Do ASCE 7 structural wall anchorage forces apply to bldg columns that support horizontal girts?

[WeezerMike1]

We have a project with a pre-engineered metal building. The exterior walls are pre-fab panels supported on horizontal cold formed Z-girts. Under seismic loads, those girts span horizontally to primary building columns, and then those columns span vertically between the foundation and the roof.

We have a disagreement as to whether the provisions of ASCE 7 section 12.11 (structural wall anchorage) should apply. Structural walls are defined by ASCE 7 as walls that are either shear walls, or bearing walls (supporting more than 100 PLF in addition to their own self weight). My position is that the panels and horizontal girts are clearly non-structural wall components. But as soon as the load path engaged primary building columns to resist out-of-plane seismic loading, those columns and the rest of the load path should be considered a structural wall with anchorage requirements per ASCE 7 section 12.11.

Am I nuts?

Mike

 

[EDB9]

Mike,

Horizontal girts are definitely secondary structural members for resisting lateral forces. They’re not for vertical support. They also provide additional attachment points for the wall coverings. That isn’t applying any vertical loading so why would that section in the code be applicable even under a seismic event? I still wouldn’t treat it as a load bearing wall unless you’re putting something there like CMU and not panels. The columns are what are transferring the loads to the foundation, that is the superstructure. I have seen designs with larger lateral loads use two columns that are placed in opposite directions to provide additional resistance for out of plane loading. Wall panels are coverings and are not providing any structural resistance except keeping it as an enclosed structure. I don’t see how that section of ASCE 7 applies here as you’re using panels.

Erica

 

[WeezerMike1]

Thanks EDB8,

The girts and panels are definitely non-structural. If those girts attached directly to diaphragms, I don't think 12.11 would apply at all. But the girts deliver wall loads to columns and then those columns span between the foundation and roof. So the out-of-plane load path for wall anchorage includes elements that are definitely bearing. The load path is roughly similar to a tilt-up building when panels are technically non-bearing spanning horizontally to cast-in-place columns that span vertically and carry roof beams. Those concrete columns are always designed per 12.11, even if the panels they support are non-structural walls.

The wall anchorage provisions apply to structural walls because anything that holds the building up deserves some resilient anchorage. That's generally described in C12.11. So it's pretty logical that wall systems that rely on primary building columns for their support would get the same treatment as walls that rely on bearing wall studs for their support.

 

[EDB9]

What’s the linear weight of the panels?

Yes, they’re are going to have some dead loads from the panels but under lateral loading, you wouldn’t get any bearing but your vertical earthquake loads could. What’s your seismic category?

In the MBS I’ve worked with, the wall panels do not get anchored at the base but that’s because they used metal panels. To make up for the out of plane loading, double columns were designed, one in the bay direction and the other turned 90°.

Have you ever reviewed the MBS literature by Alexander Newman? These are two amazing references that I would highly recommend.

Metal Building Systems: Design and Specifications

Foundation and Anchor Design Guide for Metal Building Systems


Erica

 

[PROYECTOR]

Mike,

If the exterior walls are not part of the seismic force resisting system, you must refer to Section 13.5.3 "Exterior Nonstructural Wall Elements and
Connections" of ASCE 7.

Note that some prefabricated wall panels, such as non-load bearing tilt-up panels, can be designed as part of the seismic force resisting system in metal buildings. For such cases, a very good reference is "Seismic Design Guide for Metal Building systems: Based on the 2006 IBC"

 

[WeezerMike1]

Proyector, I am not sure that's correct. Section 12.11 of ASCE 7 applies to all structural walls. Per the ASCE 7 definition of a structural wall, that can be a bearing wall that doesn't participate in the primary lateral force resisting system.

EDB9, thanks for the reference. I'll see if I can track down those documents.

 

[PROYECTOR]

Proyector, I am not sure that's correct. Section 12.11 of ASCE 7 applies to all structural walls. Per the ASCE 7 definition of a structural wall, that can be a bearing wall that doesn't participate in the primary lateral force resisting system.

Yes, you are right. However, exterior bearing walls are not quite common in metal buildings, right?. Also, your question is about "pre-fab panels supported on horizontal cold formed Z-girts". So, we are not talking about structural bearing walls here. What I was trying to say is that, although the exterior walls are non-load bearing prefabricated panels, they can still participate in the lateral force resisting system. Therefore, you must make that distinction before deciding which section of ASCE 7 you should refer to (12.11 or 13.5.3).

 

[WeezerMike1]

Yeah, I think we're on the same page. The crux of my issue is determining what actually counts as a bearing wall since that would trigger section 12.11.

The metal building resists out of plane wall forces with a system of pre-fab panels, horizontal girts and vertical columns. The wall panels and girts are clearly non-bearing and can be designed by Chapter 13, but the column is definitely a bearing element. So what's technically part of the "wall"? The girts and panels don't even sit on the slab as they rely on the column for both gravity and lateral support. So it's fair to say that the column is part of the wall assembly, if not directly included in the definition of a wall. Anytime the wall assembly includes load bearing elements, I would apply 12.11 to those load bearing elements and also anything farther down the load path, but that position is not clearly stated or disputed by ASCE 7.

