5» Precast Construction of a Factory.

The main building of а пв-л factory is about 195 ffc.long and 1<?5 ft. wide. There is also a canteen building of about 75 ft. Ъу 30 ft. The structures are partly of two stories. The first floor is designed for an imposed load of 200 1Ъ per square foot. The primary beajns are splayed in plan at the supports in order to provide sufficient reBistsncs to cocrpression without having vertical haunches which would decrease the clearance below the Ъеапш. The columns are at 20 ft. centres in both directions and neooodary beams are provided so that the floor slabs span about; 10 ft. The floor slabs are 6 in. thick and comprise 3 in. of con­crete on 3-in. precast slabs reinforced with expanded metal. The troper slabs wore cast in alternate panels to reduce the effects of s'irinking, Rlnoe the frsjao is composed entirely of precast membex'Si little shrinking of the struc­ture as a whole is expected. The external ooluxais wero precast in one piece for the full height of the two stories. The internal columns are also precast and extend to the first floor only. Connecting the oolunna end secondary beams is a steel plate embedded in the colunm and inserted in a slot in the beam. Inverted U-Ъаго projecting from the top of the precast Ъвашя above the Slote ere on the top edge of the plate. Deformed bars are welded to the top and bottoa of. the plates, through whioh holee ere drilled to provide a bond vhen the joint is concretod. The slabs adjacent to the external beams' were cast In place, end are reinforced in the top and oottoa to prevent the beams from twisting. The primary Ъеапш are supported on steps in columns and on the tops of tha Internal со- -lunme. fieinforcement projects from the external columns into the part of the primary beams which was oast; in place with the floor slab. The connections between tha primary and secondary are also by means of steel plates which are embedded in the primary beams.

6. She Reconstruction, of the Audi­torium Floor at the r.Oyal Opera Bouse, Oovetxt Garden.

This paper describes the design and constsuctioa of a new fire resisting floor to the auditorium and improve­ments to the orchestra pit and entrance staircase. Much of the work w&e carried out in the basement beneath the auditorium without interrupting regular perfo&ancea of opera and ballet.

During this period a stael framework was erected between the existing timber posts and cast iron columns, and pre­cast concrete ceiling units were then inserted. The new steelwork and ceiling were,.designed to support temporary strutting for-the existing floor, this permitting the re­moval of all the old columns without disturbing the floor Itself. Immediately after the final performance of the season, work started on the reaoval of the floor and timber lattice girders.which had been 1c position for nearly a hundred years. Theae ware replaced by hardwood strip flooring and preetx-eBaed-conarote joints resting on small steel beams --wcaI atancnioes which were bolted to the main steel framework below.

The opportunity was fcakea to increase the slope 0Ј the floor and enlarge the orchestra pit» This latter Improvement was achieved bj mounting the front row. stanchion» on rollers, l'hn «Iteration oЈ tte slope of the .floor increased the Ъя»4гоои in this pit lobby and permitted the construction of two new.refresh­ment bare ad30.in.ing the uain ataireaae.

Structural Alterations in 1955. The purpose of the re соавtraction was to provide a -new floor to the auditorium conforming to modern standards, and at the ввгае time to provide accomodation 1л the base-

• meat for a canteen "and ..much needed rehearsal rooms. In order to preserve the excellent acoustics of the Hoyal Opera House it was considered that the new hardwood floor should be a few feet above the concrete ceiling forming the f iz'e-, harrier between the basement and the auditorium. An important consideration affecting the design of the new floor was the requirement that the House Bhould remain open to the. public except for a short period at the end of the sunnier season. The construction of the concrete ceiling therefore had to bo carried on beaeeth the oxiet--ing floor whilo tt waa still in regular use. Fortunately there wae sufficient headroom available in the basement for this to be dontj, though the presence of во many columns and timber strata seriously restricted the working space.

Aa part of the general improvement to''the amenities of the House, the now floor vras'to have a slightly steeper elope than the existing one, IPbree flre-resistlag stair™ cases' lead? eg from the pit lototiy were also required*, topther with two now xefrestaent bars fi-iJoinLng the isair» entrance staircase. To provide space la the oreheetra pit for thg large оrhceBtга required for certain operas, a e portion of the,new floor to the csaHa needed to be easily removable.

7. P r t> s t г е к n e d О о d о г е t e Bridge, in Cuba w i t-h я p a n в nearly ,700ft.

About; 2 rdlee from Mondosa ne^.r tha w_ost«n: end-of Cubr»,-the Cuban branch of the Faff American 8igb»iy crosses Rio Coyag-aatje. The river is subjected to finch floods during which it can rise 25 ft. above"itn no ими level, attsininf -/Glocltie.B of more than 7 -ft, per'вес "ioca the flood flows carry much debris, including irfcol* tieea it is nor.

practicable'to use piers for a bridge crossing this river. Borings were made on both aides of the river at several poeeible locations for the bridge. In all of them was found an upper layer of soil, followed by a layer of alter­ed limestone rock full of cavitieB and cracks. Not very far below these strata lies sound, dense limestone of еэс-oellent quality which extends to a great depth. This lime­stone was selected as ideal for the foundation of the bridge and aleo proved an excellent source of coarse aggre­gate for the concrete structure. . The designers ^elected a prestressed concrete structure of about JOO ft. span for crossing. This design was considered to be feasible from the construction standpoint and competitive in initial cost with an equivalent steel structure. The bridge con­sists of a concrete^box girder 367 ft. long resting symmet­rically on two piers 298 ft. 6 in. apart. The endfl of the Bide spans are anchored down into the rock by cables. After a few tentative preliminary studies, the central ф. span of the bridge was finally designed as a two-cell hollow box-girder of uniform width but decreasing in depth from the piers toward the centre to lighten its weight. The design adopted produced a silhouette of striking elen-derness. End spans were designed to be solid end to be anchored to rock so as to provide practically full restraint for the central span at the piers. Added inertia and resi«-tance were obtained by increasing the depth and wall thick­ness of the hollow box-girder of the main span near the piers and by adding prestressing cables of variable length in the top slab over the piers.

