The structural use of plastics in building

During the past decade the plastics industry all over the world has expanded at a phenomenal rate. Various developments have taken place, resulting in the introduction of new and increasingly better plastics materials. The use of plastics in building is at present confined mainly to the non-load-bearing elements. Considerable interest is focused now on structural and semi-structural applications, in many of which plastics offer advantages which are not possible with other building materials.

In Canada a garden shelter recently built in Strathcona Park aroused a considerable amount of interest. It was designed as a dome in reinforced polyesters, with overhangs on all four sides. It is supported at four corners only. The overall dome dimensions are 54 ft. and the height is 16 ft. The dome consists of thin-skin sections reinforced with ribs, which are integrally molded with each unit. The sections were epoxy bounded and bolted on the site. The material used was a fire-resistant polyester resin reinforced with a laminate. The total weight of this structure was 3,000 lb. It proved to be remarkably rigid and shortly after the erection resisted an 80 M.P.H. gale without any ill effects.

Current trends put a special emphasis on membrane structures in which bending is either minimized or eliminated entirely. A typical example is a hyperbolic parabolic. A special mention must be made of the studies at the Massachusetts Institute of Technology on the structural use of prefabricated plastics hyperbolic parabolic structural sandwich panels in the construction of an elementary school.

It is quite possible that during the next few years some new structural systems will be developed for which plastics will be even more suitable. This review can be concluded with the statement that plastics, so often called the materials of the future, already influence to a considerable extent contemporary architecture.

 

VOCABULARY NOTES

to confine ограничивать

load нагрузка

bearing опорный; несущий

to mould формовать; отливать

resin смола; канифоль

rigid жесткий; устойчивый

to bend изгибать; сгибать

semi полу…(первая часть сложных слов)

dome купол; свод

overhang нависать; свешивать(ся)

to bond связывать; соединять; сцеплять

to eliminate устранять; исключать

contemporary современный

EXERCISES

I. Read the text and translate it. Discuss what you have learned from it.

II.Translate into Russian:

1. Plastics can be used in this case.

2. This engineer will not be able to find the suitable method.

3. Why do you want to reinforce the dome?

4. This material is to be used for structural purposes.

5. Why has he not been allowed to buy those prefabricated units?

6. These advantages are to be considered.

7. Such elements cannot have been used.

III.Get ready to retell the text and make reports on the use of modern plastics in industry.

PRESTRESSED CONCRETE STRUCTURES

STRUCTURES

A structure is the part of a building that carries its weight, and for at least half of the world’s civil engineers, structures are most of civil engineering. We should also remember that anything built is a structure. (From an aero plane engineer’s point of view, an aero plane also is a structure.) A structure may be a dwelling house, or a pyramid in Egypt, or a dam built by beavers across a Canadian river. A building is a structure with a roof and much of civil engineering structural design is the design of building structures. The building as a whole is designed by an architect, particularly in a densely populated area. Every structural design includes the foundation design. The structural design itself includes two different tasks, the design of the structure, in which the sizes and locations of the main members are settled, and the analysis of this structure by mathematical or graphical methods or both, to work out how the loads pass through the structure with the particular members chosen. For a common structure, such as a building frame, many methods have been developed for analysis, so that the design and analysis will be relatively easy and may need to be performed only once or twice.

But for any unusual structure the tasks of design and analysis will have to be repeated many times until, after many calculations, a design has been found, that is, strong, stable and lasting. For the typical multi-storey structure in a city, whether it is to be used for offices or dwellings, the most important member which the engineer designs is the floor – for two reasons: it repeats all the way up the building, and it has the greatest effect on the dead load of the building.

VOCABULARY NOTES

structure конструкция; сооружение; строение

to carry выдерживать

dwelling house жилой дом

dam дамба

dense плотный

load (dead, live) нагрузка (постоянная, переменная)

strong прочный

stable устойчивый

lasting длительный

EXERCISES

I. Find the English equivalents for the following Russian words:

здание; вес; строительный проект; по крайней мере; плотно заселённые территории; размер; расположение; относительно; один раз; дважды; расчёты; многоэтажная конструкция.