The load path is pretty equivalent to tilt-up wall construction if the concrete panels aren't shear walls (like at warehouses with repeating roll-up doors and not much solid panel). The tilt-up concrete panels can be non-bearing non-shear walls that span horizontally under out-of-plane loading. Then the load bearing columns span vertically from foundation to roof. The columns are always checked per 12.11 even though the panels themselves are non-structural walls. Although it's possible that's due to the chapter 13 footnotes that refer back to Chapter 12

Another example would be a regularly framed wood stud wall that supports roof purlins at 8ft spacing, and that isn't a shear wall. Could we argue 12.11 doesn't apply because only one stud every 8 feet is actually load bearing? Would we call those studs "posts" to avoid 12.11? That doesn't seem like an appropriate application of the provisions.

 

[PROYECTOR]

The crux of my issue is determining what actually counts as a bearing wall since that would trigger section 12.11.

I think ASCE 7 clearly defines what constitutes a bearing wall. See the attachment bellow.

So what's technically part of the "wall"? The girts and panels don't even sit on the slab as they rely on the column for both gravity and lateral support. So it's fair to say that the column is part of the wall assembly, if not directly included in the definition of a wall.

I don't think so, in my opinion the columns should not be considered as part of the wall. The architectural components (e.g., nonstructural walls) always need to be supported by structural elements (e.g., beams, slabs, columns), that does not mean that these structural elements are part of the architectural components. In this case, the wall panels can not be considered as structural elements since they do not provide support and stability for gravity and/or lateral loads.

 

[r13]

I don't know what is ASCE 12.11 talking about, but I sense some overthinking is brewing here. You should ask yourself the questions that what is the function of girts/wall panels - are they supporting the columns they attached to, and/or are they classified as essential/non-dispensable load carrying structural element for seismic/wind event, and lastly, what will be the worst outcome that the wall panel/girt to column connections have failed, lose of column, or instability of the building? Unless you are counting on the girts and wall panels for lateral stability, the connections are considered having secondary importance only.

 

[WeezerMike1]

Retired,

I think you might not have fully read my posts. I agree, the panels and girts and all their attachments can be checked by Chapter 13 for non-structural wall elements.

My concern though, is what procedures are used to check out-of-plane bending forces on the columns, and the column connections back to the roof deck. I agree with others that there's no conclusive logical path to illustrate that load bearing columns should be designed per 12.11 similar to bearing wall conditions. But similarly, I don't believe there's any conclusive logical argument that a load bearing building column should be designed for Chapter 13 provisions for non-structural components. In my mind, when there's uncertainty, it's MUCH preferred to apply the structural provisions to a load bearing element instead of applying non-structural provision to a load bearing element.

 

[r13]

Although the linked blog discusses bracing requirement, I think it is helpful in clarifying what are considered "architectural component". Below is an excerption from it.

Architectural components consist of furniture, interior partition walls, ceilings, lights, fans, exterior cladding, exterior walls, etc.

Link

 

[WeezerMike1]

Yes, that's an illuminating list. The only way a building column is included in that list is if it's considered part of the exterior wall. If it's a wall, then 12.11 applies.

 

[r13]

13.5.3 Exterior Nonstructural Wall Elements and Connections
Exterior nonstructural wall panels or elements that are attached to or enclose the structure shall be
designed to accommodate the seismic relative displacements defined in Section 13.3.2 and movements
due to temperature changes. Such elements shall be supported by means of positive and direct structural
supports or by mechanical connections and fasteners in accordance with the following requirements:

Are the wall panels and girts required for the columns to be functional during an earthquake event? If the answer is positive, use CH12.11, otherwise, use CH13.3 for the design of the panels, girts, and their connections. I hope this is convincing.

 

[dauwerda]

PEMB columns often depend on the girts for bracing to preclude buckling (and the girts depend on the sheathing), if that is the case, I would say the entire system is a structural wall. If the columns are not using the girts as brace points than I would say it does not fit the intent of 12.11

 

[WeezerMike1]

Retired,

I'm confused. I keep saying I'm only concerned about the forces and detailing of the column, and your responses keep talking about designs for panels, girts and their connections. Anyway. I think we've talked it to death. Thanks a bunch for the input.

 

[r13]

But as soon as the load path engaged primary building columns to resist out-of-plane seismic loading, those columns and the rest of the load path should be considered a structural wall with anchorage requirements per ASCE 7 section 12.11.

Your confusion was caused by misconception of the codified word "structural", a structural item is an element that carries structural loads (DL, LL, WL..), and/or an essential element in maintaining the stability of the building (bracing), and is part of the building main force resisting system, whose failure will trigger other structural elements to fail, thru structural connections as chin effect, and ultimately leads to collapse of the building. All structural elements that are part of the main force resisting system, including their connections, shall be designed for the seismic effects as stipulated in 12.4, and inertia force specified in 12.8. In addition, structural diaphragms and walls are to be designed/checked for local effects using loads and methods specified in 12.10 and 12.11 respectively. As localized effects, IMO, the back flow (to the main system) of loads from 12.10 & 12.11 should/need not be considered, whether the loads exceed the main design force or not, although could be done conservatively.

The PEMB wall panels, girts, and their connections, are not load carrying members, nor participated in resisting the seismic force, they are non-essential during such event, thus considered as architectural components, and be designed as thus per CH13. Again, the localized effect/load should not back flow to the columns they attached to, for which (the columns) are covered under 12.4 & 8.