I have an existing two-story building, with masonry shear walls along the perimeter of the building (story 1) and Braced Frames (story 2). The second story is essentially a penthouse at the roof level that covers only part of the roof area. We need to cut a big opening through the penthouse floor and need to analyze the impact of that opening on the lateral force-resisting system as well the floor diaphragm. Note the penthouse floor consists of a 4” composite metal deck supported on joists and I-beam girders, and the roof area outside the penthouse consists of a 1.5” metal deck with roofing only. Essentially two different diaphragm systems at one level.
I have modelled the structure in RAM Structural System with different diaphragms and modelled the opening through the floor. Note that for the opening we will be removing the existing joists. I have before and after models so that I can compare shear wall forces and story drifts. However, I am not sure how I can capture the localized effects, i.e., shear around the openings. A couple of questions that come to mind:
1. Since the opening is more or less in the middle of the middle, I expect the shear through the diaphragm to be small in the long direction. I can come up with the exact value by hand, but I am not sure how the shear will be distributed around the opening.
2. Will I need to revise the metal deck connections to the joists to remain around the openings? This will require chipping the concrete floor to provide additional puddle welds.
3. I could be wrong, but I think the members (joists in this case) around the parameter will go in compression and will need to be checked. Not sure how to come up with the magnitude for this load.
4. Considering we are removing the deck around some of the columns, will have to ensure that the braced length for each of the columns stays the same. Given the opening layout, the columns will be braced on one side in both the strong and weak axis.
5. Behaviour in “X” direction will be a lot different but wondering if the approach for “Y” direction will still be applicable.
6. I have thought about putting “X” bracing around the parameter essentially to avoid having the load go around the opening, but the architecture will not allow this. I guess I replace the existing joist with a new beam around the perimeter that has a moment connection at the column to essentially provide a frame but this will to big of an item as they will have to remove a larger floor area, salvage existing joists, and provide new beams with moment connections. Not to mention the column will need to be reinforced.
I am looking to see if someone in the group has dealt with this situation in the past and can provide some guidance to approach this problem. Thanks,
I have modelled the structure in RAM Structural System with different diaphragms and modelled the opening through the floor. Note that for the opening we will be removing the existing joists. I have before and after models so that I can compare shear wall forces and story drifts. However, I am not sure how I can capture the localized effects, i.e., shear around the openings. A couple of questions that come to mind:
1. Since the opening is more or less in the middle of the middle, I expect the shear through the diaphragm to be small in the long direction. I can come up with the exact value by hand, but I am not sure how the shear will be distributed around the opening.
2. Will I need to revise the metal deck connections to the joists to remain around the openings? This will require chipping the concrete floor to provide additional puddle welds.
3. I could be wrong, but I think the members (joists in this case) around the parameter will go in compression and will need to be checked. Not sure how to come up with the magnitude for this load.
4. Considering we are removing the deck around some of the columns, will have to ensure that the braced length for each of the columns stays the same. Given the opening layout, the columns will be braced on one side in both the strong and weak axis.
5. Behaviour in “X” direction will be a lot different but wondering if the approach for “Y” direction will still be applicable.
6. I have thought about putting “X” bracing around the parameter essentially to avoid having the load go around the opening, but the architecture will not allow this. I guess I replace the existing joist with a new beam around the perimeter that has a moment connection at the column to essentially provide a frame but this will to big of an item as they will have to remove a larger floor area, salvage existing joists, and provide new beams with moment connections. Not to mention the column will need to be reinforced.
I am looking to see if someone in the group has dealt with this situation in the past and can provide some guidance to approach this problem. Thanks,
