Minimum width of masonry for supporting new opening?

[Redacted]

Hi there,

I am working on a project, where the architect would like to create a new door opening for a restaurant between two existing windows, see below :

The existing building is built from CMU; it is two stories.

The door is quite wide and the cut out would only leave ~10" of masonry standing between the new door and the existing window. I am assuming the existing window lintels have a bearing seat of 4" to 6". My question is, can I design a lintel for the door alone, assuming the existing 10" nib would be a suitable bearing support? Is there guidance on what the minimum thickness for an unreinforced CMU bearing support should be? Or alternatively, should I demolish the existing window lintels and design a large beam spanning ~20', which would bear on the thicker sound wall further away (the beam would act as the lintel for both windows and the door and not be reliant on the 10" masonry nibs see below).

I guess there is also the potential to add in a column but there is a basement level and the column would most likely need to extend all the way down, so not sure if that's feasible.

 

[Liam Nesson]

I haven't alot of masonry experience so you can take what i say with a pinch of salt.

I am assuming no seismic as this is unreinforced masonry. I would be concerned about how those piers between the window and doors would handle the wind load. Essentially all wind on doors and 50% on the windows will be transferred through the piers. I wouldnt be surprised if you had a continous beam along the head of window and doors with concrete piers that span between floor above to flr below.

As for minimum bearing lengths, i didnt see any in canadian code. Minimum col dim is 200 excluding seismic.

 

[XR250]

I would replace the 10" wide masonry with HSS sections

 

[jayrod12]

I think Liam Nesson nailed a very pertinent point. You need something to deal with the wind loads. I doubt that an unreinforced masonry pier, which really in the end is only a single core, has the capacity for all that vertical load, plus the wind load.

 

[Redacted]

Hi all thanks for the responses so far.

@Liam Nesson you raise a very good point. Yes there is no seismic in my location but there is definitely wind. Typical masonry construction here includes a bond beam at the top of all exterior masonry walls to tie the walls together (which is most likely present in this building as it's a local building code requirement). The architect did offer to reduce the door size to 6' instead of 7', which would give an additional 6" (16" total pier)to the piers but I'm still not sure if that would be enough. Is there any way to stiffen the piers without putting in a new column all the way down to the basement floor level? Your last point "I wouldnt be surprised if you had a continous beam along the head of window and doors with concrete piers that span between floor above to flr below." is essentially what I was proposing to do but was wondering if there was a way to tie the new column into the existing foundation/basement wall without going through the process of knocking out the basement wall and constructing new foundations for the new columns etc.

@XR250, how would you go about tying the HSS section to the masonry foundation wall below and lintel above? Main concern with an HSS section would be the susceptibility to corrosion. The building is in a highly corrosive atmospheric environment.

@jayrod12 I agree

 

[Liam Nesson]

I am assuming this section of wall is above grade. Then you would run the column from grade elevation to flr above. For wind the lateral forces are spread to diaphragms above and below. The vertical loads would then bear onto your basement wall below. I have no experience with basement walls, but i would not want to touch it, keeping it water tight could be painful. The diaphragms and basement wall can already handle the wind and vertical loads, as these have not increased (at least i assume that) however you need to check stress concentrations due to changing the load path. You have some options to spread the vertical loads at the window cill level using a horizonal concrete beam, thereby spreading the vertical load.

 

[JLNJ]

16” seems like plenty of width to create a reinforced masonry pier to carry the wind loads.

 

[dik]

With the large tributary openings on either side, it may be more problematiic.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JLNJ]

The tributary areas for the openings either side of the doors certainly must be accounted for, but that's why they pay us the big bucks.

You can offset the rebar and go with two layers in the little pier. I know this makes for some careful bar placement, but you might be able to push the effective depth from 3 13/16" to 5". If push really comes to shove, you might be able to consider some fixity at the base of the wall to suck up some of the wind load at the window sill. Fixing the base of the pier might be possible as well. Maybe raise the f'm for the masonry to 2500 psi.

 

[Redacted]

@Liam Neeson Thanks that is helpful. Your assumption is correct there will be no additional vertical/wind loads. I guess the main question for this approach would be the method of tying this new column into the bond beam at the ceiling height (which I assume is what you mean by run the column from grade to the floor above?). This method would also require splitting the wall from grade to ceiling into 3 distinct sections to construct the columns. If I run the column from grade, how would I get fixity?

@Dik I'm not fully following/understanding, would you be able to clarify? I would have thought that if the piers were thicker it would be a stiffer element and therefore have a higher resistance?

@JLNJ Typical CMU block is 16" long so two vertical bars can get in quite easily @ 8" c/c. However, if the pier is not fixed at the bottom, would the bars be doing anything? I think my main confusion is fixity at the bottom. If the pier was to be designed as reinforced masonry, wouldn't it need to act as a column and be able to take the wind load moment and therefore need to have a fixed bottom? There is a CMU foundation wall that goes down to the basement level, which is below this door. The foundation wall would be a filled CMU wall. To give this pier some fixity, how would I tie into this to transfer the moment?

Or are you saying that it doesn't need the bottom fixity to work?

If there is 16" on either side of door, I don't think I would need to do the large beam as shown in the OP, as this would be fine for a bearing support. Strictly from a vertical load point of view, I can just put the new lintel above the 6' door. However, the wind load is perhaps still an issue that needs to get resolved.

Any help would be appreciated.

 

[Liam Nesson]

Have a chat with a hilti rep, maybe you could use something like hit-hy 200 hybrid for masonry to transfer the shear forces. Make sure you dont go through bond beam reinforcing, injecting upside down could be fun.

A neuclear option might be to break masonary and grout from bond beam reinforcement and lap in some L bars. Then repour the section of bond beam with new column. This prob is significantly more work.