Dear Dirk, Just wanted to point out that in the latest edition of the standard ASCE7-16 which is going to be adopted in the upcoming 2018 IBC, the 15% reduction of base shear for doing a modal response spectrum (MRS) analysis is no longer adopted and the standard mandates that the MRS base shear be scaled to 100% of ELF base shear. This is apparently due to buildings being designed using the ELF procedure were observed to have better seismic performance. It is funny since it reinforces the point you made during the video. I guess in the future we only stand to benefit from a more accurate force distribution for doing a MRS analysis.
Hi Laith. You are correct. Unlike the post-tensioned concrete world, the seismic codes are very "dynamic" (pardon the pun). Constantly changing the equations and the rules.
Why cant we use srss directly to combine story forces of all modes, without finding story shears and calculating the story forces again from story shears?
The dynamic base shear using only the first two modes was found to be 4,214.9 kips. This value divided by the R factor of 8 gives a scaled dynamic base shear of 526.9 kips (done in the previous Part g) ). I understand that this is confusing, and maybe I'm trying to demonstrate too many things. But first I did the calculations based upon all the translational modes, then I redid them based only upon the modes necessary to account for 90% of the mass. I apologize if this is confusing.
Hello, Professor Bondy Something related to the "jumping" modes, recently I am using AISC Design Guide 11 Chap 7 (FEM analysis) for a concrete floor vibration study (where modes need to jump). One hard thing is the requirement of getting the "mass normalized" mode shape phi. Unlike the lateral structure where I can start with lump sum assumption, for a continuous concrete slab it's hard to get the mass matrix. Just wonder have you encounter this part of chap 7 before? I tried ETABS and SAP2000 with the same geometry and the 1/3 acceleration or velocity outputs are very different as the two software normalize eigenvectors in different fashion. Many Thanks! Shaoran
Hi Shaoran, you are asking questions well above my pay grade :). I've never done what you are describing. At one point in my early career I recall published recommendations that estimated the floor natural frequency based upon a number of input values, and the idea was to make sure that equipment placed on the floor did not oscillate anywhere near that frequency. We also made sure that sensitive equipment (electron microscopes, etc.) were not sensitive to vibrations near the floor frequencies. I recall that damping had a significant effect on the system, but it's been too long for me to remember much else about that process. Good luck.
@@dirkbondy A follow up professor, we finally found ETABS mode shapes are mass normalized. There's some extra work in ETABS to plot the response spectrum per frequency in excel. Don't know what software Dr Murray used in guide 11 but ETABS normalize mode shape well.
Hi Muhammad, Actually the reduction doesn't exist anymore for any structure. It was eliminated in the ASCE 7-16 edition. All dynamic base shears must now be scaled to the full equivalent lateral load base shear. I will need to redo my videos someday :)
Great lecturer. More hearing from you i have learned alot. God bless
Dear Dirk,
Just wanted to point out that in the latest edition of the standard ASCE7-16 which is going to be adopted in the upcoming 2018 IBC, the 15% reduction of base shear for doing a modal response spectrum (MRS) analysis is no longer adopted and the standard mandates that the MRS base shear be scaled to 100% of ELF base shear. This is apparently due to buildings being designed using the ELF procedure were observed to have better seismic performance. It is funny since it reinforces the point you made during the video. I guess in the future we only stand to benefit from a more accurate force distribution for doing a MRS analysis.
Hi Laith. You are correct. Unlike the post-tensioned concrete world, the seismic codes are very "dynamic" (pardon the pun). Constantly changing the equations and the rules.
Like your lectures a nice fresh look on what I study right now. Thank you.
Sir Bondy, thank you very much again. I hope you can also give lecture on Strut and Tie Method of Analysis.
Sir Bondy, How you find ELF in table @ 30:20
Why cant we use srss directly to combine story forces of all modes, without finding story shears and calculating the story forces again from story shears?
Hello, I don't understand where you came up with 526.9k for the total Modal Base shear. @30.25
I had the same question, I think it should be 627. 0
The dynamic base shear using only the first two modes was found to be 4,214.9 kips. This value divided by the R factor of 8 gives a scaled dynamic base shear of 526.9 kips (done in the previous Part g) ). I understand that this is confusing, and maybe I'm trying to demonstrate too many things. But first I did the calculations based upon all the translational modes, then I redid them based only upon the modes necessary to account for 90% of the mass. I apologize if this is confusing.
Dear Mr Dirk,
Sir, you always mention your book. How can we get your book. I really desperately need that book which is about seismic design.
Hi Kamal, Just email Sir Bondy on his email, and he'll provide you a pdf of book.
I applied srss directly to combine story forces of all modes in exam. Am i going to fail?
Hello, Professor Bondy
Something related to the "jumping" modes, recently I am using AISC Design Guide 11 Chap 7 (FEM analysis) for a concrete floor vibration study (where modes need to jump). One hard thing is the requirement of getting the "mass normalized" mode shape phi. Unlike the lateral structure where I can start with lump sum assumption, for a continuous concrete slab it's hard to get the mass matrix.
Just wonder have you encounter this part of chap 7 before? I tried ETABS and SAP2000 with the same geometry and the 1/3 acceleration or velocity outputs are very different as the two software normalize eigenvectors in different fashion.
Many Thanks!
Shaoran
Hi Shaoran, you are asking questions well above my pay grade :). I've never done what you are describing. At one point in my early career I recall published recommendations that estimated the floor natural frequency based upon a number of input values, and the idea was to make sure that equipment placed on the floor did not oscillate anywhere near that frequency. We also made sure that sensitive equipment (electron microscopes, etc.) were not sensitive to vibrations near the floor frequencies. I recall that damping had a significant effect on the system, but it's been too long for me to remember much else about that process. Good luck.
@@dirkbondy A follow up professor, we finally found ETABS mode shapes are mass normalized. There's some extra work in ETABS to plot the response spectrum per frequency in excel.
Don't know what software Dr Murray used in guide 11 but ETABS normalize mode shape well.
thanks
85 percent reduction is only for regular buildings, is it not?
Hi Muhammad,
Actually the reduction doesn't exist anymore for any structure. It was eliminated in the ASCE 7-16 edition. All dynamic base shears must now be scaled to the full equivalent lateral load base shear. I will need to redo my videos someday :)
Really appreciate that you replied. Thank you sir.
new codes have taken out the 0.85Velf benefit and require to use 100% Velf.
Yes, that's true. But I have decided not to delete this video.
@@dirkbondy yeah don't delete them. I think I re-watched some of them 5 times already. good stuff, thanks for posting.