jfmann

In my 34 years experience in structural engineering......including the last 18 as a consulting engineer.......I have found that (other than when a collapse occurs) it is quite difficult for the vast majority of those involved with buildings to grasp the reasons we "overdesign". Also.........this "overdesign" tends to encourage practices (by contractors, owners and even architects and engineers) that counteract or "use up" the margin of safety, before it is needed. Many contractors (and owners) take the approach that, as long as the building or structural element (component) is standing now....supporting only the weight of the building.........it must be fine. However that is the wrong standard for judging structural capacity. In general, we design and construct buildings to last, literally, a lifetime (or more). We expect the building to support and resist not just "normal" everyday loads........but also extra-ordinary loads, such as, for example, 2 feet of snow on the roof.......a hurricane........crowds of party-goers packed tight on a deck.......or on a walkway in a Kansas City hotel (1981).......and even (at least in the US) earthquakes. In the event of hurricanes and earthquakes......thousands of buildings could be destroyed at the same time.......with severe injury and death (see; Haiti earthquake). On a more down-to-earth level for a particular house......many conditions that we assign to the "defect" category can be lived with.......sometimes for many years........because that defective element never has to be tested by conditions that we design for, based on long experience (over many years that can exceed the life experience of any single person) that shows ......one day........an extra-ordinary loading is bound to eventually occur. The problem is that some of these defects will eventually have to face the test.........yet we can not predict where and when.

As to undermining shown in photo.......which is relatively minor......simply fill in void with concrete. Any such condition should always be noted in an inspection report. To lower basement floor.....basic approaches are; (1) Bench wall, if loss of space for new (lower) floor is workable, or (2) Underpinning existing wall / footing with new concrete. I have been called out on several foundation collapses caused by contractors exposing too much soil along foundation walls........trying to underpin a wall and installing a drain line. In one case, the contractor explained how, suddenly, a very large segment of thick stone wall "exploded" into the basement, nearly crushing himself and coworker.........he was obviously shaken and would not go back into the basement to install heavy timber shoring that was eventually installed (by others) to allow for major repair work. In another case, the base of a long concrete foundation wall simply slipped inward and downward into the basement, breaking away near each end. Amazingly (though this happens), the wood-framed house wall remained intact, although it did sag about 2 inches. The wall essentially acts as a "deep beam" (at least for a while!)

Great photos. Hopefully first floor has not already collapsed! Strange assembly ........with two layers of concrete "brick" in first photo. In second photo........pier in distance appears to consist of 3 layers of concrete brick between four full block........makes one wonder. In first photo.....is top block at least filled solid? Any footing that may be wider than the pier?

For bearing of floor joists on wood girder, I recommend the direct approach.....constant-thickness wood shims filling the entire gap. Plywood is perfectly acceptable assuming the gap is at least 1/4 inch thick. Bearing pressure will be very low. You may need to lift joists very slightly during installation. Lifting the girder seems completely out of line unless there is an obvious sagging problem............in which case there should be additioanal supports under the girder.........especially if the joists are not bearing on the girder to being with! It is important that the top of the girder is braced laterally.........by connection to at least a few of the floor joists at regular intervals....say 4 feet or so. At the girder, there should also be some bracing to prevent bottom of girder from "kicking" out.........especially if the girder is relativley deep and narrow. If there is space available.........wood "keeper" blocks usually work just fine, nailed to treated wood plate on top of support pier. For bearing of a girder on block pier........treated wood plate is best for new construction, with block pier filled solid. If foundation piers are tilted noticably........they were probably built on compressible soil that was loosened during construction but not removed. Or......the girder is applying highly eccentric load.....off-center. Either way, the pier might have to be replaced..........or, another pier installed nearby.

Another reason for roof failures due to recent snow may be the occurrence of snow drifts on the roof.............which may not have occurred before, at least to the current extent. A roof should be designed for "unbalanced" snow ........in accordance with standard code provisions (ASCE 7).......to take account of drifting potential.

