THE ULTIMATE-LOAD METHOD OF DESIGN

At present structural members are designed to resist the moments and forces caused by the loads that they are designed to carry without the stresses produced in the material being greater that the safe working stresses. To ensure safely these working stresses are chosen as a fraction of the ultimate strength of the material, the ratio of the ultimate strength to the working stress permitted being the factor of safety.

In many simple structures the stresses produced in the material increase in direct proportion to the load and, for such structures, if the factor of safely were 3 the structure would carry three times the design load before it failed.

There are, however, many cases in which the stresses produced do not increase in direct proportion to the load. For example, a load column may bend slightly as the load increases, causing the maximum stress in the material to rise more rapidly than if it remained straight.

On the other hand, in the case of a reinforced concrete frame-for a building the required to cause collapse might be greater than that expected from the factor of safety of each part of the building, because settlement of foundations and yielding of the members would result in the load carried by highly-stressed members being shared with adjacent members which were not so highly stressed. For this reason it is probable that for most structures the method of design using the actual loads and safe working stresses calculated from the ultimate strengths of the material for each part and the factor of safety results in some parts of the structure being capable of carrying a greater load than others. Such a design is inefficient; for example, if column collapses the capacity of the-beam which it supports to carry even a greater load is of no value.

The aim of the ultimate-load method of design is to produce a “balanced” design so that all members would fail simultaneously. The true measure of the safely of a structure is the ratio of the ultimate load causing collapse to the working load.

In the ultimate-load method of design the load that would cause the failure of each member of the structure is decided by multiplying the working loads by a factor of safety, and the sizes of the members are based on the ultimate strengths of the materials and theories of the behavior of materials when they are in a plastic condition.

The ultimate-load method is claimed to enable economies to be made in the design of many parts of a structure, for example in the case of concrete members with compressive reinforcement, and to make possible the building of structures which have a uniform factor of safety throughout.

In order to obtain values for the internal forces or stresses in terms of the applied loads and the dimensions of the member one has made some assumption, just as Galileo did, with regard to the stress distribution in the body. Frequently, this assumption is based on knowledge of the strain distribution which is obtained experimentally. The stress distribution can then be found by use of Hooke’s law which states that stress is proportional to strain. For most engineering materials, this law is a good approximation over a wide range of stresses, although the specific range of validity has to be determined experimentally for each material.

 

VOCABULARY NOTES

1) load – навантаження

2) to ensure safety –гарантувати безпеку

3) ultimate – головний

4) reinforced concrete frame – залізобетонна рама

5) to come collapse – статус причиною падіння

6) simultaneously – одночасно

7) Hooke’s law – Закон Гука

8) internal forces – внутрішні сили

9) to multiply the working load - помножити робоче навантаження

10) stress – напруга

11) strain – на тяжіння

 

EXERCISES

1) Find the English equivalents to the following word combinations:

чинили опір; напруга, що виробляють матеріали; збільшувати; з іншого боку; фактор безпеки; неефективний; навантаження , що не є цінним; міра безпеки; інженерні матеріали; 

2) Answer the questions:

1) How are the structural members designed?

2) What is the reason for choosing the working stresses?

3) Why, in the case of a reinforced concrete frame the load might be greater then of each part of the building?

4) What’s the aim of the ultimate – load method of a structure?

5) What’s the true measure of the safety of a structure?

6) What are the sides of the members based on?

3) Translate into English :

1) В багатьох простих спорудах напруга, що виробляють матеріали, звеличується в прямих пропорціях до навантаження.

2) Існує багато випадків, в яких напруга не звеличується в прямих пропорціях до навантаження.

3) Метою основного вантажного методу дизайну є виробити «збалансований» дизайн так, щоб всі елементи впали одночасно.

4) Часто, припущення базуються на знаннях розподілу на тяжіння.

5) Закон Гука звучить: «Сила напруги пропорційна силі натяжіння ! »

 

 

Text 14

 

⇐ Предыдущая234567891011Следующая ⇒

Поделиться: