structural3
Structural
- Aug 10, 2005
- 19
We are working on a project where the building needs to be evaluated due to the crane upgrade, assuming that crane load will be increased from 50 ton to 80 ton, the building was built back in 1940s and have span of 2x80' span (2 bays in tansverse direction), building heigth is about 60' and roof has wood truss containing steel rod as tension rod as web member.
On longitudinal direction there are bridge type wood trusses connecting the lattice wood columns which spaced at 25' o.c. in the center line and 12.5' o.c. at two perimeter lines. Trusses are located at mid-height of column where the crane girders are supported, slender trusses are also built at higher elevation to connect columns in the longitudinal direction.
It is strange that the roof diaphragm is not at the same elevation on top of roof, for every 50' length, the roof elevation drops a truss height (~10') with inclined sash
for one bay length (25'). I don't know how the structural engineer considered the load path at that time. I guess I need to propose lateral rod braces at the bottom chord of trusses as diaphragm.....
Based on Chapter 34 of CBC 2007, the crane load caused vertical load increase of more than 5%, seismic load has also exceeded 10% increase, I think that the building needs to be seismically upgraded. At this time I could not figure out what lateral resitance system can be proposed.
1. There is no ordinary braced frame for wood frame
It seems that according the current code, the only lateral resistance system for wood construction is wood shear wall, is there any possibility to propose other system, since we are located in SDC D/E, I just don't see any other systems work for wood construction. (intermediate truss moment frame is prohibited in high seismic zone, let long for wood construction)
2.Provide lateral bracing outside of the building in transverse direction, but only X braced frame or V type frame will work, single brace shall not be allowed since there is no redendancy and it's similar to K type brace. what do you think?
3.Eccentric brace? put knee braces to support existing "truss moment frame". Where exactly in the code will I be able to find requirement for this design. Again "wood"?
4. Can I avoid doing seismic upgrade and just look at crane upgrade?
5. Shall I use ASCE 31/41 or FEMA publication, I could not find any information that can be helpful at this time from those pucblications.
Thanks so much for your comments!
On longitudinal direction there are bridge type wood trusses connecting the lattice wood columns which spaced at 25' o.c. in the center line and 12.5' o.c. at two perimeter lines. Trusses are located at mid-height of column where the crane girders are supported, slender trusses are also built at higher elevation to connect columns in the longitudinal direction.
It is strange that the roof diaphragm is not at the same elevation on top of roof, for every 50' length, the roof elevation drops a truss height (~10') with inclined sash
for one bay length (25'). I don't know how the structural engineer considered the load path at that time. I guess I need to propose lateral rod braces at the bottom chord of trusses as diaphragm.....
Based on Chapter 34 of CBC 2007, the crane load caused vertical load increase of more than 5%, seismic load has also exceeded 10% increase, I think that the building needs to be seismically upgraded. At this time I could not figure out what lateral resitance system can be proposed.
1. There is no ordinary braced frame for wood frame
It seems that according the current code, the only lateral resistance system for wood construction is wood shear wall, is there any possibility to propose other system, since we are located in SDC D/E, I just don't see any other systems work for wood construction. (intermediate truss moment frame is prohibited in high seismic zone, let long for wood construction)
2.Provide lateral bracing outside of the building in transverse direction, but only X braced frame or V type frame will work, single brace shall not be allowed since there is no redendancy and it's similar to K type brace. what do you think?
3.Eccentric brace? put knee braces to support existing "truss moment frame". Where exactly in the code will I be able to find requirement for this design. Again "wood"?
4. Can I avoid doing seismic upgrade and just look at crane upgrade?
5. Shall I use ASCE 31/41 or FEMA publication, I could not find any information that can be helpful at this time from those pucblications.
Thanks so much for your comments!
