Design of masonry infill walls 1

[MikeE55]

Just wondering if anyone else has noticed that Building Code Requirements and Specifications for Masonry Structures has added an appendix on design of masonry infill walls. Masonry infill walls are not allowed to be used as shear walls if they have any openings at all in the wall. Actually, the Masonry Designer's Guide has an example of a small mechanical louver in a wall which disqualifies it as part of the lateral load resisting system. These new provisions create a huge issue for stability of the typical steel frame building which uses masonry infill for stability. I don't understand why the Code committee chose such a drastic requirement - other than to take their explanation that inadequate research has been done on performance. I have seen this type of building used successfully for many years, and the Commentary itself says masonry infill has been used for over a century. Are we having failures occur that I don't know about? Sorry, this may not be a question as much as it is me venting about this requirement.

 

[a2mfk]

http://www.structuremag.org/article.aspx?articleID=1391

I don't have a copy of the 2011 ACI 530. But maybe the systems you are designing are not exactly "infill walls" as covered by this section of the code, and described in this article, which are basically portal/moment frames infilled with CMU. The new codes I believe address these as a composite system, where the infill wall (with no openings) acts like a diagonal brace. Pretty ingenious actually, I had never thought of it behaving quite like that. But now I understand if you are designing it like a composite system, you cannot have openings.

The way I read it, this would not restrict you from engineering "normal" CMU infill walls as shear walls, the way you always have. This section is meant to provide guidance if you are modeling your system as a composite frame and CMU system.

 

[MikeE55]

The system as I typically design it has structural frames which are not rigid. Stability is provided by the diaphragm roof distributing lateral loads to the endwalls, which are masonry infill - typically 8" cmu with a brick veneer. This section of the Code says you must identify which walls are "participating" and which are "non-participating." The participating walls are not allowed to have openings. I don't think they are referring to moment resisting frames which work compositely with the masonry. However, I think the solution may be to design all the steel as moment resisting, or to add x - bracing.

 

[a2mfk]

If I understand you correctly, I disagree. These are two different structural systems. The system you describe is typical and how most low rise steel and CMU buildings are designed in my experience. The steel frame is for gravity and is used to collect shear and transfer it into CMU stairwells and/or elevator towers, or into sections of CMU walls. In this system, by the code definition of infill, these are NOT infill walls, so thus would be "non-participating". They are not part of a composite frame and CMU system such as the one described and detailed in the article.

Infill wall systems, such as those described in the article, are composite systems with the frame and CMU working together. These WERE common 25+ and more years ago here in Florida in big-box and warehouse type structures with concrete beams and columns. I do not quite understand the article's explanation on lateral out-of-plane loads, I still think these structures would perform poorly in lateral loading if unreinforced.

 

[matth35]

My understanding of the MSJC 2011 Appendix B and the NCMA TEK 14-23 is that any and all masonry walls that fill in the open space of a concrete or steel framed building is considered "infill." The definitions of "infill" and "bounding frame" in the MSJC 2011 are pretty straight forward. This is a major design change to a very common building system. Why haven't we heard more from the design community on this issue? From what I have been reading, it seems to me that this is driven by seismic research. Maybe this should have been a seismic design category "D" requirement rather than for all infill construction.

I design a lot of schools that use steel gravity frame with masonry infill, and there are virtually no walls without an opening. In the past we have been designing all the walls that butt up to a column as a shear wall, but the new requirements for no opening in a participating wall is a very difficult design obstacle to overcome with a typical architectural layout. For the most part, I have no shear walls available to me. It seems to me that I am being forced into designing a braced or moment framed system.

All of my options negatively impact the architectural layout and the building costs. Either architects will have to provide walls with no openings or work around more bracing and larger columns. The owners are not going to be happy with these design restrictions and the additional cost impact.

 

[JLNJ]

I do not have firsthand experience with this new code provision, but I agree with a2mfk.

There are many older existing buildings with infill walls but the manner in which they act was know well known or quantified. It seems like the new standard is attempting to address the additional capacity due to the confining action of the bounding frame which most intuitively knew existed, but could not justify per the code.

I don't think this new provision keeps us from using masonry shear walls between other gravity structural elements unless we are trying to take advantage of their collective and composite confining properties.

 

[a2mfk]

Here is a treatise on the subject:

http://www.ncma.org/etek/Pages/ManualViewer.aspx?filename=TEK 14-09A.pdf&apf=1

I think you will see that this is a hybrid system that uses a steel or concrete frame around the CMU, and the two systems are complimentary. This is entirely different then a steel gravity frame system (pinned connections, no lateral resistance) that uses CMU shear walls (outside of the steel frames) such as stair and elevator towers for the lateral system.
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Frankly, I do not understand the economics of this system. Outside of the explanation of redundancy in the article in reference to progressive collapse (fire, explosions, etc), it seems like an overly-redundant system for your average low rise building at least. If you have CMU infill walls on a building, why not eliminate the steel columns and beams and make all the CMU structural? You can use concrete tie-columns and tie-beams when the masonry needs the "help". This is the system I am more familiar with for low rise where CMU exterior walls are desirable. Otherwise, most low rise in Florida that I am familiar with uses CMU in the stair and elevator towers and maybe a few other places for lateral, and steel gravity system.

I have been driving by the local university that is putting up a new 3 story building with a steel frame and CMU infill, and EVERY CELL is reinforced, and I still cannot figure this out. We are only in a 120/130mph wind zone ( new strength speeds), and these appear to be infill only, so this appears to be WAY over-reinforced. Unless they are designing it as a hurricane shelter or for explosions or something...

 

[concretemasonry]

a2mfk -

The reinforced core construction sounds exactly like the FEMA suggestions for projectile penetration. These suggestions are based on actual tests of walls subjected to a 12' long 2x4 fired out of air cannon (135 or 145 mph). Both CMU and poured concrete walls have steel at 8" on center. They have been doing these type of tests at Texas Tech (I think?) for many years. These type of walls are often used in "safe cells" in the tornado belt. After many many years a wall consisting of a steel plate sandwiched between layers of 3/4" plywood was able to have no penetration(life safety and not strength criteria).

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[MikeE55]

Many thanks for the thoughtful comments. a2mfk and JLNJ, I think you are right about the intent of the masonry infill section of the code.

 

[DCBII]

Masonry infill walls are generally considered a seismic no-no. Under moderate lateral loads they tend to separate from the frame (especially steel frames). They can cause stiffness irregularities and unanticipated load paths in the structure. They also add significant seismic weight to the structure. If you don't watch the wall boundary connections, they can also fail out-of-plane and leave you without a lateral system. An excellent reference for designing masonry infill walls for seismic loads is "Seismic Design of Reinforced Concrete & Masonry Buildings" by Paulay & Priestley. I would be cautious using this system for seismic resistance, paying special attention to the details.