CloudSan1
Civil/Environmental
- Jul 27, 2021
- 9
I am investigating an ongoing project that includes a cantilever column and a parallel chord roof truss as shown in the attached photos.
Currently, all concrete works is already complete and they are currently fabricating the trusses, however, it seems to me that the columns are too small to be able to carry the truss especially during earthquakes.
I first assumed that the trusses will not be used as an earthquake resisting member so I analyzed the steel truss and the concrete structure separately (superimposing the truss vertical and horizontal reactions to the top of the concrete column) and found out that the columns are too weak to carry the lateral superimposed load from the truss (Demand/Capacity Ratio = 2.0+ using ETABS). This is assuming that R=2.2 for Cantilevered column elements.
After that, I tried modeling the entire building with the trusses included in the earthquake analysis, this time I used R=6.5 (Special Truss Moment Frame) and the column still failed the demand/capacity check but by only a little (D/C Ratio around 1.1), the bottom chord of the truss also failed with D/C ratio around 2.0 in some parts.
I have not analyzed the entire structure with wind loads yet since the location is not really exposed to huge wind loads, but will try to analyze it later. For now, I would like to focus on earthquake situations first.
Questions:
1. Did I use the right R value in my 2nd analysis (R=6.5)?
2. Can I analyze the entire structure (including the truss) assuming it will be a Special Moment Resisting Frame (Steel or Concrete) with R value either 8.0 or 8.5?
3. Can anyone recommend a different approach in the analysis?
4. Any recommendations for the actual site situation?
Excited to know different opinions and suggestions. Please also point out if I have done something wrong in the analysis. Thanks a lot!
I tried to model the entire structure in ETABS to give you an idea of how the whole structure looks. Please see links below.
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Currently, all concrete works is already complete and they are currently fabricating the trusses, however, it seems to me that the columns are too small to be able to carry the truss especially during earthquakes.
I first assumed that the trusses will not be used as an earthquake resisting member so I analyzed the steel truss and the concrete structure separately (superimposing the truss vertical and horizontal reactions to the top of the concrete column) and found out that the columns are too weak to carry the lateral superimposed load from the truss (Demand/Capacity Ratio = 2.0+ using ETABS). This is assuming that R=2.2 for Cantilevered column elements.
After that, I tried modeling the entire building with the trusses included in the earthquake analysis, this time I used R=6.5 (Special Truss Moment Frame) and the column still failed the demand/capacity check but by only a little (D/C Ratio around 1.1), the bottom chord of the truss also failed with D/C ratio around 2.0 in some parts.
I have not analyzed the entire structure with wind loads yet since the location is not really exposed to huge wind loads, but will try to analyze it later. For now, I would like to focus on earthquake situations first.
Questions:
1. Did I use the right R value in my 2nd analysis (R=6.5)?
2. Can I analyze the entire structure (including the truss) assuming it will be a Special Moment Resisting Frame (Steel or Concrete) with R value either 8.0 or 8.5?
3. Can anyone recommend a different approach in the analysis?
4. Any recommendations for the actual site situation?
Excited to know different opinions and suggestions. Please also point out if I have done something wrong in the analysis. Thanks a lot!
I tried to model the entire structure in ETABS to give you an idea of how the whole structure looks. Please see links below.
[
https://res.cloudinary.com/engineering-com/image/upload/v1633051708/tips/Truss_full_model_gothf0.jpg
][]
