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Foundation Design and Calculations for Architects

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Structures 3 (ARC 323/323A )

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Academic year: 2016/2017
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California State Polytechnic University Pomona

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Foundation Design and Calculations for Architects As an architect, you will not likely size any foundations except for some small projects that you might have occasion to do during your career. Nonetheless, the architect pretty much decides on the foundation type based on the design and conditions where the building will be placed. Ching has some very basic rules of thumb for foundation types that you should already be familiar with. You are required to design and calculate your foundations for Except for the systems integration class, you will probably never size a foundation quantitatively after You will however, be expected to show appropriate foundation types and reasonably approximate sizes of foundations for all future design projects. The following example shows a student design that does not recognize the importance of foundations or basic structure as they relate to architectural design. Figure 1 Student design example showing a lack of understanding for basic structural requirements in architectural design and foundation requirements. Foundations are not simply horizontal lines beneath our buildings as indicated in Figure 1. Your choices for foundations include two basic categories: q Shallow Foundations Slab on Grade with Thickened Edge Raised Foundations with a Spread Footing Pad Foundations Grade Beams q Deep Foundations Piles Caissons While these are the two basic types, there are others: q Foundations Sidewalks Roads Fences Flag Poles, etc. q Retaining walls retaining walls such as garden walls Building retaining walls such as the Eames House The basic foundation types that Ching has provided cartoons for are shown in Figures 2A and 2B. Figure 2A Basic foundation types presented Ching 2 Shallow Foundations In reality, shallow foundations behave in a manner similar to beams. Where they differ is in their reactions. the reactions under shallow foundations are continuous with either point loads or uniform loads from above. Beams have specific reaction locations that results in their beam action. Most shallow foundations are made with reinforced steel and concrete. Like a beam, the reinforcing steel needs to be placed where tension and shear are expected. How can a foundation perform like a beam? Figure 3 shows two basic beam types and typical concentrated loads on the beams. this loading results in flexure of the beam with tension and compression. Figure 3 Typical beams showing the location of tension and compression Figure 4 shows how the loads on a foundation act like beams. Figure 4 Foundation and their respective tension and compression due to flexure 4 Notice in Figure 4 that not all walls that retain earth act like retaining walls. The diagram in the bottom right of Figure 4 shows a basement wall where the earth is retained the basement wall like a simple beam with a uniformly increasing load die to the equivalent fluid pressure of the soil. Therefore, the flexure and reactions of the basement slab and floor framing create a simple beam condition that is into the vertical position. Therefore, the tension is on the basement side of the wall which is opposite of a cantilevered retaining wall where the tension is on the earth side of the wall. Because have their welded wire mesh (WWM) placed at the neutral axis of the slab, the WWM is not considered reinforcing steel. Instead, it plays a role as temperature steel which helps to control cracking in the SOG due to the differing thermal properties of the soil, concrete, WWM and the inside air within the building. now, you should appreciate the lack of moment for WWM being placed at the neutral axis of the SOG and be able to explain this in detail on an exam. You should be able to extrapolate this moment philosophy to other structural conditions such as concrete walls, CMU walls and solid brick walls as well. When soils are very poor below they may be reinforced with cables. Never saw through post tensioned cables. have thickened edge footings. In the early part of the 20th century, when building codes were in their infancy, always have thickened edge footings. Road House is an example of an SOG without a thickened edge footing (Figure 5). Figure 5A and 5B Road House photos courtesy of Professor J. Sheine Road House is still standing after approximately 90 years, however modern codes no longer allow a simple SOG without footings at the edge of the slab and at interior bearing walls. Today, the footings would appear as shown in Figure 6. Note that the perimeter footing bottom must be a minimum of 12 inches below grade. If the building is in a frost area, the bottom of the footing must be below the frost line as established the Authority Having Jurisdiction (AHK). The AHJ is the new terminology for the Building Official. The bottom of the footing typically rests on undisturbed subgrade soil. The top of the slab can never be any closer than 4 inches from the soil and all wood attached to the perimeter of the slab must be pressure treated if it is less than 6 inches above the soil. The soil must slope away from the SOG at a minimum of The SOG has WWM placed at the center of the slab (the neutral axis) which means that it is not reinforcing steel since there are no bending forces at the neutral axis. The footings (both perimeter and interior) 5 Spread Foundations There are three basic types of spread foundations. These are: q Column pad foundations q Wall spread foundations q Mat foundations Column pad foundations take concentrated loads from columns and deliver the loads to the soil via the pad footing as shown in Figure 7. The pad footings are shown on the left and cartoons for a variety of pad footings is shown ion the right. Figure 7 Column pad footings from: wsuzana.wordpress We have looked at structural details for column pad footings since the beginning of the quarter. Figure 8 shows how the anchor bolts are set when the pad footing concrete is placed. Figure 8 Column pad footing concrete and anchor bolt placement from: 7 Figures 9A and 9B show two different methods of placing wall spread footings. Figure 9A shows where the entire footing is formed with wood formwork and Figure 9B shows the more common case where the spread wall footing has been placed using the earth as formwork. When we design for wall foundations, we need to determine the load carried one linear foot of wall including any tributary loads from above such as roof loads and upper floor loads. Figure 9A St. Monastery Bakery spread footing before setting the CMU wall above the ftg Figure 9B More common spread footing will be covered soil following ICF wall placement Mat foundations are very expensive, fairly rare and only needed where soil bearing values are very low where there is a chance of soil liquefaction during an earthquake. Figures 10A and 10B shows typical mat foundations. Figure 10A Mat foundation under construction from irvinegeotech Figure 10B Placing concrete for a mat foundation from irvinegeotech Deep Foundations Deep foundations are only needed where structure loads are very high or poor soil conditions exist. They are relatively expensive options. Deep foundations can have end bearing conditions where the deep foundation passes through poor soils and bears directly on competent rock below the poor soils. They can also have side friction bearing conditions where the soils have sufficient side shear strength or they can have a combination of end 8 Figure 12A Pile driver in operation along 10 freeway in Fontana note the diesel smoke drifting away from the exhaust Figure 12B Driving an H section with a vibratory hydraulic pile driver these can also be used to extract temporary piles Piles can be placed together in a matrix with a pile cap taking the load from a column and distributing the load to each pile uniformly (Figure 13). Column Pier Pile Cap Piles Figure 13 Pile cap detail Whereas piles are driven into to ground, caissons are constructed in place. Caissons have been around for hundreds of years, but not as long as piles. In the early years of constructing caissons they were a very dangerous proposition. People died due to and later as pressurized cells were developed people died as a result nitrogen narcosis when they left the pressurized workspaces too quickly. It was called 10 Today, it is called which is a scuba diving term. The construction of the Brooklyn Bridge was fatal or depilating to a multitude of workers. Figure 14 Deplorable working conditions inside a caisson on the Sioux City bridge 65 feet below the waterline from: blairhistory Caissons have been constructed with masonry, concrete, timber and steel. Reinforced concrete is the most common today. Figure 15 Showing different types of caissons from: 11 Example: A square pad foundation needs to support a load of lb. The soil bearing value is based on a soils report. The foundation measures x Is the foundation acceptable? If it is not acceptable, what size foundation is needed? Solution A: f ft x 2 ft) NOT OK Solution B: f (25 5 ft x 5 ft We used the square root because the foundation is square in shape. NOTE in Solution B, the correct answer is 5 ft x 5 ft, NOT 25 ft2 or 5 ft if you provide an answer such as this, you will lose points due to the incompleteness of your answer. Always provide complete answers. The correct structural diagram for the example is shown in Figure 11: Figure 11 Diagram for Example Note, I have not drawn the soil in the structural diagram. It is optional to draw or not draw the soil in this case. If the soil will obscure information, leave it out. If it helps to explain the condition, insert it. When we are sizing wall foundations, they footings are seldom square. The first step in sizing wall foundations is to get the load of one linear foot of wall. This means that we will need to know the tributary weights for the roof and any upper floors (Figure 12A) as well as the tributary weight of the wall and foundation. We clearly know the tributary weight of the foundation because it is yet to be designed. Therefore, we need to make an educated guess based on common sense. We get the tributary weight of the roof, upper floors and the wall itself based on calculating the weights of the components of our design. You will need to prepare a list of weights for your project based on a similar format to Figure 12C. Determining the unit foundation loads is a laborious process. You are the designer and should have a clear understanding of what your building weighs. I have provided you with common unit loads on BlackBoard. You can also look up common values on the Internet, in Ching, etc. Make certain that you reference your source for the loads. Also, make certain that you work with the correct unit values and convert where necessary to your project. 13 Figure 12A Unit rood and upper floor loads Figure 12B Unit wall and guess at foundation load Figure 12C Unit foundation loads are derived from total building unit loads 14 Figure 14 shows a sample foundation calculation. Figure 14A Setting up the foundation calculation requires preliminary calculations, explanation and appropriate structural diagrams 16 Figure 14B are some default values from ARC341 for SOG perimeter footings. Figure 14B SOG perimeter footing sizing based on code defaults Figure 14C is the same as 14B except for wall type spread foundations. Figure 14C Wall spread foundation sizing based on code defaults 17 Figure 15 Calculating pad foundation sizes 19 Figure 16 shows the calculation sequence and sizing the foundation for steel framed building on a SOG foundation and a wall spread foot type foundation Figure 16 SOG and wall spread foot calculations 20

