cutting an opening in unreinforced cmu wall

huangyhg

cutting an opening in unreinforced cmu wall
hi guys,
i have the situation that has come up where a client wants to provide a new 9'(2700) wide doorway through and un-reinforced block. my job is to strengthen this wall so a lintel can span across the opening.
my initial thoughts were to strengthen the opening with steel and then rely solely on the steel to span the opening.
has anyone had problems cutting unreinforced masonry to provide a doorway and what is the best detail i can use for this application?

it seems not to have significant loads. one might overengineer these things. you may drill for some bars supported on props temporarily discharge the loads, or not even this, work 1st one side and use steel l and then the other, or make an incision from one side and insert rebar, there are many solutions from the more risky to the absolutely safe, that would be like for an structural bearing wall cut.
i have had problems in the past with workmen whom over cut the corners instead of drilling a large hole and cutting from the hole, thus that crack don't continue.
the crack continued for only a few mm before i had them drill a hole and stop the crack. however they did have to patch the job. i also have this problem when they cut holes in tilt panels. i now have a heavy spec in this regard.
i personally don't have any problems with the steel idea, there are many ways to do this, i personally like to over cut the door in height, install lintel blocks & reo  and reinforce the existing masonry, seal it all up with concrete, looks good, and architects likes it.
  
when in doubt, just take the next small step.

i use the following detail a lot with success.
1. rake out mortar joints on either side of wall at the elevation of the top of the doorway/window.
2. slide in an angle on either side. make sure length of angle is 16" longer than opening for 8" bearing each side.
3. remove wall below.
4. weld bottom closure plate to horizontal leg of angles. this keeps debris in the wall and makes all three steel shapes into one composite shape.

i have also through-bolted a channel each side, then remove the wall below, and then welded the bottom plate to the channels.
you have a lot more to deal with than just creating the header. the block on the sides of the opening has to be designed to carry not only all loads above the opening, but the lateral wind loads on the door as well.
rowingengineer,
i typically do the steel angle thing too.  your method sounds great though -- especially if architects dig it.  can you list the construction steps in more detail though?  i'm not entirely sure that i understand.  also, do you need to shore the opening using your method?
one more too: are you proposing that reinforcement be added to the existing masonry wall above the opening?  if so, i wasn't aware that was possible / practical.  if it is that's great news.  i'll have a lot more options available to me when renovating masonry.
thanks.
you can call for saw cutting of the opening.
however, a couple of additional comments:
1. check if the lintel assembly will have to be fire rated, e.g., 1hr, 2hr, 3hr, etc.  this may require the istallation of fire-proofing plates and use of a w8 or larger beam.
2. 9'-0" is an odd number when dealing with cmu.  try to keep to 8" modules.  so the rough opening (r.o.) would be either 9'-4" or 8'-8".  this will ensure that you will have a cleaner installation.  same goes for the head height.
3. will a finished door be installed in the opening or will it be a pass-thru opening.  if something else will be installed inside the opening make sure you take account of build-out clearances.
4. you may have to specify that the bearing cells be grouted solid.  hollow cmu is typically good for 45psi on the gross area based on empirical design methods.
5. you seem to be concerned with construction means and methods, some will leave it up to the contractor to provide shoring and sequence design by his/her engineer. what is common is the use of needle beams or temporary channels bolted to both sides of the wall to span the temporary opening.  re  
thanks for the input guys, i am heading out there this morning so i will tell you how it all goes.
kootenaykid,
yes you need to shore up the existing masonry when you use this detail, and yes this is tricky. generally i sort this out with the builder, as he will have some great ideas, but the method that i specify on the drawings, is fairly similar to vandede427, but a touch different
general spec.
1. drill out edges of opening, using a 50mm bit.
2. cut out top of opening, install angle each side of opening (i like 150x75x5 ua) weld some temp props in place, how many props decides the angle size.
3. finish cutting out wall and install lintel blocks (horizontal reo at the same time this must be doweled back into existing masonry 300 odd mm). middle lintel block to have large hole installed, about 100mm will do the job (if a longer lintel is require you will need a few more holes and back prop under lintel (can now remove the 150x50x5 ua)
4. depending on existing masonry, if hollow; reinforce with reo, by drilling a large hole for feeding in reo and concrete, if corfilled check strength of existing if ok, do nothing is not i like to use a bit of carbon fibre (cost $$$, but again if an architect is involved it will be worth it).
5. using a fast flow concrete mix (generally 10mm agg) check with concrete supplier, fill all. there are some special techniques involved to make sure that the lintel is fully core filled, an experience concreter will know them, but generally a bit of flexible pipe will do the job
6. install new door.
note, make sure the side of the wall you put the holes is either going to be rendered or covered. otherwise, you will need to use masonry/block reflection formwork ($$$).
as i said this is like by architects but one problem is that it is $$$$. i have been doing a lot of reno's for million dollar houses so it wasn't a great problem, even when i asked for carbon fibre.
   
when in doubt, just take the next small step.

thanks rowingengineer.  when i use your detail, i'll re