Intro to Masonry-Timber Design, CE 3253.001, 12/5/2007

Group Members

• Alejandro Diaz
• Cliff Rasmussen
• Epifanio Ruiz
• David Wagner

1. Cover sheet.
2. Table of contents.
3. Introduction.
4. Project summary.
5. Construction procedures.
6. Conclusion.
7. Acknowledgments.
8. References.
9. Appendix I: Answers to questions.
10. Appendix II: Site location map and site photographs.
11. Appendix III: Drawings.
12. Appendix IV: Miscellaneous.

This report is about the replacement of a McDonald's fast-food restaurant with, unsurprisingly, another McDonald's fast-food restaurant. Unusually, the building is a load-bearing masonry structure. Although it is neither grand nor elegant, this erection at the corner of Valley Hi Drive at Cedarhurst Drive does appear to be an economical way to provide McDonald's USA, LLC with a continuing presence near Lackland Air Force Base. In addition, this structure does have its points of interest.

In addition to chatting about the project and answering some basic questions, Mr. Marbach let us have free run of the construction site for a few minutes. We were able to examine the structure in detail, then return to add a few photographs to the ones provided by Sergio Aranda when he visited the site a few weeks earlier. Unfortunately, it took until December Third to get in contact again with anyone involved in this project.

After our initial visits, Mr. Marbach no longer returned our calls. In addition, it took a very long time for McDonald's to route our request for information. We hope it is not too much of an inconvenience, but this report would be little more than speculation had we not been able to include what we learned from Mr. Pensy, McDonald's Project Structural Engineer, just yesterday.

There is little danger of being overwhelmed by the grandeur of McDonald's upcoming replacement of its southwest San Antonio location. This modest structure barely qualifies for this project since it doesn't even use any timber or lumber, but a load-bearing masonry retail building seemed unusual enough to justify this examination, and the structure does have its points of interest.

Most notably is the massive foundation necessary to support a structure in this part of San Antonio, Texas. The soil is bad, very bad clay, and the water table comes near the surface from time to time. This presents quite a bit of variable loading as the soil shifts around the building, and although analysis of the reinforced concrete foundation design is beyond the scope of this report, Doug Marbach the Site Supervisor, surprised us when he described the forty-two inch (42”) deep reinforced-concrete foundation beams sitting on similarly-comprised piers buried twenty-two feet (22') deep into the soil. Even the 6” double-mat slab is a bit stronger than is usual for a small one-story retail structure. Both Mr. Marbach and Paul Pensy, Project Structural Engineer, confirmed our conjecture this massive foundation was necessary because of the poor clay soil at this site.

Sitting on this impressive configuration of concrete and steel are what are essentially two separate structures. These two structures, tied together with a steel frame, attempt to reconcile the windowed-look McDonald's prefers for its establishments to present to the world with the reality of the lateral force resistance needed of the building. In the front part, those few customers who do not use the drive-through will wait to order, interact with the staff, wait for their orders, and may stick around to chow down. All this time they can use the large windows provided to stare at passing cars, and perhaps think about how they would rather be elsewhere. If this makes, on average, fewer people spend spend less time inside, McDonald's can serve more burgers from smaller structures on less real estate, and so be more profitable. Meanwhile, this relatively flimsy portion of the building is subject to external loads, which it transfers through a steel 'K' frame to the back of the building, where the work is done.

The rear of the building is more solid and enclosed so as to resist both the loads imposed on it and the loads transferred from the front, where the walk-in customers are serviced. The rear of the structure is made of shear walls, and they are grouted as necessary to resist the loads imposed. Most of the building is grouted at a standard forty-eight inches (48”) on-center, where reinforcement is located. The rearmost wall is exceptional. It bears a somewhat greater load and is grouted every forty inches (40”). This extra reinforcement is necessary to resist the shear forces transferred from the rest of the building all the way to the back.

The front part of the structure, where the pedestrian public encounters this building, is little more than a facade providing a flimsy umbrella under which customers huddle briefly with the staff. Wind and seismic loads applied to the front walls are transferred through the steel 'K' frame to the rear masonry walls. Similarly, loads applied to the roof also transfer through steel members to the 'K' frame and into the rear section's masonry walls. The rear roof and walls are also under load, and these combine with those transferred from the front. The longitudinal walls are long enough to resist the in-plane shear applied to them, but the hindmost needs a bit more than the standard grouting to withstand the loads ending up all the way back.

The masonry used in this structure is hollow load-bearing block as required by ASTM-C90 and are grade N1. The mortar used below grade is is at least as strong as 2500 psi, and is of type M. The mortar used above grade has a minimum strenth of 1800 psi, and is of type S. In addition, its prism strength is at least 1500 psi.

But the public will not see a dull gray concrete block structure looming with fries inside. Most of the blocks used are textured and colored that fake 'Santa-Fe' adobe red with iron oxide and perhaps some proprietary pigments. Some blocks of contrasting color are arranged to give this stone box some 'interest', and perhaps to prevent it from looking too much like the sarcophagus one may enter prematurely if one attempts to sustain one's bodily functions with too great a proportion of the dead animal parts provided here. Nor will the public see the steel roof system and interior steel framing, the analysis of which is beyond the scope of this report, and of little interest, but is mentioned here for completeness of this, the project summary.

