A Little Gem About Steel Building Foundations
Your steel building will need some type of foundation on which to anchor the columns and thereby tie the whole structure together. Some agricultural buildings are designed to have dirt or gravel floors and consequently use concrete piers (footings) instead of a full slab; however, a concrete slab is best for most situations.
See also About Concrete.
Whatever foundation you use, we strongly recommend that you employ an experienced local foundation engineer to design it. Your erection drawings will include an "anchor-bolt setting plan with reactions," which will give a qualified engineer the necessary data to design a suitable foundation for your building.
You should note that the engineer will also take a good, hard look at the soil. The soil conditions are of utmost importance and should under no circumstances be ignored.
To give you a slightly better insight into this aspect, read a little about ground conditions below. And be sure to also read our section on "How Does This Relate to Steel Structures" further down on this page.
If a building is to be constructed in an area with clay soils one should be aware that clay soils have been found to expand 23 cm or more if subjected to long cycles of drying or wetting, thus producing powerful forces that can shear foundations and lift lightweight buildings. Some soils with high organic content may, over time, compress under the building load to a fraction of their original volume, causing the structure to settle. Other soils tend to slide under loads.
Soils that have been modified in some way often perform differently, especially when other soil has been added to or mixed with existing soil, or when the soil has been made wetter or drier than normal, or when cement or chemicals such as lime have been added. Sometimes the soil under a proposed building varies so greatly over the entire site that a building simply cannot be constructed safely or economically.
Soil and geological analyses are necessary, therefore, to determine whether a proposed building can be supported adequately and what would be the most effective and economical method of support.
If there is sound bedrock a short distance below the surface of the construction site, the area over which the building loads are distributed can be quite small because of the strength of the rock. As progressively weaker rock and soils are encountered, however, the area over which the loads are distributed must be increased.
So, if you are perhaps starting from scratch and looking for some land to build on, do not purchase an Erf if you have even the slightest suspicion of soil condition problems - it can cost you a small fortune. Remember; the cheapest stand may not always be the least expensive.
Take a look at this little story:
An important institution and had the desire to build a fitting monument to its achievements. They decided that a tower of great architectural beauty would be built in close proximity. Since the site was close by, ground conditions were well known and were taken for granted. Besides, soil investigations always take time and add considerably to the cost. Was this a recipe for failure? Of course it was.
Construction commenced but when the tower reached four storeys tilting became evident and construction was halted. Years later, construction was resumed only to be halted after adding four more storeys. More years later, a penthouse is added off-center in an attempt to redistribute the load. Even so, the penthouse floor was so out-of-level that one side had to be raised by over 30cms (12 inches). Under the added load the tower continued to incline, and then became several degrees more out of plumb, and was consequently never used for its originally intended purpose.
Today, the tilting continues to progress, albeit very slowly; but by now the tower has engendered a great fondness so that every effort is made to save it. Many and various are the efforts to stabilize the tower and even to right it, but more often the condition is aggravated rather than improved. Needless to say, dignitaries and politicians wade in with threats, counter threats and law suits.
If you haven't recognized it, this is the story of the Leaning Tower of Pisa in central Italy. Begun in 1173, the tower was finally finished in 1370 and still exists to this day. The tower is renowned world-wide and thousands of tourists come to see it every year. Recovery has engaged the best architectural, engineering and geo-technical minds in the business, and hopefully the delicate engineering works currently underway will save it from final collapse. That the building has survived for over 800 years is remarkable in itself, considering the short life span of most of our modern buildings. By these measures, this initially disastrous project has become a valuable asset and an outstanding success.
Our point is however that all this drama occurred ONLY because the soil conditions were never properly checked.
For a detailed history of the Pisa tower, visit: http://www.endex.com/gf/buildings/ltpisa/ltpisa.html
Purely as a matter of interest, its lean is 4 metres (13 ft) from the vertical. Legend has it that Galileo took advantage of this inclination to carry out experiments with weight in free fall, dropping weights from the top of the tower.
How Does This Relate to Steel Structures?
The portal frames are the main structural elements of the building, being so to speak its "skeleton". They consist of S355JR columns and rafters, and the column (footings) foundations. The frame joints at B, C and D are shop welded and site bolted, and are designed and fabricated to achieve a rigid structural unit.
The frames are designed for the following loads:
A = Roof loads such as workmen, snow or hail.
B = Wind - from the sides or as an up-draught.
Vertical loading on the frame as depicted in figure "A" above results in the columns (poles) A and E tending to be pushed outwards. If the foundation cannot resist this horizontal push, outward movement will occur, and the frame will lose structural strength.
Wind (as depicted in figure "B" above) subjects the portal frame to uplift forces (the roof tries to "fly-off - just like an aircraft wing), to overturning forces on the sides and ends of the building, and also to "drag" forces on the roof and sides of the building.
The effect of wind on a steel building cannot be overemphasized. The destabilization it causes is a major design consideration, and in this context, foundations can be regarded as the building’s "anchors".
Trust our years of positive construction experience and craftsman expertise to confidently guide your specific building project smoothly to sure success.
You may just experience enormous benefits.
And please always remember John Ruskin's famous words:
The common law of all business prohibits paying a little and getting a lot - it cannot be done. Although it is unwise to pay too much, it is far worse to pay to little - for when you pay to little, you sometimes lose everything. Therefore, if you deal with the lowest bidder, it would be wise to add something onto the price for the risk you are running; and - if you do that, you had enough to pay for something better in the first place.
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