-
Skip These Ads
-
Skip These Ads|
|
 |
Click here and add this page to your favorites!
|
Encyclopedia Britannica - Main :: FLA-FRA |
|
|
FOUNDATIONS , in building. The object of foundations is to distribute the weight of a structure equally over the ground. In the construction of a building the weights are concentrated at given points on piers, columns, &c., and these foundations require to be spread so as to reduce the weight to an average. In the preparation of a foundation care must be taken to prevent the lateral escape of the soil or the movement
York
of the footings shall be deemed to safely sustain the following loads to the superficial foot: per sq. ft. Soft clay . . 1 ton. Ordinary soft clay and sand, together in layers, wet and springy . . 2 tons. Loam, clay or fine sand, firm and dry . 3 tons. Very firm coarse sand, stiff gravel or hard clay 4 tons. A comparison of the pressure exerted on an ordinary founda- tion by the walls of the several thicknesses and heights provided for by the London Building Act of 1894, and a corn- Load on parison of a few of the principal authorities, will be tonnda- tlon found useful in helping us to arrive at a decision as to . what can safely be allowed. Take as an example a wall
wall
Another factor must, however, be taken into consideration, viz. the weight distributed from the loaded floor and from the roof. In this case there would be at least six floors, and the entire weight could hardly be taken at less than 6 tons, which would give a total weight of 10.05 tons on an area of 4.41 ft. super. or a load of 2.28 tons per foot super. This is on the assumption that no extra weight has been thrown on the foundations by openings or piers, or by girders, &c., in which case, in addition to the work being executed in cement, the foundations should be increased in area. Piers always involve a great increase of weight on the foundations, and in very many instances this increased weight, instead of being provided for by increasing the area of the foundations and so reducing the weight per foot super., is only partly met by the improper method of merely increasing the depth of the concrete, while keeping the same projection of concrete round the footings as for the walls. As an example take an iron column to carry a safe load of 8o tons, standing on a York
If we compare a section of wall of the dwelling-house class, as prescribed by the London Building Act, we find that, taking a wall 50 ft. high and having a thickness at base of 222 in. as for the warehouse wall to which we have referred, we have a wall weighing 3.75 tons per foot super. on an area of 4.41 feet super., or .85 ton per foot without weight of floors and roof as against the 9 ton in the warehouse example. To this must be added the weight of, say, 5 floors and roof at a total of 3 tons per foot run of wall, and we then have an aggregate of 6.75 tons per foot run and 1.50 tons per foot super. as against 2.28 tons in the warehouse class. If we turn from the act to text-books we find that Colonel Seddon in the Aide Memoir gives the load which ordinary foundations will bear as a safe load per foot super. as follows: tons. Rock, moderately hard . . 9 Rock of strength of good concrete 3 Rock, very soft . 1.8 Firm earth . r to 12 Hard clay I to 12 Clean dry gravel and clean sharp
spreading sideways I to IZ Most of the work in London may be classed under one of the latter heads, and according to this table we have, when we erect walls in accordance with the building act, to overload our foundations. As to the possibility of spreading weights, we have as an example the chimney at Adkin's Soap Works in Birmingham,312 ft. high, so arranged that its pressure on the foundations is only 12 tons per foot super.; also the great St Rollox chimney at Glasgow, which has a pressure of 1-1 tons; the weight of the Eiffel Tower (7500 tons) is so spread over 4 bases, each 130 ft. square, that the pressure is only 117 ton, or 21 cwt., per foot super. The Tower Bridge has a load of r6 tons per foot on the granite bed under the columns of towers, reduced by spreading to an actual pressure on the clay foundation of 4 tons. The piers under the Holborn Viaduct have a load of 2; tons only, those of the Imperial Institute 24 tons, and those of the destructor cells and chimney shaft at Great Yarmouth 4 tons 64 cwt. per foot super. From these various examples it would appear that on sound clay or gravel foundation a load of from 24 to 4 tons may