WHAT ARE SERVICE LOADS IN STRUCTURAL ENGINEERING? BASIC CONCEPTS

In designing structural members, designers should use whichever is larger of the following:

1. Loadings specified in the local or state building code.

2. Probable maximum loads, based not only on current site conditions and original usage of proposed building spaces but also on possible future events.

Loads that are of uncertain magnitude and that may be treated as statistical variables should be selected in accordance with a specific probability that the chosen magnitudes will not be exceeded during the life of the building or in accordance with the corresponding mean recurrence interval.

The mean recurrence interval generally used for ordinary permanent buildings is 50 years. The interval, however, may be set at 25 years for structures with no occupants or offering negligible risk to life, or at 100 years for permanent buildings with a high degree of sensitivity to the loads and an unusually high degree of hazard to life and property in case of failure.

In the absence of a local or state building code, designers can be guided by loads specified in a national model building code or by the following data:

Loads applied to structural members may consist of the following, alone or in combination: dead, live, impact, earth pressure, hydrostatic pressure, snow, ice, rain, wind, or earthquake loads; constraining forces, such as those resulting from restriction of thermal, shrinkage, or moisture-change movements; or forces caused by displacements or deformations of members, such as those caused by creep, plastic flow, differential settlement, or sideways (drift).

SCRAPER PARTS AND DETAILS BASIC INFORMATION AND TUTORIALS

Scrapers ~ these machines consist of a scraper bowl which is lowered to cut and collect soil where site stripping and levelling operations are required involving large volume of earth.

When the scraper bowl is full the apron at the cutting edge is closed to retain the earth and the bowl is raised for travelling to the disposal area. On arrival the bowl is lowered, the apron opened and the spoil pushed out by the tailgate as the machine moves forwards.

Scrapers are available in three basic formats:

1. Towed Scrapers † these consist of a four wheeled scraper bowl which is towed behind a power unit such as a crawler tractor. They tend to be slower than other forms of scraper but are useful for small capacities with haul distances up to 300.

2. Two Axle Scrapers † these have a two wheeled scraper bowl with an attached two wheeled power unit. They are very manoeuvrable with a low rolling resistance and very good traction.

3. Three Axle Scrapers † these consist of a two wheeled scraper bowl which may have a rear engine to assist the four wheeled traction engine which makes up the complement.

Generally these machines have a greater capacity potential than their counterparts, are easier to control and have a faster cycle time.

To obtain maximum efficiency scrapers should operate downhill if possible, have smooth haul roads, hard surfaces broken up before scraping and be assisted over the last few metres by a pushing vehicle such as a bulldozer.

Typical Scraper Details