the Paper Owl
Structural
- Sep 22, 2021
- 22
Good Afternoon all,
I am tasked with the preliminary design of a fireplace renovation in which the header will be revised from a dropped condition to a raised condition. This is in part due to the new gas burning fireplace that will be installed in the fireplace insert that will need more than the current 3'-4" clearance height that exists between the first floor subfloor and the bottom of the header. The clear span needed for the header is 3'-11 1/2".
I have re-run the calculation for the header, which will remain the same size ( (2) 2x10 ) with 2 jacks and 2 kings at each end, but due to the location of the header I am trying to create a helpful solution in regards to the temporary shoring that will be involved with the renovation.
The existing header is on a first floor exterior wall that runs parallel with the second floor platform joists as well as parallel with the roof trusses above. There is an 10'-0" (measured from the sub-fascia to the exterior wall) 4:12 wrap around first floor porch roof assembly that bears on the wall (through the use of jack trusses). The house has a lower level, making any first floor temporary shoring techniques that extend to existing grade too long and slender to use.
I have calculated the vertical loading that exists at the exterior wall which consists of the following:
- Second Story Dead Load + Live Load (for 1 foot tributary area <- increasing from actual 8" tributary area that it receives as the gable end wall for the floor above, running parallel with the floor system).
- Main Roof Dead Load + Wind Load (again using a 1 foot tributary area).
- Wrap Around Porch Roof Dead Load + Wind Load (using a 5 foot tributary area that takes half of the jack truss loading, the other half being supported by the dropped beam / post system to foundation at the edge of the porch).
My preliminary solution is to essentially create a face mounted board ( (1) 2x12 ) with a fastening schedule that will rely on the shear capacity of SPAX Powerlags (1/4" x 4") to transfer the force from the double top plate / king studs to the board, then back to the king studs, reminiscent of a moment frame.
My force transfer interaction is as follows:
- 406 plf vertical loading along top plate transferring to (7) 1/4"x4" powerlag screws spaced at 9" through lateral shear.
- powerlag screws (each taking 304 +- plf) lateral shear and transferring to the 2x12 board
- 2x12 board transferring lateral shear to (8) 1/4"x4" powerlag screws that are driven into each king stud (2 screws per stud - 4 on each end).
- king stud transferring newly allocated load onto first floor rim board / lower level stud wall / foundation.
What I do not feel great about with this solution:
- Each 1/4"x4" SPAX screw will be placed dead center of each stud / plate, leaving only 5/8" from hole edge to stud edge.
- The absence of slanted studs that would take the vertical loading and transfer it to the first floor floor platform. I am entertained the solution of running slated studs, 3'-0" from the exterior wall in question, that would lap (similar to a ceiling joist and rafter) to a horizontal stud that would run into the fireplace insert and anchor to the stud wall. This solution tries to avoid tearing up the hardwood flooring that runs up to the fireplace insert.
Thank you for your time reading this post and feedback. Please refer to the attached sketches for information.
I am tasked with the preliminary design of a fireplace renovation in which the header will be revised from a dropped condition to a raised condition. This is in part due to the new gas burning fireplace that will be installed in the fireplace insert that will need more than the current 3'-4" clearance height that exists between the first floor subfloor and the bottom of the header. The clear span needed for the header is 3'-11 1/2".
I have re-run the calculation for the header, which will remain the same size ( (2) 2x10 ) with 2 jacks and 2 kings at each end, but due to the location of the header I am trying to create a helpful solution in regards to the temporary shoring that will be involved with the renovation.
The existing header is on a first floor exterior wall that runs parallel with the second floor platform joists as well as parallel with the roof trusses above. There is an 10'-0" (measured from the sub-fascia to the exterior wall) 4:12 wrap around first floor porch roof assembly that bears on the wall (through the use of jack trusses). The house has a lower level, making any first floor temporary shoring techniques that extend to existing grade too long and slender to use.
I have calculated the vertical loading that exists at the exterior wall which consists of the following:
- Second Story Dead Load + Live Load (for 1 foot tributary area <- increasing from actual 8" tributary area that it receives as the gable end wall for the floor above, running parallel with the floor system).
- Main Roof Dead Load + Wind Load (again using a 1 foot tributary area).
- Wrap Around Porch Roof Dead Load + Wind Load (using a 5 foot tributary area that takes half of the jack truss loading, the other half being supported by the dropped beam / post system to foundation at the edge of the porch).
My preliminary solution is to essentially create a face mounted board ( (1) 2x12 ) with a fastening schedule that will rely on the shear capacity of SPAX Powerlags (1/4" x 4") to transfer the force from the double top plate / king studs to the board, then back to the king studs, reminiscent of a moment frame.
My force transfer interaction is as follows:
- 406 plf vertical loading along top plate transferring to (7) 1/4"x4" powerlag screws spaced at 9" through lateral shear.
- powerlag screws (each taking 304 +- plf) lateral shear and transferring to the 2x12 board
- 2x12 board transferring lateral shear to (8) 1/4"x4" powerlag screws that are driven into each king stud (2 screws per stud - 4 on each end).
- king stud transferring newly allocated load onto first floor rim board / lower level stud wall / foundation.
What I do not feel great about with this solution:
- Each 1/4"x4" SPAX screw will be placed dead center of each stud / plate, leaving only 5/8" from hole edge to stud edge.
- The absence of slanted studs that would take the vertical loading and transfer it to the first floor floor platform. I am entertained the solution of running slated studs, 3'-0" from the exterior wall in question, that would lap (similar to a ceiling joist and rafter) to a horizontal stud that would run into the fireplace insert and anchor to the stud wall. This solution tries to avoid tearing up the hardwood flooring that runs up to the fireplace insert.
Thank you for your time reading this post and feedback. Please refer to the attached sketches for information.