The length chosen between piers and anchor cables / 54 ft. 5 in./ provided the moat economical solution considering the cost of concrete, reinforcing steel, forme, rock excavation and high-etrength steel wire cables at the time of construction.

The 29 ft. 6 in. width of the bridge provides for two lanes of traffic and a 5 tt. sidewalk on each side. The two-cell box girder, whioh has a uniforn width of 23 ft. 6 in. , has three vertical walla 8 in. thick, a top roadway 8 in. thicfc ( which overhangs the side walls by 3ft.) and a bottom slab IX In. thick at the centre of the span. The depth of the holler» girder et the piers is 12 ft.'tin., and it decreases according to a parabolic curve to 6 ft;.4in. at the centre of the bridge. The ratio of bridge depth to clear-span length at the centra of the bridge is about I»50» Concrete with a strength of 5,000 psi 1л 28 days was used in the box seotion or the bridge. Coarse aggregate was made at the site and cilicer «and came froa the river. Although the superstructure ie but eiightly reinforced, 46 tons of structural reinforceaeat went into it. At fall load the Eiaxi-шшв atxess in the concrete is about 1,000 psi, and maximum diagonal tension is 120 psi. Temperature stresses are negli­gible as in Cub» the temperature ranges only from a minl.nmct of 60 deg.F. to a xsaxiHum of 95 deg.F.

С Е М В Н Т .

A bard dense material Is obtained winea cement i milted -with water. Sons inert material ( sand , gravel > are usually added to sucn mixtures which are called mor tara. The mortars set and harden well. Tha addition of some water-pk-oofIng material inajtew tho cement products even usable under water. Cement has different properties," most important being the strength of its products. This property is affected by such factors ая type , proportion and grading of aggregates ueed , water cement ratio and even workmanship.

The strength of cements increases after tr­act , end the rate of strength gain ie different ferent types.

The main types of cements are i ordinary Port; Kapid Hardening Portland Cement , White and Cole; Portland Ceijents , Portland Blast-furnace Oenent Alumina Cement. The process of manufacture of tfc; ments Is rather similar , the difference being In . r.e degree ox fineness ( aa with Rapid Hardening .Portland Cement ) and in ueing different materials i for inatanct Chinfi-ciay for white cement , blast furnace Blag for Blast-furnace Cement , or bauxite in High Alumina Cement The usual three stages of manufacture are » 1) grinding end mixing the raw materials in a for» of a wet slurry , 2) burning the mixture to hard elinkere , 3) crushing end grinding the clinkers to a very fine powder.

The raw materials— limestone , obalk , elat» , clay used in aanufacture of cenent undergo chealcal deoomposition and combination during heating.

fieady cement ie mixed with little gyp⁸"* ^{or cal}" olum sulphate to prevent too rapid setting»

Cements , except white and coloured , usually give grey appearance , Cement products are difficult to decorate with oil paints > Cement nuLsturee should not Ъв exposed to temperatures helow f>6^e F. , Ъесаиве в re­duction of strength gain takes place-. Cold weather effecta High Alumina Cements not во roach aa they li­berate more heat during setting than other types . This type of oeaent has also high fire resistance , though special fire cement is availahle , Shrinkage of cement products ie also the most important property of cements as it is the cause of cracking .This de-.penda од the proportion of cement and the quantity of mixing water . Also the method of curing may effect the amount of shrinkage : the slower the drying , the less is the shrinkage .

The engineer mist allow for all those faetore af­fecting cement mortars and cement productв .

OORCEEIE and REIKFOSCED 0 0 H С К В

Concrete ie a mixture of cement , Band _; ччйлг ю aggregates such as crushed brick , gravel or cruehed . stone . Chose are called heavy aggregates .To obtain lightweight concrete aggregates such as blast lurnace Blag , coke | breeze are used . Concrete ia a denes m ttrial , it» density depending upftn the density of ,the aggrugates sud upon, their sizes aa well. Different types of concrete can be obtained while tising fine or coarse aggregates..Sometimes "no fines" concrete without fine aggregates is prepared. If a foaming agent or a gas foraing voids is used, then the resulting material is called cellular coacrete - a ma­terial extremely necessary for soac Mjids of buildings. Coacrete differs not only in its quality but also in the way of working it. So there are in-eitu and precast con­cretes, the difference being in preparingthe first on a building site or making prefabricated units at a factory,^ Sometimes concrete is used exposed be facing -work but some-t.vroes it in covered with other naterinls. It 1b interest top; to note, that concrete ia not a modern building materiajt it was already used by the Воюатш. There are шдку factors which-Influence the'strength of concrete. •Those are t weter-eeasent ratio, tba nature of the «and and coor-ве aggregates, the temperature of placing and the reten­tion of moisture in IS durir-e; plecirn-. Proper nixing also occv.piefi an important place among¹ th" above aentioned factors and so does proper cuxin: -