Is having Bad Modal Mass Participating Ratios Okay?

[sky1379]

i'm designing a 2 to 3-story monolith concrete structures. i'm gonna use alumunium formwork and cast in place to do this. but i dont use all shear wall for all side, i just use 1 sides to have shear wall and the other is beam and column. because of that i'm having a Bad Modal Participating Mass Ratios because it have torsional irregularities type 1b per ASCE 7-16, but in the codes it is okay to have torsional irregularities as long as :
1. use redudancy factor 1.3 for the load combination and drift became lower
2. i use accidental torsion (5%) + eccentticity (Ax)
3. my building is in seismic design category D which is permitted in the ASCE 7-16

and i reach 90% of the modal mass in each direction

soo the question is what do you think of this types of buildings? is it okay or not? because i think i complied every requirement in the codes

there is a shear wall in structure is because i use alumunium formwork so it can do it fast and the other side is just beam-column because if the owner wants to renovate this part of sides. and i also model the stairs.

 

[JoshPlumSE]

In the Y direction, I have no concerns. The behavior looks really good. Have mode 22 and 36 which control the behavior.

The X direction is kind of messy to for me. You've got one real shear line at column line 5. My design emphasis would be on a few things:
a) Seeing if I could stiffen up the moment frames in this direction (presumably column line 2).
b) Making sure my diaphragm and moment frames can handle the expected deflections without loss compromising the vertical load path in any way. It looks like you've got a lot of openings in the diaphragm too. Close to where the load transfer is most important. The load transfer around the openings will be a major design consideration.
c) I'd really like to have some form of shear walls in another shear line. Maybe right on the other side of those opening. Even if there were only two.

That being said, if this is a single story structure in a low seismic zone, then I don't think it's necessarily a problem.... if you're sure you've followed the appropriate code provisions.

 

[driftLimiter]

OP said 2 - 3 story structure in SDC D. Frankly there are some red flags from the OP that I think should be addressed.

As Josh said, I would really try to get a second line in x-direction with some shear wall.

If you can't:
You have to check the level of torsional irregularity and apply the correct provisions from ASCE 7.
You say using rho = 1.3 changes the drift, but it should not. Check the ASCE 7 provisions.

You can still make this work I think but that torsional behavior is not desirable and we should strive to avoid it whenever possible.

 

[sky1379]

OP said 2 - 3 story structure in SDC D. Frankly there are some red flags from the OP that I think should be addressed.

As Josh said, I would really try to get a second line in x-direction with some shear wall.

If you can't:
You have to check the level of torsional irregularity and apply the correct provisions from ASCE 7.
You say using rho = 1.3 changes the drift, but it should not. Check the ASCE 7 provisions.

You can still make this work I think but that torsional behavior is not desirable and we should strive to avoid it whenever possible.

I already check the torsional irregularity and this structure belong to type 1b. According ASCE 7-16 :

in 12.3.3.1 Structures assigned to Seismic Design Category E or F that have horizontal irregularity Type 1b of Table 12.3-1 or vertical irregularities Type 1b, 5a, or 5b of Table 12.3-2 shall not be permitted. Structures assigned to Seismic Design Category D that have vertical irregularity Type 5b of Table 12.3-2 shall not be permitted. (Soo for SDC D Type 1b is OK)

in 12.3.3.4 For structures assigned to Seismic Design Category D, E, or F and having a horizontal structural irregularity of Type 1a, 1b, 2, 3, or 4 in Table 12.3-1 or a vertical structural irregularity of Type 4 in Table 12.3-2, the design forces determined from Section 12.10.1.1 shall be increased 25% for the following elements of the seismic force-resisting system: 1. Connections of diaphragms to vertical elements and to collectors and 2. Collectors and their connections, including connections to vertical elements, of the seismic force-resisting system. EXCEPTION: Forces calculated using the seismic load effects, including overstrength of Section 12.4.3, need not be increased.

in 12.3.4.2 tor, ρ, for Seismic Design Categories D through F. For structures assigned to Seismic Design Category D and having extreme torsional irregularity as defined inTable12.3-1, Type 1b, ρ shall equal 1.3. (i'm using 1.3)

in 12.7.3, model 3D

in 12.8.4.3

in 12.12.1.1 For seismic force-resisting systems solely comprising moment frames in structures assigned to Seismic Design Categories D, E, or F, the design story drift (Δ) shall not exceed Δa∕ρ for any story. ρ shall be determined in accordance with Section 12.3.4.2. (so for the drift divide by 1.3 and the drift limit became lower)

Tabel 12.6-1. using response spectrum

in 16.3.4 Inherent eccentricity resulting from any offset in the centers of massand stiffness at each level shall be accounted for in the analysis. In addition, where a Type 1a or Type 1b horizontal structural irregularly exists as defined in Section 12.3.2.1, accidental eccentricity consisting of an assumed displacement of the center of mass each way from its actual location by a distance equal to 5%ofthe diaphragm dimension of the structure parallel to the direction of mass shift shall be considered. The required 5% displacement of the center of mass need not be applied in both orthogonal directions at the same time.

SO i already acount all this consideration. I know the MPMR is not great but i checked all the boxes in ASCE