. However due to the added cost and the inefficient use of not being able to use the existing masonry columns to resist lateral I have been looking for an alternate solution.

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.

 

[a2mfk]

ahhhhh- EXISTING COLUMNS? Or you mean they will be there (new construction), so why not use them? I agree, I have just done a lot of canopies where they had CMU wrapped around steel columns, because the CMU transitioned into brick or EIFS or something else that was cladding and of no use...

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.

 

[bgsmith]

Sorry if I didn't state it clearly but the primary design issue is the design of the footing. The masonry column is of sufficient size to resist lateral but the footing fails in overturning and soil bearing. For a simple example the column is 12 feet tall and has 1040 s.f. of tributary area to column with 18 psf roof dead weight. The Sds value for the site is 0.70. If you use R = 1.5 then Cs = 0.70/1.5 = 0.47. The total lateral load would be 0.47 * 1040 * 18 = 8,800 lb. The resulting moment at the base is 8,800 lb * 12 ft = 105.6 k-ft. Then if the overstrength factor is required that increases to 211.2 k-ft. Running a quick calc that makes the footing around 18 feet square for just one column.

 

[sandman21]

The code sections states for piers and columns.

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.