be employed with safety. One of the first and most important requirements in preparing drawings for a large building is to ascertain the nature of the sub-soil and strata at different levels over the proposed site, 7y9a1 so as to be able to arrange the footings accordingly at the boring. various depths and to decide as to the various forms and methods to be employed. For this purpose trial holes or borings are sunk until a suitable bed or bottom is found, upon which the concrete foundation may safely be put. If no such solid bottom is found, as often happens near the water side, special foundations must be employed, such as dock, gridiron, cantilever and pile foundations, &c., all of which will be described hereafter. As examples of the varying subsoils we may mention the following, in which will be noticed the great depths dug before getting through the made ground: At the Bank of England there were 22 ft. of made ground resting on 4 ft. of gravel. Some of the made ground was of ancient date, and preserved relics of Roman occupation. In some parts the subsoils have been excavated for ballast or gravel, as at Kensington, or for brick earth, as at Highbury, and the pits filled in with rubbish. Rock, which forms an excellent and unchanging foundation in one situation, may prove a dangerous foundation in another. Thus chalk forms a good limestone foundation in certain positions, but when it dips towards a slope or a cliff with an outcrop of the gault or underlying clay, it is a very unsuitable foundation for any building, as the landslips in the Isle of Wight and on the Dorsetshire coast bear witness. A boring made in Tallis Street, near the Thames embankment, showed: (I) 18 in. ballast, dirty; (2) 6 in. greensand, wet and dirty; (3) 2 ft. peat clay; (4) 6 in. greensand; (5) 52 ft. peaty bog; (6) 9 ft. running sand; and (7) 4 ft. clean ballast, resting at a depth of 23 ft. below the ground line upon blue clay. A boring at Highbury New Park gave: (I) 2 ft. made ground, (2) 18 ft. loam, (3) 9 ft. sand, (4) 4 ft. peat, and (5) 8 ft. gravel and sand. These examples show that while trial holes should always be made before designing a foundation, to ascertain the nature of the subsoil, care must be taken not to calculate upon uniformity. Thus at the block
block
The London Building Act and the model by-laws prohibit the erection of buildings on sites that have been used as " shoots " for faecal matter or vegetable refuse, and in such cases the Constrncobjectionable material must be removed prior to the Con commencement of building operations, and the holes from which it was taken filled up with dry brick or other rubbish well rammed. Foundations are usually executed by excavators or navvies, and the tools and implements used are boning rods, level pegs; lines, spirit level, pickaxe, various shovels, wheel-barrow, rammer or punner, &c. In digging the ordinary trenches and excavations, should the ground be loose, planking and strutting have to be employed. This consists of rough boarding put along the sides of the trenches and wedged tight with waling pieces and struts; this work is done by navvies. Figs. I and 2 show the general forms of planking and strutting for the different soils. In very large works of excavation in soft soil a steam digger is used for the bulk of the work. It consists of a large steel bucket with a cutting edge; this is lowered by means of a crane into the excavation, and on being withdrawn cuts off a portion of soil which is hoisted and deposited in carts for re- moval to any desired posi- tion within the radius com- manded by the crane. The work of trimming the exca- vation to a regular shape must always be done by , i manual
rbvn to Concrete for filling into 1of4ga41 the foundations is usually mixed by navvies; for large works it is sometimes mixed by machinery. In order that the work of excavating and constructing the foundations may proceed in a water-logged site, pumps have to be employed, and where the inrush of water is great it is usual to sink a sump hole lower than the depth required for the foundations, and to use a steam pump kept going day and night. The foundation of a wall is required to be as follows in accordance with the London Building and Amendment Acts: " The projection of the bottom of the footings of every wall on each side of the wall shall be at least equal to half of the thickness of the wall at its base, unless an adjoining wall interferes, in which case the projection