r и о i i о. я .

1. Р о un : . t Ion s, П ill М о с h a n i с fl

and Site Explorations ■

The problems to lie polved In tho dosign and construction of foundations are i

1. required depth, of foundation,

2. safe bearing pressure, 5. amount of settlement, 4. eco nomical type of foundation. The first three of these can be found 'by site and laboratory investigation of the subsoil conditions, while bho fourth involves relative coate of the different types of foundation.

Various methods are enployed in sit© investigations, the most ■ .consnon being the -undlstiirbed boring and the loading tesij. Other methods widclx are not in general use in this country are i 1) resistivity method, nxid 2) the Beiamio method.

Another important factor to"bo deteratitied is the ground sater level. Ground water is generally caused by water which enters the soil during rains or thaws,and fills up tho pors sjace.of tho soils, sediments, etc.: up bo a cer­tain levsl called the "zone of saturation" and the zone above - the zone of aeration. ,

- The water-table is not/посввза-гЛу- ho^'isorrtral. .'Grotxnd water Пев t-elo'ffthe Tratcr-tablo, but there вау be a local zone оГ saturation above tho water-table. There is a cer— .-, tain amount of moisture above the water-table duo to oa~ pillary actipu eua this Гохтаз the capillary fringe» The oapillasy rise depends on the number of poree and is Inversely proportional to the pore diameter.

it вау be necessary to lower the level of the ground either teispoxarily or permanently. This con be dong, by,

driving sheet piling and pusaping. In certain caees,i.e. graving-docks, the lOTrerlrig snist bo porssanent on aocouni: of buoyancy and this is done by letting down perforated pipee into the surrounding aitbaoil and pumping.

2. А г о а в а .

Arches can Ъе classified under two- main headings* a) non~elastic and t>) elaatic arches.

Non-elastic arches are incapable of resisting bending and are visually built in masonry or brickwork. * -¹ It is one of the earliest forms of construction for bridges or roofs.

Elastic arches. This type waa first built in. iron. The elastic arch is designed•to x-eaist bending moment. It is necessary to understand the difference between , the arch rib ( that part of the arch which carries the ., load to the supports) and a beam curved or shaped in ela- -vation. In the latter case the.reactions ara vertical un­der loading. For an arch rib the tendency of the rib to spread under loading must be resisted by the horizontal cojaponenta of the reactions. Elastic arches can be,sub­divided thus: 1. three-hinged, 2. two-hinged and 3.binge--lese or fixed arches. Тура I is_Kstatically determinate, i.e. such erch.es c=jn ta analysed directly by the first principle? of mechanics. Јype 2 is statically indeter­minate, i.e. the value of the horizontal thrust cannot be found directly end depends on the elastic properties of the rib. -

Type 3 tea fixing moments at the supports ( like fixed beams) цо.prevent rotation of the aroh rib about the supports. Types 2 and 3 are subjected to utreseaa due ■ to changes of temperature; they are also subject to shrinkage stresses. Deflations for arches of type I are greater than those for type 2, which. 1л tura have greater deflexions than type ?.

Arches can also bo claaaified as "open" or "eolid" spandrel typas. In tha latter саве the Бр$с« between the loaded area and the rib is filled in . In-'the "open type the lotto, is 6rani5.ferre9 to fctie arch rib by coltama or walln.

3.Preetrensed Precast 'Beans with. Pre-oaat and in- a I t u Blabs,

The Gilding of a Secondary School at tflint,-Horth' Wales, has conpoaite precast and in-sltu floor and roof slaba supported on precast secondary beams, and prestress-ed precaet main beams. The main beams are supported on brick piers. Steel roof trusses were used over the assemb­ly hall and gymnasium*-

Generally the main beam span 26 ft. and are at 8 ft. in centres. The imposed load on the roof is 25 lb. per square foot and on the floors 120 lb. per square foot. The main beams are of rectungular егоза section, 10 in. wide and I ft. 8 in. or I ft. 3 in. deep. The heaviest beam weighs 3 -1/2 tons and is 27 ft. in long.Theae beams,ar« prestr*s-eed by 0.2 in high-tensile wires; beams X ft. 8 in. deep contain 56 wirea. Because of a restriction in the overall depth of construction', pockets were formed in the upper facee of the main beams to receive secondary beams at about 4 ft. centres, _, . , The secondary beams are 8 in» deep Ъу 5 in. wide and ijre of precast reinforced concrete. The pockets were I~ln. wider than the beams to allow mortar to be packed in the joints. These beams weigh about 31/2 cwt each» The beams support precast reinforced concrete slabs *

1. 3/4.in. thick on which is a sand-cement topping 1 ¼ thick. The slabs are 2 ft. wide and vary in length from

2. ft. 6 in. to 5 ft. 3 in», most of thorn being 4 ft. long. The upper surfaces of the Blabs were toughened to bond with the topping. Because of the possibility that the packing of the mortar between th« ends of the aecondery beams and the sides of the pockets in the rain beams would be unsatisfactory, the compressive strength of

the main beams was calculated on the net area..One main bean.-was tested to failure and had a moment of resistance of 2,095(000 in» ~.lb, eonpared with a computed, moment of jsesiatance of 1.614.000 in. lb. The defl-jcM-on before failure was 2-in precast and in-f'3tu sl.-iba.

4. Precast Construction.

When precast concrete structural members are to be used tbe construction methods must be considered from the start. It is therefore desireble to mention briefly the advantages' of precaEt structures.

JUaong the advantages of precast construction are i

1. Precasting can be done by mass-production methods, with considerable saving of shuttering (moulds).

2. The possibility of more thorough control of the concrete, with poseible higher allowable stresses and saving of materials.

3. Precasting under cover is independent of climate conditions, and machinery can be used extensively.

4. The construction period is reduced because the members of the superstructure can be made while the foundation is being built.

5. The effects of temperature and shrinkage are less impoi*tant, due to the presence of joints and because naich of the shrinkage will take place before erection.

6. Deflections can be more easily controlled,

7. The aembers can be dismantled and the.shuttering can Ъе re-used.

8. As precast construction is based on standartization a further saving can be(Obtained by planning a building ao that there are sufficient members of the same eize to give the moet economical production.

The design of prgcast buildiDgs must be completed before starting to make the members. Stresses during the strip­ping of the mould3 and handling the/members must be low, and the method of erection must be decided before a de­sign is made in order that the stresses shall not be ex­cessive during handling. Јhe casting procedure Etust be considered when designing the members. Slabs must be of euch a shape that the moulds can be removed easily and quickly) upper surfaces should be plane. Reinforcement at connections Bust be designed to facilitate the removal of the moulds'ее well es the erection of the members.

The repeated use of moulds make possible the uee of Ъеаия with cross eectiong which are more economical than are rectangular shapes.

Greater accuracy in fixing reinforcement can be obtain­ed ooEtpared with in~sitru worlj:, and consequently the cover nay in some cases be reduced. The method of keeping the reinforcement in position during vibration of concrete must be specified.