Increase of snow density with depth of snow is taken care of (in ASCE equation).......indirectly.........by using ground snow load as a proxy for snow depth on a roof. More important is the fact that "wet" snow is much more dense / heavy than dry snow....which is not entirely reflected in the ASCE equation. The following quote from a July 2000 research paper (Density of Freshly Fallen Snow in Rocky Mountains is interesting; "From studies that have been published, the density of freshly fallen snow in the United States varies from 10 to about 350 kg m#8722;3 (Diamond and Lowry 1954; Wilson 1955; LaChapelle 1962; Judson 1965; Grant and Rhea 1974; McGurk et al. 1988; Doesken and Judson 1997). Other pertinent contributions to the literature came from Oda and Kudo (1941), Bossolasco (1954), Power et al. (1964), Gunn (1965), Stashko (1976), and Meister (1986). Most of these publications are not in mainstream meteorological literature, and the results have not been widely distributed among meteorologists. For this reason, and because of the general dependence of operational meteorologists on gauge data, snow density has not been available to most operational forecasters."

Density of snow can vary greatly. The governing code for determination of building loads (ASCE 7) includes a basic equation (7-3) for density of snow (in pounds per cubic foot)....[ 0.13 times "ground snow load" + 14 (not to exceed 30 pcf) ]......so that, if ground snow load (per standard snow map) is 20 psf, snow density is 16.6 pcf ........and 12 inches (one foot) of snow weighs 16.6 psf. In New Jersey......ground snow load varies from 20 psf to 50 psf per standard map in the NJ UCC (Bulletin 94-. Failure from snow is more likely due to inadequate connections at low end of rafters........than by failure of the rafters in bending. For standard rafter construction.........outward thrust force at low end of rafters can be quite substantial........especially for low-slope roof (4 on 12 or less). Typical nailed connections between low end of rafters and attic floor joists can and do fail .........especially if there has been decay of wood due to roof leaks. New article (with diagrams) about gable roof framing will soon be on structural101.

Cracks in any material are due to movement of some sort. Concrete and masonry (brick, block, stone) can crack from very very small movement (especially masonry)......and also from "normal" shrinkage, which occurs as excess water in the mix (concrete and mortar) evaporates. In general.........horizontal cracks (which are indicative of excess lateral pressure pushing inward) in block or brick walls should always be noted as a defect...........with further action to be taken. Most often (but not always), horizontal cracks occur along with inward movement (vertical curvature or "bowing") of the wall. Such movement can most easily be observed by holding a 4-foot carpenter level (plumb) against the wall. With one end of the level on the floor slab, you will see a gap between wall and level if the wall has been pushed inward. You will usually see this at top of wall also with end of level pushed up against bottom edge of a floor joist. You might also see the wall tilted inward (instead of bowing inward). Horizontal cracks are caused by excessive tension stress on inside face of wall (at mortar joints which are notoriously weak in tension) due to bending of the wall in the vertical direction (between floor slab and first floor; though adequate lateral bracing from first floor can be problematic!) Near ends of wall (at corner)........lateral pressure causes "step" cracking instead of just horizontal cracking..........as the wall tries to bend both vertically and horizontally. Vertical cracks are (in general) due to vertical settlement or "normal" shrinkage.........though some vertical cracks (such as those formed as part of "step" cracking) can be caused by lateral pressure. Unless a vertical crack is relatively wide (say, more than thickness of a quarter or nickel), it is likely that settlement (or shrinkage) has stabilized. However, an inspector should note all obvious cracks. Downward settlement of a foundation must cause downward movement of the entire house......unless a gap develops between top of foundation wall and the house elements above (which can happen). Such downward movement usually causes house floors to slope. Therefore, if the house floors are not sloping in the vicinity of the suspected foundation settlement problem.........there really can not be all that much of a settlement problem.