Shear Wall vs Portal Frame Systems/Foundations

[StrEng007]

This is a long one with lots of questions...

I'm trying to determine the advantage/disadvantage between two different types of lateral systems on a 2-story building with a CMU/Concrete Tie-Beam shell. Both buildings use a stem wall type foundation where top of footing is located 1'-4" below grade. My issues are specifically to do with the 1st floor and supporting foundation.

The building experiences high winds, no seismic, and is built on non-cohesive soils (sand). Shallow foundations are the accepted practice.

System #1:

The first floor is comprised of three shear wall segments made from reinforced CMU. Unfortunately, the wall openings don't allow for balanced proportions of each shear panel and the middle segment takes on a majority of the lateral loading. Each shear wall ties to a 24" deep concrete tie beam at the 2nd floor diaphragm. When determining the lateral force distributions for each shear panel, I utilized the approach where the top of the wall is considered to be fixed (Relative Rigidity of Fixed Walls/Piers). The headers at each opening are framed with pre-cast lintels that allow for a modular system and alleviate some need for formwork.

The foundation is a shallow wall foundation modeled as a beam on elastic foundation. After carrying down all my gravity loads and lateral loads, this wall footing ends up being much larger than I anticipated. Part of the issue, from my understanding, is due to the elastic beam not 'being allowed' to have any tension along it's length. Understanding that soil cannot resist tension loading (performed via Enercalc) the foundation either conservatively works or doesn't work at all.

System #2

The second system considers four individual beam-columns that are monolithic to a portal frame that supports the 2nd floor/roof. My approach is to design the portal with pinned bases to alleviate some of the moment imparted to the top of foundation. This approach yields isolated footings that are easier to digest than the large wall footing required for system #1.

Although system #2 provides more desirable foundations, I'm not sure how reasonable my design assumptions are. The questions I'm trying to answer for myself are:

1. Is there an alternate way to design the foundation for system #1? I tried looking at each shear panel with an individual spread footing, but the footprint gets so long that they all merge together. Also, shallow foundations will be the only accepted approach as far as the GC/construction. Deep foundations are out of the question.

2. How reasonable is it to assume that system #2 will achieve pinned bases? I typically fully develop all vertical bars with equivalent hooked bars placed in the foundation. Logic tells me that each beam-column is neither fully pinned or fixed. Without the aid of advanced FEA software, how do I rationally go about determining what's actually happening? I've seen a lot of engineers call a concrete column pinned at the base but still shown full development of bars.

3. For system #2, if the middle segment of infill CMU is not isolated from the concrete with some sort of joint, will it become a shear wall segment? My details typically consider the horizontal joint reinforcing (ladder joint) to be continuous into the adjacent concrete column. The shell contractor will erect CMU first, then pour the concrete columns against the existing CMU.

4. For both systems, how do you all handle the out of plane loading requirements while designing your MWFRS lateral systems. My understanding of ASCE 7 requires dual loading on these walls. Let's say this LFRS line is carrying transverse wind loading, then logically there would be side wall loading the shell/doors/windows with out of plane negative flexure/shear.

5. I see a lot of building similar to this where I practice... most of which would have utilized a 24" wide x 12" wall footing (Allowable soil bearing 3,000PSF). I'm not sure what approach these engineers are taking, short of ignoring the lateral demand all together. For a location that experiences such high winds, I'm often surprised at how small the foundations are. Similarly, how often chord steel requirements are not called out on CMU shear walls.

 

[Aesur]

Typically I would design this as a shear wall with piers and distribute loading based on rigidity, you will probably find that most of the load will go to segment 2. There are rigidity tables out there in some publications that simplify this design method. For the footing, I would consider burying the foundation deeper and relying on overburden to resist some uplift and overturning.

When you say experiences "no seismic" can you explain that a bit more, every building experiences seismic, and I have found that in many cases Masonry buildings can be controlled by seismic due to mass of the building even in low seismic country. I see you appear to be located in SC, so maybe you are coastal and have hurricane winds in which case maybe wind still controls?

I believe the Masonry shear wall module in EnerCalc can design foundations as well. What is your force into this wall line? I have seen plenty of buildings with sim footings as you described, however many are single story and have more shear walls, it is my opinion that 24" x 12" for a 2 story building with the layout you show above may be small.

If you go with system 2, I'm curious of the impacts on costs of the building as it appears you would be introducing concrete moment frames into the design. This seems like a potential construction sequencing concern and potentially added cost versus a larger foundation. It is my opinion that it would be the more expensive option. As you mention in #3 I believe without isolating the masonry from the concrete you are essentially back to a shear wall from a stiffness standpoint, but with more complexity with 2 different systems.

For question 4 - I would consider analyzing the wall for the loading and maybe consider a unity equation for out of plane due to MWFRS in combination with shear wall loading.

 

[StrEng007]

By no seismic loading, I mean this building is located in a location with SDC A and 175MPH wind velocity. Wind governs.

The masonry shear wall module in Enercalc does design foundations... but like I mentioned before, trying to design an individual footing below the center shear wall will require a footprint long enough that it picks up the other shear wall along this gridline. The total base shear for this gridline is approximately 30K, with an OTM of 540 K-FT.

Good idea for point #4.

 

[Aesur]

That's what I figured regarding loading. For the foundation, have you considered using the combined footing module? You will end up with top and bottom reinforcing, but I believe that would be fairly negligible costs compared to other options.

 

[StrEng007]

The combined footing module is valid for only (2) columns on a single combined footing. This is (3) individual points of loading.

I have accepted the beam on elastic foundation approach as I've verified the results with some misc. hand calculations. It's conservative and accomplishes what I need.

Going back to my list of questions:

1. Resolved by accepting the beam on elastic foundation approach.
2. I went with the first method using (3) individual shear wall/piers. However, my question about fixity still stands and I'm still searching for the right approach.
3. I was able to find some documentation from NCMA regarding 'non-participating infill'. Still giving this an overview.
4. An applied unity equation seems like the most logical approach. Currently, none of the US standards/specifications illustrate how to do this type of analysis.
5. I'm sure we could talk about this all day. I figure those engineers who have designs that are clearly lacking don't spend as much time on this forum as the rest of us?

 

[Aesur]

To be 100% honest with you, most of us, including myself would have put all the load into the middle pier, designed that as the only shear wall and designed an appropriate footing, probably something like 4' wide x 24" thick that extends 8' or so each side and called it a day; fees are not high enough to waste a lot of time on fine tuning little things like this.

As for the combined footing, you could input as moments, design just two piers as point loads with moments and you would have a conservative design IMO. The wall itself just add stiffness and strength to bending in the footing. When designing you could take the extension past the piers as CL of the pier to CL of the opening for 2 of these, and then just copy the design for the length of the additional pier.

 

[StrEng007]

I totally understand what you're saying. This is a constant balancing act in a jurisdiction that is extremely litigious.

The jurisdiction this building is located requires calculations to be submitted. The BO who review these jobs are also licensed PEs and at time require explanations to designs. I've even had to do it face to face on several occasions. I've found it best to always tackle these items with proper calculation techniques that you can prove.

Just as a side note, the 24" thick with 8ft extension each side doesn't even begin to scratch the surface for what is required. The length is much longer. I appreciate all the feed back though!