In short, and to sum up this summary (as it were), the main load-bearing system is made of the rear masonry shear walls. Loads on these, the roof, and the loads transferred from the front of the building and through a steel frame, are resisted by the fairly standard masonry structure of the rear part of this building. A massive foundation is then necessary to transfer the loads to the very poor soil at this site without allowing the masonry to crack and gradually or suddenly fall to pieces before enough profit has been made to justify the expense.

Among the expenses in evidence at this construction site on the southwest side of San Antonio near Lackland Air Force Base was the labor being done by masons, the site supervisor, and other assorted people making up the duly contracted and employed workforce constructing this building and detailed at exhaustingly tedious length in the next section, Construction Procedures.

We saw only a bit of how this simple building is being put together by a part-skilled, part-'unskilled', and presumably underpaid workforce. (This is San Antonio, Texas, a location not noted for the extravagant compensation provided those who do real work.) When we were there, we saw masons and mason's helpers selecting concrete masonry units from among the pallets of blocks delivered to the site. These masonry units were transported, sometimes by hand, sometimes in a wheeled vehicle, to a lift, since the masonry work being done was up near the roof. Masons and mason's helpers, and perhaps some 'unskilled' laborers used the lift to raise the raw materials up to near the roof, where the masonry work was being done while we were there to observe it.

In addition, a field mortar mixer rotated to keep the mortar pliable so it could be spread with ease (easy for a person skilled in the application of mortar, that is). The mortar was placed on hods to be carried to the lift where it was raised up to the level where it was in use while we were there to observe its use in constructing this load-bearing masonry structure.

Unfortunately for us, the observers, it was difficult to see to the top of the building, where masons applied the mortar to the bricks. We did, however, observe the crusty old master mason in charge of the masonry construction (with whom we later spoke) hang with apparent ease at odd angles off of the walls of this structure up near the top, at the roof level, where the masonry work was being done while we were there to observe it.

In addition to the masonry work we had come to observe, other duly contracted workers and laborers of other sorts (the description of which, strictly speaking, is a bit beyond the scope of this report but is described here for completeness) were at work on the front of the building. These gainfully employed people were working on steel framing at the front of the building, and used various hand and power tools to construct of steel parts a steel frame. This steel frame was neither timber nor masonry, nor was it of great importance to the structural integrity of this building, but rather it was being built by skilled and 'unskilled' workers to support wall panels meant to be more decorative than functional. Perhaps they did some work on the 'K' frame or another part of the load-transfer structure, but if they did so in our presence, we did not recognize the work as being on the load-transfer structure not on the simple steel framing components.

We did observe, and had occasion to speak with, the site supervisor, one Doug Marbach, in his site-supervisory trailer near the rear of the site. Although not involved directly in the construction of this building by laying hands on tools or on structural components, we observed the work he did indirectly on this structure. While we were there to observe, and waiting with patience for him to finish so we could have a word with him, Mr. Marbach spoke with contractors about construction of this building. During these conversations, the parties involved consulted the sheaf of engineering drawings spread out on the desk in the site-supervisory trailer. There was much pointing, some gesticulating, and occasional swearing, the exact words of which are beyond the scope of this report, though the expression of said words undoubtedly improved, albeit indirectly, the construction of this load-bearing masonry structure by stressing the importance of the topic under discussion.

When we talked to Mr. Marbach for what may have seemed to him the gajillionth time, he explained how the plans were often just plain wrong. He even showed us how we could see for ourselves how the fire hydrant was across the street as plain as day, and the plans showed it on this side of the street. This, of course, caused consternation and some expense, since water from the main that should have been accessible on this site is needed, among other things, for keeping the mortar in the mortar mixer moist enough to be spread easily by someone skilled in the spreading of mortar. Perhaps Mr. Marbach was sick of us after what may have seemed to him the gajillionth time we spoke with him, because after this he did not return any of our calls and we could not arrange a follow-up visit to this construction site.

We were able to verify by examining the photographs taken by our group and earlier by Sergio Aranda that the voids grouted in the concrete masonry units were likely to have been the same voids into which rebar embedded vertically in the foundation was placed. However, this gets into speculation and away from reporting what we actually observed being constructed during our field visit to a masonry construction site.

This simple load-bearing masonry structure had a few small surprises for us, including the massive foundation necessary because of the poor soil in this part of San Antonio and the odd combination of two substructures for public and work areas. The 'K' frame steel structure used to transfer loads from the relatively weak, glassy front of the building to the much more utilitarian rear part was of some interest.

Talking with the site supervisor (while he was willing to do so) was of some help, and speaking with the master mason on site was interesting. We learned from them how engineering plans are sometimes used more as guidelines than the legally binding documents they are nominally, and how this can be necessary just to make sense of them when they depart from reality.

Although late in coming, the project engineer was of great assistance, as is obvious from the direct quotations included in this report. It is, unfortunately, sometimes difficult to get from experienced engineers simple, clear, concise answers to the questions of naive engineering students. But perhaps it is like a paraphrase of the old saying, “Clear, concise, timely: pick two.”

We would like to thank those who helped with this project.

• Doug Marbach, Site Supervisor, for showing us around and introducing us to the unnamed master mason mentioned herein.
• Paul Pensy, Project Structural Engineer, McDonald's USA, LLC for answers to our questions.
• Sergio Aranda for showing us the site location and providing earlier photographs.

Besides some googling and Wikipedia references for jogging our memories, you can probably tell if you have read this far that we used no references other than our course materials to prepare this report.