Podium slab design for amplified uplift force from cold-form steel shear wall 6

[king12345]

Hi Guys,

I am working on a project which has a 4-story cold-formed steel light-framed structure over a concrete podium slab. Based on the AISI 400, the uplift force of the Holdown for the shear walls need to be amplified with a overstrength factor or expected shear strength factor. My question is do I need to design the podium slab with the amplified uplift force or not?
For example, the uplift force is 20k, the overstrength factor is 3, amplified uplift force 20x3 = 60k. Do I design the podium slab for 60k uplift force or 20k?

Thank you for help,

 

[Eng16080]

A bunch of us on here were debating/discussing this a little while back. My understanding is that the overstrength factor is applicable in certain situations where you have either horizontal or vertical structural irregularities. For example, if you have a LGS shear wall supported by the podium which is offset horizontally from the LFRS below the podium, then you would have a Type 4 horizontal irregularity per ASCE 7-16, Table 12.3-1. From there, the table references Section 12.3.3.3 which requires that the podium be designed for the "seismic load effects, including overstrength." (See EDIT #2 below) However, the code does not require that the hold-down connection be design for overstrength. Figure C12.3-5 illustrates the requirements.

EDIT #1: Based on driftLimiter's response below, a further clarification is necessary. While ASCE 7-16 does not require that the overstrength factor be applied to connections, the requirements of ACI 318-14, Section 17.2.3.4.3 need to also be considered for the seismic design of concrete anchorage in tension. That code Section gives 4 options for designing the connection, one of which (option d) is to use the overstrength factor.

EDIT #2: While ASCE 7-16, Section 12.3.3.3 requires the podium to be designed for the "seismic load effects, including overstrength," Section 12.4.3.2 allows the seismic force to be "calculated with the capacity-limited horizontal seismic load effect, Ecl" which is used instead of omega. Further down on this thread, driftLimiter describes how this would be the case if, for example, the anchorage was designed per ACI 318-14, Section 17.2.3.4.3, Option a.

Now, if somehow, the LGS shear wall happened to be perfectly aligned with a steel frame, or some other LFRS below (unlikely, I know), then my understanding is that the overstrength factor would not apply in that case.

One other thing to note, which may or may not be applicable in your case is that there could be an additional amplification factor (unrelated to overstrength) which is applicable if you're using a two-stage analysis per Section 12.2.3.2. I mention this because this analysis method is common for podium type structures. That factor is discussed in Section 12.2.3.2(d).

 

[Eng16080]

One other thing to mention, in case you're not aware:
Per Table 12.2-1, footnote b, you may be able to reduce the overstrength factor from 3 to 2.5 if you have flexible diaphragms. If you're doing a two-stage analysis, I would take this to mean the diaphragms of the upper floors, not the concrete podium.

 

[driftLimiter]

I think Eng16080 sums it up pretty well. The only caveat is that the anchorage design of the hold downs falls under ACI Ch 17 and for seismic loading the anchorage itself has to meet a ductility requirement. That requirement is often satisfied by simply designing anchorage for earthquake loads including overstrength. But there are other options as well. Ref: ACI 318-14 Section 17.2.3.4.3.

 

[Aesur]

My understanding of cold formed steel is that if you are using the higher R value for sheathed cold formed steel shear walls, then overstrength must be included in the chords and holdown designs, including connection. I'm not sure if the podium would require this for design of the podium, however I believe you will may need to look at amplifying the loads for 2 stage analysis, I have to read that section every time I do these as I don't do them often enough. I typically use the exception of R=3 (not specifically detailed for seismic) to avoid dealing with over strength in CFS design.

 

[Eng16080]

driftLimiter, thanks for the response. I'm going to edit my post above accordingly.

 

[king12345]

Thank you for the responses. They are very helpful.

Let me know if I have this right: I have rigid floor diaphragm for the upper light-framed structure, so this building cannot be considered as two separate structures and use two-stage analysis. I was thinking using the amplified uplift force to design the holdown to slab connection, such as checking the concrete breakout force, pullout force of the slab. Then I can use the uplift force without the overstrength factor for designing the podium slab/beam for bending and shear force similar to foundation design. Since this building cannot be considered as two separate structures, this is like holdown on a discontinuous beam and the overstrength factor must be applied for the podium/beam design.

 

[Eng16080]

Let me know if I have this right: I have rigid floor diaphragm for the upper light-framed structure, so this building cannot be considered as two separate structures and use two-stage analysis.

I don't think this is necessarily true. The criteria for providing a two-stage analysis is listed in ASCE 7-16, Section 12.2.3.2. I don't see any requirements which necessarily disqualify the structure if the upper diaphragm is rigid. In general, the requirement is that the lower portion is much more (at least 10 times) stiffer than the upper portion. I think you would need more information than just the upper diaphragm stiffness to make that determination.

I was thinking using the amplified uplift force to design the holdown to slab connection, such as checking the concrete breakout force, pullout force of the slab.

If you're designing the connection for the uplift force with the overstrength factor, then that would be in accordance with ACI 318-14, Section 17.2.3.4.3, design option (d). That would be correct.

Then I can use the uplift force without the overstrength factor for designing the podium slab/beam for bending and shear force similar to foundation design.

No, you should use the overstrength factor in designing the podium slab/beam, as required by ASCE 7-16, Section 12.3.3.3.

Since this building cannot be considered as two separate structures, this is like holdown on a discontinuous beam and the overstrength factor must be applied for the podium/beam design.

This contradicts your previous sentence, but yes, you should be using the overstrength factor as noted above for the podium slab/beam design. Also, as far as I understand, the use of the overstrength factor has nothing to do with whether or not you happen to use a two-stage analysis. Make sure you're not confusing overstrength with the amplification factor between the upper and lower parts of the structure for a two-stage analysis. That is a separate factor which serves a different purpose.

To summarize, I think you're doing the analysis correctly if you're designing the connection to the podium using the overstrength factor and if you're also designing the podium slab/beam using the overstrength factor.

If you find the code requirements confusing, you're not alone.

 

[king12345]

@Eng16080

Quote:
I don't think this is necessarily true. The criteria for providing a two-stage analysis is listed in ASCE 7-16, Section 12.2.3.2. I don't see any requirements which necessarily disqualify the structure if the upper diaphragm is rigid. In general, the requirement is that the lower portion is much more (at least 10 times) stiffer than the upper portion. I think you would need more information than just the upper diaphragm stiffness to make that determination.

Yes, you are right. I need to look deeper into it.

Quote:
No, you should use the overstrength factor in designing the podium slab/beam, as required by ASCE 7-16, Section 12.3.3.3.

If the building can be considered as two separate structures, I would think the podium is the foundation of the upper structure. Based on the AISI S400 Section E1.4.1.3, "the required strength shall be determined from the seismic load effect and need not include the overstrength factor nor consider the expected strength of the seismic force-resisting system unless otherwise specified in the applicable building code", the overstrength factor does not need to apply for the foundation (podium) element.

 

[Eng16080]

I'm not familiar with AISI S400, so my forthcoming comment might be wrong, but based on the section of the code that you referenced, I would interpret "unless otherwise specified in the applicable building code" to mean that the overstrength factor is required because it is specified in the applicable building code. If the building code is IBC 2018 (I'm guessing in your case), IBC 2018 (with a few exceptions) adopts the entirety of ASCE 7-16 as the code to use for seismic loading requirements. Thus, ASCE 7-16, Section 12.3.3.3 is applicable, or rather, it is "otherwise specified."

Either way, I wouldn't think of the top of the podium as being the same as a foundation, or having the same associated risk as a foundation. My understanding is that the whole purpose of the overstrength factor is to safeguard a premature failure of the gravity force resisting system below. The commentary to ASCE 7-16, Section C12.3.3.3 states: "The purpose of requiring elements that support discontinuous walls or frames to be designed to resist seismic load effects including overstrength is to protect the gravity load-carrying system against possible overloads..." The purpose is to prevent a premature collapse of the podium level. I don't think there would be the same level of concern if it were a foundation.

