Building the Sun Cottage Part IV: Walls and Beams
(Page 2 of 3)
November/December 1983
By Angus W. Macdonald, M.Arch.
STRUCTURE
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In any situation where backfill will be more than 4 feet above floor level, standard building codes carefully restrict the length of an unbraced wall to 12 feet. This means that you must provide a cross wall or buttress, at that interval, along the rear wall of the home (see Fig. 2). Furthermore, whether they're built from concrete or masonry, these buttresses must be linked (with reinforcing steel) to the wall itself, since they'll
have to counteract the force of earth-called unbalanced fill in the code-piled outside of the wall.
Furthermore, there are other construction techniques that can help contribute the strength necessary to resist such stresses. For Example, have you ever noticed that it's much harder to push someone over from behind than it is from the front? That's because the length of a person's feet, sticking out in front, helps counteract the force from the rear. For this reason, we'll build the rear wall of our building on the outer edge of its footing. This will help the wall resist lateral forces resulting from backfilling, since the right-angled starter bars projecting from the footing tie the wall and footing together . . . forming a concrete version of a foot and leg. Thus the sideways force of unbalanced fill would have to lift up the entire wall and footing to push it over. As you can see, then, the actual weight of the wall and the roof (equivalent to our hypothetical person's torso and head) helps to stabilize our earth shelter's structure.
Special steel reinforcement, which we'll detail in a few paragraphs, is also required for retaining walls where unbalanced fill exceeds 4 feet in depth. Other walls (such as the south one and, perhaps, the east and west) need not be reinforced, although it's good practice to use web like lateral reinforcing material in every third course. (Don't put it in retaining walls, as it would get in the way of the vertical steel and grouting required for their reinforcement.) Finally; if you're building in California or Alaska (earthquake country), treat all walls as if they were retaining walls.
UP GO THE BLOCKS
If you're using solid concrete, the footings will be 16 inches wide . . . but the block work I'm going to describe here will require 24"wide supports. End walls, bearing partitions, and the front wall will all be centered on their footings-because unbalanced fill at these locations is normally less than 4 feet above floor level-while the rear wall will be set to the outer edge as described previously.
Before you begin, make a story pole from a straight piece of wood, and mark it clearly at 8" intervals to indicate the positioning of the block courses. (If you're dry-stacking, use 75/8" intervals.) Also mark the windowsill heights, door and window head elevations, and the bottom and top of the belt beam (I'll discuss more about this later). By varying colors, you'll be able to tell more easily which item is which. As you build, refer to this pole often-by setting it on the slab against the wall and checking course heights-to remind yourself of all the openings and features in the wall.
Type M mortar cement is best for use on the earth-sheltered walls, since it's much stronger than the type N used for the front wall and partitions. Mix one part cement to three parts clean builder's sand . . . and add clear water that's free from silt and debris. Use just enough water to form a workable, putty like mix, and don't settle for a soupy or a granular, dry mixture. Lay the blocks with full bedded joints (mortared all the way around), and scrape the excess mortar off the inside and outside so that the surfaces are flat.
Don't throw any excess mortar into the cavities, as it could later block the flow of vertical reinforcing grout. Also, don't use reconstituted (remoistened) mortar, because the added water reduces its strength . . . and never lay block or pour concrete at less than 40°F, since freezing temperatures will inhibit curing.
As you begin stacking, starter bars should be sticking out of the footing top (it was actually covered with one course of block in Part III) at intervals of 16 inches along the lines where the retaining walls will go. Carefully lay out the first two rows of block over the starter bars, and mark the locations of reinforcement-which are in the same cavities as the starter bars-on the side of the wall with a grease pencil, so you'll be able to recognize the right cavities in which to insert more steel rods later. The 8" end walls and buttresses will interlock into the 12" rear wall, so you should plan on cutting blocks to avoid creating long vertical joints at these intersections.
Start each course at a corner, always overlapping blocks so there are no continuous vertical joints, and use a plumb bob to insure that the intersections are vertical. A mason's line should also be nailed up temporarily from corner to corner for each course to help you keep the wall straight. When you reach window or door tops, set precast lintels over the openings. Lintels should overlap (bear) the walls at least 8 inches at each end.
If you plan to use recessed electrical service in your house, you'll have to choose from several options available for use with masonry and concrete construction. For one, the service lateral can be clamped to the outside face of the wall before you tackle the waterproofing and backfilling. Then holes can be formed in, or punched through, walls to accommodate the boxes. Conduit may also be inserted into the form work and secured, with the boxes in their correct positions, before you cast the concrete. Or conduit may be placed in a groove made in block webs . . . and left cemented in place in the masonry wall, again with the boxes positioned properly. The mason will chip or cut blocks to fit around recessed boxes.
Once the walls have reached the point where the top must be formed and the cores poured, insert No. 6 steel bars into the marked cavities in the rear wall and No. 4 bars into alternate cavities of any end walls where the floor is more than 4 feet below grade. Position all vertical rods to the 1 interior side of the cavities, about 7-1/2 inches from the exterior block face.
Concrete grout rated at 2,500 PSI can now be poured into the cavities directly from the truck hopper, if you have a good solid berm behind the house (and a cooperative driver). Concrete grout has finer aggregate than does normal concrete-which should lessen the chance of stones getting caught between the steel and the cavity walls-and it may be mixed with more water than usual for ease of placement. In this case, it's best to avoid a stiff mix, or the cavity bottoms won't be filled. Too much water, however, will make the grout too weak. In any event, have at least one other person on hand to rod (vibrate) the cavities to insure complete dispersal of the concrete.
A bond (or belt) beam must be formed around the top of all earth-bermed or - sheltered walls to tie the block together. To prepare the beam, create reinforcing cages consisting of four No. 4 bars linked with wire to each vertical reinforcing bar, and anchor them between 2 X 12 forms tied and braced to the top of the wall. These forms must also be built with a slope that conforms to the roof pitch on the end walls, as shown in Fig. 3. Stuff heavy paper into the cavities that you wish to keep hollow (to prevent wasting concrete grout), fill the beam forms with 3,500 PSI concrete, and level the tops.
Finally, embed 3/8'' X 10" bolts upside down into the belt beam or the top of the poured wall, in such a way that they project between 2 and 3 inches and can act as anchors for the treated sills. The fasteners should be 8 feet or less on center (o.c.) and no more than 2 feet from any corner or end. To keep them in place during the pour, the bolts may be wired to the reinforcement.
It isn't necessary to form a bond beam for the top of the front wall, on solid concrete walls, or with interior walls. In addition, when dealing with the end walls you may lay special U-shaped bond beam blocks on the top of the wall and place a pair of No. 4 bars, linked in ladder configuration, in the trough. Also, if you plan to pour a concrete roof slab, a bond beam won't be required, because the roof itself will act as a diaphragm, tying the top of the walls together.
On an earth-sheltered house, raising the walls is a particularly important stage. Getting the job done correctly will guarantee years of security (and assure that your floors will stay dry!). And the next step-erecting the roof supporting structural frame-will put a shelter over your head. Having a place to work out of the weather is a landmark for any builder, so be sure to be with us for Part V!