II. Name the part of speech of the following words:

building; structural; is designed; includes; mathematical; graphical; relatively; may; calculations; greatest.

III. Put up 5 questions to the text.

IV. Make up a summery of the text (using questions of ex. III).

REASONS FOR PRESTRESSING

The development of reinforced-concrete construction has been very rapid. Its application in engineering structures and buildings began toward the end of nineteenth century. Within the period of about 50 years, it has reached a high qualitative level and applied to the most important engineering structures.

Up to present, steel construction, rather than reinforced concrete, has succeeded in holding the field of long-span bridges and other long-span engineering structures. The main reasons why reinforced concrete has been lagged behind steel structures may be found in the following:

a) The dead load of structure increases with the length of the span. This causes a sharp drop in economic efficiency of the structure when a certain length of span is exceeded.

b) Materials cannot be used efficiently in reinforced-concrete members subjected to bending. The use of high-strength reinforcing steel in unprestressed reinforced concrete is limited because of the considerable elongation of the steel under high tensile stresses. No economy can be achieved by applying high- strength steel in conventional reinforced concrete.

c) One of the fundamental principles of conventional reinforced-concrete theory is the proper transmission of stresses from concrete to steel so that the steel may be considered as an incorporated part of the concrete section.

In view of the consideration outlined above, steel structures are in a privileged position in the field of long-span construction, and reinforced-concrete structures of conventional design are unable to compete with them. To place reinforced-concrete construction in a wider competition with steel construction, it has been necessary to develop new ideas and systems for the application of reinforced concrete which are more economical and technically more refined than those previously used. One of these ideas was prestressing. Although not very new idea (the first attempts to apply prestressing were made in 1886), prestressing was not applied successfully in early days of its development. First of all, as the physical properties of concrete, such as shrinkage, plastic flow, etc. were unknown and compression strength was then less than 3,000 psi.

PRINCIPLES OF PRESTRESSING

 

The basic principle of prestressing is the induction of stresses in a concrete member before the dead and live loads are applied, so that these stresses act in the opposite direction to those developed by loading (dead, live, temperature). When the loads are applied, the resulting stresses from loading will be superposed on the prestresses. In this manner a more economical stress distribution over the cross section is obtained, and cracking in the tension face of the member can be better controlled.

Prestressing converts a concrete structure into a more homogeneous state with improved elastic capacity; the stresses and deformations caused by load can be computed quantitatively and qualitatively with satisfactory accuracy.

Another important advantage of prestressing is that, if the design load on the prestressed member is exceeded and cracks occur, the latter will be closed when the excess load is removed. Thus, as compared to conventional reinforced-concrete structures, maintenance costs are considerably reduced and the life of the structure is increased. So it is obvious that prestressed reinforced-concrete structures are now able to compete with steel structures in wide range of spans and applications.

 

VOCABULARY NOTES

prestressing предварительное напряжение (бетона)

span пролет

long-span (beam) длинная балка

load нагрузка

dead load постоянная нагрузка

live load переменная нагрузка

stress напряжение

tensile stress растягивающее напряжение

shrinkage усадка

plastic flow пластическое течение

to compress сжимать

compression strength сила сжатия

to superpose совмещать; накладывать (одну вещь на другую)

to crack образовывать трещину

cracking растрескивание

elastic capacity мощность на растяжение

excess load перегрузка

EXERCISES

I. Read the texts “Reasons of Prestressing” and “Principles of Prestressing”. Translate them into Russian.

II. Answer the following questions:

1. When did the application of reinforced-concrete structures begin?

2. What are the main reasons for using steel structures?

3. Why was it necessary to develop new ideas and systems?

4. What are the principles of prestressing?

5. What is the main advantage of prestressing?

III. Make the following sentences simple.

1. Up to present, steel constructions, rather than reinforced concrete, have succeeded in holding the field of long-span bridges and other long-span engineering structures.

2. The use of high strength reinforcing steel in unprestressed reinforced concrete is limited because of the considerable elongation of the steel under high tensile stresses.

3. One of the fundamental principles of conventional reinforced-concrete theory is the proper transmission of stresses from concrete to steel so that the steel may be considered as an incorporated part of the concrete section.