Creating a large opening in a existing brick masonry load bearing wall 1

[smitty9898]

I recently worked on a project where I was asked to develop reinforcing to create an opening in a brick masonry load bearing wall. The opening was approximately 9’ and the cross section of the wall was triple wythe brick masonry. We were looking at using the “pinch beam” method (as found in prior posts), but it was determined that the stresses in the “thru-bolts” were too high.

I’m trying to figure out how other engineers analyze this problem. More specifically, what is the load path?
Here is the load path that I assumed:

1. The vertical dead loads within the triangle above the opening are bearing on the thru bolts. (i.e. the bolts between the steel channel that sandwich the brick masonry.

2. The masonry creates bending and shear stress on the thru bolts which are transmitted to the steel channels on each side of the wall.

3. The concentrated loads from the thru bolts are transmitted to the steel channels by bearing.

4. The loads from the thru bolts create bending and shear stresses on the steel channels which are transmitted to the ends of the steel channel.

5. The loads from the steel channels are supported by thru bolts that are located outside of the opening

6. The loads in the bolts are transmitted to the masonry wall through bearing.

7. Loads in masonry wall are transmitted to the foundation.

Since the “pinch beam” in my situation resists floor live loads on both sides of the wall, the stresses in the thru bolts were high, and were inadequate.

Does anyone assume that the steel channels act compositely with the brick masonry?

I’m trying to obtain other engineer’s opinion on this topic, and how the loads were analyzed.

 

[JAE]

We sequentially weld bottom cover plates on the channels to take all the load and don't rely on the bolts to support the vertical weight. The bolts just hold the channels together.

So you install the channels with the through bolts, prior to removing any masonry. Then you sequentially remove a small section of masonry (say 2 ft wide) and then weld on a 2 ft cover plate. Remove the next 2 ft of masonry, weld on the next plate, rinse/repeat.

 

[FinnB]

Personally I wouldn't be inclined to use through bolts with masonry where loads are significant. I have done it before on lightly loaded walls but on a heavily loaded wall it places heavy local stresses on the bolts where the bricks bear on the bolts. It can cause cracking in the masonry.

I have put large opes in masonry many times but I place steel beam(s) directly under the wall over the opening. What I do is as follows:

1)To form the lintle beam bearings break out pockets either side of the top of the new ope and install precast concrete padstones or insitu padstones, the insitu option is normally too much trouble.

2) Needle temporary steel beams perpendicularly though the wall over the location where the ope will be formed. Needle beams at approximately 3-4 foot centre over the length of the ope. Do a calc to confirm.

3) Prop needle beams.

4) Break out new ope in wall. The wall above is supported on the needle beams.

5) Slide in a new steel beam or a number of parallel beams depending on the witdth of the wall directly above the ope. Steel beams rest on the padstones.

6) Pack up the area between the top of the new beam and the wall above by ramming in non-shrink dry pack (sand/cement).

 

[a2mfk]

We've always done it pretty similar to JAE, but not too often with brick bearing walls, mostly CMU. I see nothing wrong with his method. I am feeling too lazy to understand Finn's method but that may work too.

 

[smitty9898]

@JAE – Do you continue the installation of the steel plates at bearing as well? My biggest concern regarding the “pinch beam” method is the load/shear transfer at bearing. I’m assuming that the weight of the supported wall is being transferred by the thru bolts that are located beyond the opening.

@JAE and @a2mfk - What are your thoughts on the load path?

@FinnB – My analysis determined that the stresses on the masonry from the bolts at bearing were too high, and would result in cracking at bearing. My situation is a little unusual, since it is a live load bearing wall.

 

[cap4000]

FinnB

I have done it both ways described above. Based on your large loads go with the needle beam and the temporary shoring of them. No more than 3 ft c/c for the beams. Take it slow as you proceed.

 

[FinnB]

smitty9898

The method I outlined does not require bolts to to drilled through the brickwork and therefore there are no heavy localise stesses created by bearing.

In the method I proposed a beam is installed directly under the wall over the ope. Temporary propping allows the opening in the wall to be made without the wall above collapsing.

The fact that you have live loading has no bearing on the solution I have proposed which is, by the way, a very standard method of forming opes in masonry walls.

 

[smitty9898]

@ FinnB - I was agreeing with you. Heavy loaded walls will create high stresses in the thru bolts, which makes the "pinch beam" impractical for some cases. We are looking at using the "needle beam" method, however this will be a slighlty more difficult method for the contractor.

 

[hokie66]

Whether you needle the wall to install a beam under the opening or use channels each side, you have to take the reaction at the end of the opening. FinnB uses "padstones", and you must check the bearing and capacity of the masonry jamb. If that doesn't work, you could use steel columns. With channels each side and bottom plates as JAE described, you would need columns beside the wall, I think.

 

[FinnB]

smitty9898

Using needles shouldn't be more difficult for the contractor. It is a standard construction method and once the wall is supported on the needles it is easy to break out the opening in the wall quickly.

Using the method JAE described means that only a small section of the wall can be demolished at a time while plates are welded between the 'pinch beam' channels. No welding on site is needed with the needle method.

 

[JAE]

Either way works (needle or channels). Agree with FinnB that needle beams don't necessarily mean more work or difficulty.

The negative of needle beams is that you sometimes don't have good points to support the needle beams from. You disrupt and damage (temporarily) the wall above.

The negative of channels is that you have a permanent, somewhat ugly, set of channels running over and besides your opening. This usually demands covering or furring out to make more aesthetic and protect from fire.