Confused about ASCE 7 Vertical Structural Irregularity 4

[GamerSE]

Hello fellow engineers,

What do you guys consider as "In-plane Discontinuity in Vertical Lateral Force-Resisting Element Irregularity" per ASCE 7 Table 12.3-3, Type 4.
For example: For a two story building with X-braced frames in its transverse direction as lateral force resisting system, if the first and second floor X-braces are offset from story to story (see Grid A of attached diagram), is that considered a "In-plane Discontinuity in Vertical Lateral Force-Resisting Element Irregularity"?
Table 12.3-3 states, "... is defined to exist where there is an in-plane offset of a vertical seismic force-resisting element resulting in overturning demands on supporting structural elements"
Overturning demands, to me, can be the vertical and horizontal components of the braces. So does that mean the columns will have to be checked for the overturning demands?
Per ASCE 7 commentary: "Vertical lateral force-resisting elements at adjoining stories that are offset from each other in the vertical plane of the elements and impose overturning demands on supporting structural elements, such as beams, columns, trusses, walls, or slabs, are classified as in-plane discontinuity irregularities (Type 4)."

Thanks!

 

[r13]

See the link for a graphical explanation of each kind of vertical irregularity. Link

 

[r13]

My view.

 

[HTURKAK]

Overturning demands, to me, can be the vertical and horizontal components of the braces. So does that mean the columns will have to be checked for the overturning demands?

No...not really. For the system in Fig. ( in your post) , the offset produces overturning moment demands on the column shown in figure below in cloud. This column is subject to the requirement of Section 12.3.3.3, so shall be designed with load cases which include the overstrength factor omega 0.

 

[JLNJ]

I agree that the column circled needs to be checked for the overstrength loads combinations.

How about the connections which drag the forces into the adjacent bay?

 

[HTURKAK]

I agree that the column circled needs to be checked for the overstrength loads combinations.

How about the connections which drag the forces into the adjacent bay?

The relevant sections of ASCE 7-16 for Vertical Structural Irregularities
Table 12.3-2 TYPE =4 ,Reference Section =12.3.3.3,12.3.3.4,Table 12.6-1

12.3.3.3 '.... The connections of such discontinuous walls or frames to the supporting members shall
be adequate to transmit the forces for which the discontinuous walls or frames were required to be designed.'

12.3.3.4 ' Increase in Forces Caused by Irregularities for Seismic Design Categories D through F. '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.'

The commentaries C12.3.2 and C12.3.3.3 are also very useful to see the case.

 

[JAE]

Yes that definitely is a vertical irregularity Type 4 per ASCE 7.

What HTURKAK states above is correct - most everything in that overall system - both X-braces, the columns below the offset brace, need to be "kicked up" by Omega. Collectors and diaphragm connections need either the Omega or the 1.25 factor of 12.3.3.4.

 

[GamerSE]

Thanks everyone for confirming.
I have several people who disagree because the columns are continuous, but that to me means nothing, because the LFRS is offset.