Wednesday, May 23, 2012

Strength of Materials: Simple Stress

Simple Stress - is the intensity of force inside a solid material. The object is influenced that may lead to breakage or change on it's physical form. 

It is force per unit area, which is basically Pascal (Pa) Newton per square meter or Megapascal (MPa) which is Newton per square millimeter.

The stress is spread out in the entire cross-section of the item that reacts to the external force or load applied.

Different Kinds of Internal Forces:

  • Axial Force - pull and or push action that is perpendicular to the cross section. Pull represents tensile force that tends to elongate the subjected member while Push represents compressive force that tends to shorten the subjected member, load usually denoted as P.
  • Shear Force - total resistance to sliding the portion to one side of the exploratory section past the other in vertical or horizontal manner, usually denoted as Vx for horizontal shear force and Vy for vertical shear force.
  • Torque - twisting the member either clockwise, counterclockwise or both, usually denoted as T.
  • Bending Moments - resistance to bending the member about any axes, and are often denoted as M or Mx for moment about x-axis or My for moment about y-axis.
Studying material's strength is relevant to guarantee that the structure to be used will be safe against maximum internal forces that is produce by combining different kinds of loads. 

Monday, May 21, 2012

Surveying: Stadia Method

Stadia - came from Greek word for a unit of length applied in measuring distances for athletic contests. It denoted 600 Greek units, or 184 meters 93 centimeters or 606 feet and 9 inches as calculated by present-day international standards.

Stadia is the plural of stadium. It is applied to the cross hairs and rod used in making measurements and method. Readings can be taken with almost all surveying instruments such as engineer's level, alidade, theodolite and engineer's transit.

Stadia consists of telescope with two horizontal hairs called stadia hairs and a graduated/stadia rod. Distances can be measured rapidly by stadia method. By observing through  the telescope the apparent locations of the two stadia hairs on a vertically held rod. From the observed interval, the distance from the instrument to the rod is readily computed.

This method can also adapted in mapping requirements and is widely used for locating details and contour points in topographic surveys. More rapid than taping, and under certain conditions could be made as precise. It requires the employment of fewer survey personnel.

Saturday, May 19, 2012

Surveying: Tacheometry

Tacheometry - a procedure to obtain horizontal distances and differences in elevations based on the optical geometry of the instrument employed.

It uses subtended intervals and angles observed with an instrument like transit or theodolite, on a graduated scale or rod as a rapid and indirect way of measurement.

A relative accuracy of 1/300 up to 1/500 can be obtained for most horizontal measurements, and differences in elevation to within plus or minus 3 centimeters. This type of measurement also have a lower order of accuracy as compared to taping and differential leveling.

Usage of subtense bar for tacheometric measurements consists of a 2meter long bar mounted horizontally on a tripod aligned perpendicular to the line by means of a sighting device on top of the bar. The horizontal angle subtended by the two sighting marks on the bar is hence read by a transit or theodolite and by trigonometry the distance is computed.

This method may also employs the sighting of a telescope of an instrument in reading a small angle along a vertical plane and in determining the length which the angle subtends on a graduated rod held vertical on the distant point.

Tachemetric methods are used
  • to check the more accurate taped distances to uncover gross errors and mistakes
  • to determine differences of elevation between points
  • to carry lines of levels where a relatively low order of accuracy is required
  • to measure lengths of traverse lines
Its most general use is found in compilation of planimetric and topographic maps, in field completion surveys for photogrammetric mapping and in hydrographic surveys' location of details.

Monday, May 7, 2012

Foundation Engineering: Functions of Piles

When the soil bearing capacity is too low, then the land is too weak or too compressible to provide adequate support, hence the loads including dead and live loads are to be transferred to a more suitable material at a greater depth by drilling in piles of piers. 

Piles - structural members of small cross-sectional area compared to their length and are usually installed by a vibrator or hammer. Piles are grouped into clusters or rows each containing enough members to support a load delivered by a single column or wall

Piers - usually larger in cross section capable of transferring entire load from a single column to the supporting stratum

Columns with not much load can require just one single pile. However, field conditions should be considered too like the actual position of a pile that may be several inches away from planned location hence eccentricity of load may occur and can hardly be avoided. Consequently, the heads of piles are usually braced in two directions by grade beams. 
If only two piles are needed, heads may be connected by a concrete cap braced by grade beams in only one direction perpendicular to the line joining the two piles.
Three or more piles clustered together shall be provided with reinforced cap and are considered stable even without the support of grade beams.

Aside from its original purpose, vertical piles can also resist lateral loads such as winds and lateral earthquake at certain point. But when larger lateral loads are to be resisted, an inclined or batter piles are more applicable. A batter of four horizontal and twelve vertical represent about the greatest inclination that can be achieved with ordinary driving equipment. Economically, smaller inclinations are more favorable even if it requires more piles to be battered.

Foundation Engineering: Soil and Rock Definition

Engineers always use terms soil and rock in implicating a clear distinction of two different kinds of foundation materials.

ROCK - is a natural aggregate of mineral grains connected by strong and permanent cohesive forces

SOIL -  is a natural aggregate of mineral grains that can either be with or without organic constituents and can be separated by gentle mechanical means such as agitation in water or strong winds

However, on actual site condition, one can never sharply distinguish rocks from soil because they usually come together. Even the strongest and most rigid rocks may be weaken by natural processes such as weathering and some highly saturated soil may exhibit strengths that is comparable to those of weathered rock.