 

[driftLimiter]

The woodworks document on 5 over 1 podium plainly states the podium slab, beams, and other elements supporting discontinuous vertical seismic loading should be designed for the amplified seismic loading. It's not quite the same as a foundation because the load path is still offset due to the irregularity.

 

[ggcdn]

I'm on team Omega. Both the anchorage and the supporting members should be designed for amplified seismic forces.

Regardless of code wordings, it seems conceptually wrong to be transferring seismic loads without overstrength factor. The transferring members are not calibrated for the same R-factor as the LFRS. Overstrength in the LFRS system will increase the loads received by the podium. Without capacity design, you are potentially creating gravity failures as your limit state.

 

[driftLimiter]

Team Omega I like it!

Here is a test report done by simpson. The goal of the supporting member design is to avoid yielding of the supporting members. Good seismic design suggests that you want to guarantee there will be no yielding (capacity design).

The results of the test indicate that the loading indicated in ACI 17.2.3.4.3 (a) (i) or 1.2 Nsa was sufficient in all cases to avoid yielding of the supporting member.

Based on a project I'm working on I found this 1.2 Nsa to be often quite a bit lower than Omega x E. This is an important savings that may help significantly.

For example I have a 1-1/8" Anchor rod anchored to the podium. The Omega x E for that specific anchor is up to around 90 kips, but the 1.2 Nsa anchor is 55 kips. A significant savings.

Also a sensible one, because excessive yielding is required of the ductile anchor rod so you really cannot develop the Omega x E level load in that anchor.

 

[Eng16080]

driftLimiter, In the example that you give above with the anchor meeting ACI 318-14, Section 17.2.3.4.3(a), I understand that the connection would be designed for 55 kips. Would the podium framing also be designed for that same 55k force then? I'm guessing the answer is yes with the reason being that it's impossible for the connection to transfer more than 55 kips to the podium framing. If that's the case, then my response at the top should probably be revised again.

Also, it's probably worth noting that there's a list of criteria that needs to be met to satisfy ACI 318-14, Section 17.2.3.4.3(a). It's more than just meeting 1.2 Nsa.

 

[driftLimiter]

Yes My interpretation is to design the framing for the 1.2 Nsa limiting load assuming the other criteria of subpart (a) are satisfied.

 

[Eng16080]

Alright. Thanks. ASCE 7-16, Section 12.4.3.1, which discusses applying the overstrength (omega) factor, states at the end of that section that the seismic load "need not be taken as larger than Ecl where Ecl=the capacity-limited horizontal seismic load effect..." So in this case, Ecl would be based on the 1.2 Nsa and would control the design. Section 12.4.3.2 seems to state the same thing.

 

[Michel60]

I believe the question about elements in the system to which the over-strength (OS) factor is applied is discussed in the ASCE7-16 Commentary and Figure C12.3-5. The design philosophy being to encourage ductility in those elements capable of it and avoiding it in those that are not and could fail suddenly, or might lose their gravity load carrying ability.

With that approach the shear wall would be designed for the standard lateral load combinations and a hold-down and anchor rod selected based on those results. Since the hold-down/anchor rod provides some amount of the overall system ductility we'd like to promote that behavior. The selected anchor rod embedment would be that necessary to allow for yielding of the rod or for the level of force limited by the system capacity or OS factor force level combinations. Designing the anchor rod for itself for OS factors would result in a larger size anchor with reduced overall system ductility since it would allow for larger forces to be generated prior to obtaining the desired ductile behavior.

The podium slab would then be designed for the OS combinations or system limit level forces in both uplift and compression since it could loose gravity load carrying capacity if the force levels were sufficient to push the slab into the ductile range. To me this would include the slab reinforcing design as well as slab to column connections for punching/flexural shear.

 

[CURVEB]

This topic is a couple of months old but I have a project where this issue came up. I am in agreement that the podium slab needs to be designed either for the Omega factor or the capacity-limited horizontal seismic load effect per ASCE7. For the capacity of cold-formed strap-braced shear wall would you use AISI400 section E3.3.3 (clipped below) to determine the capacity of your system? Essentially you would evaluate the forces in the tension/compression chords based on the expected brace force per the equation below, and that could provide an upper-bound limit on the seismic force applied to the podium.