San Antonio and Austin

Foundation Engineering,  

Concrete Press Piles Foundation Repair: Building Code non-compliance Part 4
b) Terminology Explained

          b.1) Lateral Load

Many people (and engineer’s prior to the 1950s) think there are primarily gravity loads. Gravity loads are vertical loads and the most common kind of gravity loads is the dead load (stuff you can’t move around, like the weight of the structure, walls, ceiling, etc.) and live load (stuff that you can move around, like people load, furniture, etc.).

There are also lateral loads. The most common types of lateral loads are Wind Loads and Seismic (earthquake) loads. In San Antonio, Austin, Dallas, and Houston (and almost the entire state of Texas) the controlling loads are Wind Loads so we will just talk about that.

Wind loads, as the name implies, are loads from wind simply pushing against a structure, and they are also determined according to the building code. The wind loads are different depending on many factors, such as where you are (in the hill country, or city, or near the beach or lake), what is the height of the structure (higher floors in a building are subjected to more severe wind loads), etc. Although even while living closer to the city the wind loads are probably not very frequent, their force against the structure is pretty high. For a typical one story house in the city (away from lakes or the ocean), the force exerted in one plane of the house could be around 6000 lbs (that’s about the weight of two small cars). All of this force eventually needs to find its way to the foundation. So, whether you have a new foundation or are doing a foundation repair, you need to remember a huge amount of lateral load will be pushing against your house.


           b.2) Moment, Shear, Deflection, and Buckling

All of these terms are related to each other mathematically. Moment, also known as bending, can be best visualized if you imagine grabbing a stick and trying to flex it (bend it).

Shear is a little difficult to visualize, but imagine having a bar of butter and holding half on the counter and the other half sticks out in the air, if you press against the bar with your hand closer to the part where the counter is, then the butter piece will shear off (if you would push against the free end of the bar then the bar would crack and it would be a more of a bending failure instead of a shear failure).

Deflection is what people commonly refer to as sag. If you hold a cable at both ends without applying tension it will sag, it will deflect.

Buckling is similar to deflection, but it is the deflection caused by an axial compressive load. Grab an uncooked spaghetti stick and push the two ends towards each other, the spaghetti will start to buckle (deflect laterally).

Any sound structural piece (and we mean any, not just used for foundation repair) is a piece that needs to be able to somehow resists moments and shears, and for serviceability purposes, these structural members need to be able to not deflect or buckle excessively (a cable resists moments and shears indirectly through internal tension in the cross section). A member may be structurally sound, but if it deflects too much or buckles too much, then the structure may not be useable. For example, your floor joists may be able to resists moments and shears but if they sag (deflect) too much, then although structurally they are acceptable, in terms of serviceability the joists would not be acceptable because people could feel unsafe walking on them. Think of a fishing rod, when the fish grabs on to the line, the rod flexes considerably, but doesn’t break; the moments and the shears are resisted by the rod, but the deflection is excessive.  

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