may be omitted where that wall adjoins, and the diminution of the footings of every wall shall be formed in regular offsets and the height from the bottom of such footing to the base of the wall shall be at least equal to two-thirds of the thickness of the wall at its base." (See BRICKWORK.) The base of a wall is the thickness above the footing; the footing is the brickwork built directly on the top of the concrete and diminishing in width in every course. Thus: " The projection of the bottom footing to be equal to one-half the thickness of wall on both sides " means that a 132-in. wall would require to have three courses of footings, the bottom one being 27 in. wide. " The height from the J^`' bottom of such footing to the base of the wall shall be at least equal to two-thirds the thickness of wall at its base " means that in the case of a 132-in. wall the height of footings would have to be 9 in., or three courses of brick-work, each measuring 3 in. The New York Building Code enters more fully into the require- ments for the foundation of walls as regards depth than that in use in London. Section 25, Part 5, requires that every building, except buildings erected upon solid rock, or upon wharves and piers on the water front, shall have foundations of brick, stone, iron or concrete laid not less then 4 ft. below the surface of the earth, on the solid ground or level surface of rock, or upon piles or ranging timbers when solid earth or rock is not found. Piles intended to sustain a wall, pier or post, shall be spaced not more than 36 in. nor less than 20 in. on centres; they must be driven to a solid hearing if practic- able, and their number must be sufficient to support the super- structure proposed. No pile shall be used of less dimensions than 5 in. at the small end and to in. at the butt for short piles, or.piles 20 ft. or less in length. No pile shall be weighted with a load exceed- ing 40,000 Ib. When a pile is not driven to refusal, its safe sustaining power shall be determined by the following formula: twice the weight of the hammer
The New York Code, Section 26, further goes on to say that foundation walls shall be constructed to include all walls and piers built below the curb level or nearest tier of beams to the curb, to serve as supports for the walls, piers, columns, girders, posts or beams. Foundation, walls shall be built of stone, brick, Portland cement concrete, iron or steel. If built of rubble stone or Portland cement concrete, they shall be at least 8 in. thicker than the wall above them to a depth of 12 ft. below the curb level, and for every additional 10 ft. or part thereof deeper, they shall be increased 4 in. in thickness. If built of brick, they shall be at least 4 in. thicker than the wall next above them to a depth of 12 ft. below the curb level, and for every additional to ft. or part thereof deeper, they shall be increased 4 in. in thickness. The footing or base course shall be of stone or concrete, or both, or of concrete and stepped up brickwork of sufficient thickness and area to bear safely the weight to be imposed thereon. If the footing or base course be of concrete, the concrete shall not be less than 12 in. thick; if of stone, the stones shall not be less than 2X3 ft. and at least 8 in. in thickness for walls, and not less than so in. in thickness if under piers, columns or posts. The footing or base course, whether formed of concrete or stone, shall be at least 12 in. wider than the bottom width of walls, and at least 12 in. wider on all sides than the bottom width of said piers, columns or posts. If the superimposed load is such as to cause undue trans-verse strain on a footing projecting 12 in., the thickness of such footing is to be increased so as to carry the load with safety. For small structures and for small piers sustaining light loads the cornniissioner of buildings having jurisdiction may, in his discretion, allow a reduction in the thickness and projection specified for footing or base courses. All base stones shall be bedded and laid crosswise, edge to edge. If stepped-up footing of brick is used in place of stone above the concrete, the offsets if laid in single courses shall each not exceed 11 in., or, if laid in double courses, then each shall not exceed 3 in. offsetting the first course of brickwork back one-half the thickness of the concrete base, so as properly to distribute the load to be imposed thereon. It will be seen by the foregoing that the