Enclosing a Car Porch with CMU walls

[driquer1274]

Hi,

I was looking for some advice on enclosing an existing residential car port sharing 2 existing CMU walls with the single family dwelling. The house has a gable roof that extends past the exterior wall and over the car port and is supported by 2 CMU columns on a corner and side of the carport.


My plan is to enclose the 2 open sides by constructing CMU walls between the existing columns and walls of the home. I would like to design the walls as non-load bearing walls with out of plane lateral loads (for wind) to avoid connecting the new walls to the existing MWFRS and avoid having to consider footings for the new walls.

Is this the best way to go about this? If so, is there any special consideration needed to ensure the new CMU walls are not connected to the existing columns, walls, and beams to ensure the walls act as separate structural members?


I attached a floor plan and elevation. Any advice is much appreciated.

Thanks

 

[JAE]

Just to make sure we understand - you want the new walls to not be load-bearing (in terms of vertical load) but would be tied into the existing structure for lateral wind resistance?
Or are you wanting the walls to be totally free from the existing framing both vertically and horizontally?

If the former (no vertical load on walls but lateral bracing from the house)
1. Simply provide a slip connection along the top of the masonry walls - this could be via a series of steel angles will vertical slotted holes and bolts into the top bond beam of the wall.
2. Even without vertical load bearing capacity, the walls will still need proper foundations - minimum per code and below frost line if applicable.
3. The vertical joints between columns and walls can be simple clean expansion joints with sealant or flashing.

If the latter (no vertical load on walls and no lateral bracing from the house).
1. The walls then would have to cantilever off of footings.
2. Expansion joint on sides and top of walls.
3. The wall footings will have to have some width to them and be designed much like a cantilever retaining wall - only instead of soil pressure you have wind/seismic.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Ron247]

driquer,
Are you turning the open carport into an enclosed garage or an enclosed habitable room? The final use would make a big difference to me as to how I approached this. I am not sure about your local code requirements but carports converted to habitable rooms in my area tend to have future problems when selling the house. Here are some issues but I do not know if your area has similar characteristics:

1. Carport slab may be a slab on grade without a perimeter foundation of any size. It may only have a slight turndown around the perimeter. Placing block on a slab eddge that does not have a perimeter footing tends to become a future problem. Our local inspector's won't allow it without a letter from an engineer okaying it. You need to make sure what the perimeter consists of.
2. Water tends to seep under the walls since carport slabs are at a lower elevation than the house. Code requires floor slabs to be at least 4" above exterior grade. I have seen this a 100 times or more. The driveway is level with the carport slab. If the drive slopes slightly to the carport the problem enhances.
3. I would not worry a lot about connecting the top of wall to the existing. Right now, the wind blows through the openings onto the house wall that adjoins the bedroom and kitchen. Connecting the new wall transmits the load to the roof diaphragm which in turn still pushes on the shear walls. You are changing the load path but not the final resistance. You may be able check the load path to see if it makes any real difference.

 

[driquer1274]

JAE,

Thanks for your reply. The existing carport does have a monolithic slab poured already which is why I was considering the second option, no vertical load or lateral bracing, but thinking about it more, having a cantilever wall is probably not the most economic way to go about this as the footing would probably need to have a higher capacity to what is there already.

If I am going to connect the wall to the beams above for lateral support I will revisit having it be load bearing.

What is the best way to make connections/bond existing CMU walls/columns to new addition CMU walls without expansion joints?

Thanks,
Daniel

 

[driquer1274]

Ron247,

There is an existing monolithic slab/footing with rebar reinforcement, and the plan is to raise the existing slab 4 inches to meet the house slab height and be compliant for an enclosed habitable room.


Thanks on the load path comment. The overall resistance should be very similar as the same wind loads are just being transmitted to the diaphragm by the new walls and not the ones that were open before.

 

[EdStainless]

One concern is that if build a wall on the slab it is tied into the columns because they both sit on the same support.
Looking at this long term wouldn't you be ahead building real walls and taking out the columns as you go?
If these various parts moved independently from each other in future it would cause some real issues.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[jayrod12]

Why masonry? Why not wood or steel stud infill? Less of a concern regarding overloading the existing slab.

 

[OldBldgGuy]

Frost heave? Where are you? I'm not in a particularly cold area in Canadian terms, but it was -20C here this morning...

 

[jayrod12]

I'm in an area that was significantly colder than that this week. Either way, I'd still be considering infill wood or steel stud with appropriate slip joints to dead with the seasonal movement. That way I wasn't concerned about the weight of the infill wall overloading the existing.

But I am a fan of using wood where possible, so I could be considered biased.

 

[driquer1274]

Thanks for the replies. The location is south Florida. Freeze and frost heave are not a concern at all, but wind is. With strong wind loads CMU is usually the most common to resist overturning and uplift. The problem with wood is the existing foundation is not very deep. It is 12 inch x 10 inch continuous monolithic footing (with slab) with 2 #4 reinforcement rebar along one of the proposed patio walls. It still has not been confirmed if the shorter proposed wall opening even has a footing, so that has to be checked.

 

[jayrod12]

A small foundation is better for wood in my eyes, again from a vertical load standpoint. And the added dead load of the block wall seriously can't be helping overturning that much for an average height wall. I usually would neglect self weight from the overturning calcs if it was me.

Either way, if you don't connect the wall to the roof at the top, you've got to design your wall as a cantilever. There's an extremely high likelihood that your existing footing is woefully inadequate for overturning anyway, therefore I'd be providing some form of slip connection at the top that will still allow for lateral support.

 

[driquer1274]

I will look into wood walls more closely. Thanks.

 

[masonrygeek]

Being a masonry guy, stick with the masonry and its better missile impact resistance and use steel angles on either side of the top of the new wall as mentioned by JAE to provide a gap on top, but provide a pinned end instead of figuring it as a cantilever. Similar to this detail:

http://imiweb.org/02-010-1202-bond-beam-at-top-of-wall/.