American acts are far more extensive than in London. The London Building Act mentions that the footings of a wall shall rest upon the solid ground or concrete or upon other solid substructure. The building act amendment says: " The foundations of the walls of every house or building shall be formed of a bed of good concrete not less than 9 in. thick, and projecting at least 4 in. on each side of the lowest course of footings." Various Types of Foundations.The most natural foundations for walls are those constructed where the walls are built directly upon the ground; this is only possible where the ground is very hard or consists of rock, and in either of these cases the ground is simply levelled and the building commenced. The next and most universally recognized method, which might safely be said to be adopted in 95 % of all modern buildings, is the system of placing a bed of concrete under the walls, digging trenches where the walls are to come until a solid bottom is reached, and in these laying the concrete. The London Building Act requires this concrete bed to be at least 4 in. wider than the bottom course of footings on each side of the wall, but it is generally made 6 in. wider on each side and in general circumstances the depth of the concrete is varied according to the weight placed upon it. Where a site is in close proximity to a river or old water-course, &c., where deep basements are excavated, or where the ground lies low, naturally water is met with, and where water is the ground is soft. It is here that special foundations are required. In certain cases it is necessary to use concrete legs or stilts. These are placed in such positions as to take the weights of the building, and sunk to depths of 40 ft. more or less as the case may Concrete require according to the nature of the ground; and on the tops of these stilts concrete arches or lintels are fie"' turned over (fig. 3). As an example of the stilt principle, s Ita or mention may be made of some premises at Stratford and a church at South Bermondsey
Pile foundations, used in the case of soft ground, for small works, consist either of stout scaffold poles or of timbers varying from 6 in. to 12 in. square according to requirements (fig. 4). The bottom ends of these timbers have an iron shoe with a point, so as to be easily driven into the ground, and the tops of the timbers have an iron band round, so that when the timbers are being driven in the band prevents them from splitting (fig. 5). The methods of driving these piles are various. The usual plan is to erect a tempor- ary structure, upon one side of which is a guide path faced with sheet-iron so as to give a smooth face. Up and down this guide path a heavy iron weight, called a monkey, is worked; the monkey is hoisted to the top of the guide path by means of a crab worked by hand or steam, and II,1_//tiY/llf~llJ7l/,~/,/ir~~r;rrr ,r , /MO maw 9I1/, pm, l iu //Y////Iiw I a ~ =~t%i%%L'~fl~flF~1!J..`~11~1/%~lJl1%~%YfL1f:r1 ,if '~ff~ilyTRAN5VER5e End of Article: FOUNDATIONS If you wish, you can link directly to this article.
<a href="http://jcsm.org/StudyCenter/Encyclopedia/FLA_FRA/FOUNDATIONS.html"> FOUNDATIONS </a> |
|
|
(Previous) FOUNDATION (Lat. fundatio, from fundare, to found) |
(Next) FOUNDING (from Lat. fundere, to pour) |
Jesus Christ Saves Ministries, P.O. Box 70696, Pasadena, CA 91117JCSM is a 501(c)(3), non-profit organization. Copyright © 1997-present. |
Free & Cheap Cell
Phones |
Cheap Long Distance
Phone Service Carriers |
Talk America Local Phone Service
|
Ztel & MCI - Unlimited Long Distance
Compare
Cell Phone Plans & Companies |
International Calling Cards & Prepaid Phone Cards |
Voice Over IP Broadband Internet Phone
Service | Wireless
Phone Plans & Cheap Cell Phones
|
_____________________________________________________________________________
Online First Aid and CPR Certification . The Online Christ Centered Ministries . The Skeptic's Annotated Bible: Corrected and Explained . The Inerrancy Discussion Board . Free Email Accounts . Home Equity Loans . JasonGastrich.com . The Missions, Apologetics, and Creation Bible Conference . Young Earth Creation Science . San Diego Music Lessons . 10,000 Wise Quotes and Spiritual Sayings . Gastrich.net . Maximizing the Internet: 12 Keys to Success . Louisiana Baptist University . NKJV Web Hosting and Services . Michael Newdow . San Diego Soccer Training . Christian Guitar Lessons . Jesus Christ Saves Ministries . Eternal Security