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Foundation Design and Calculations for Architects

Course: Structures 3 (ARC 323/323A )

9 Documents
Students shared 9 documents in this course

University: California State Polytechnic University Pomona

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L-23%–%Foundation%Design%and%Calculations%for%Architects%
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As#an#architect,#you#will#not#likely#size#any#foundations#except#for#some#small#projects#that#
you#might#have#occasion#to#do#during#your#career.###Nonetheless,#the#architect#pretty#much#
decides#on#the#foundation#type#based#on#the#design#and#site/soil#conditions#where#the#
building#will#be#placed.##Ching#has#some#very#basic#rules#of#thumb#for#foundation#types#that#
you#should#already#be#familiar#with.##You#are#required#to#design#and#calculate#your#
foundations#for#ARC301/504.##Except#for#the#systems#integration#class,#you#will#probably#
never#size#a#foundation#quantitatively#after#ARC301/504.##You#will#however,#be#expected#to#
show#appropriate#foundation#types#and#reasonably#approximate#sizes#of#foundations#for#all#
future#design#projects.##The#following#example#shows#a#student#design#that#does#not#
recognize#the#importance#of#foundations#or#basic#structure#as#they#relate#to#architectural#
design.#
#
#
Figure%1%%Student#design#example#showing#a#lack#of#understanding#for#basic#structural#requirements#in#
architectural#design#and#foundation#requirements.#
#
Foundations#are#not#simply#horizontal#lines#beneath#our#buildings#as#indicated#in#Figure#1.#
#
Your#choices#for#foundations#include#two#basic#categories:#
#
q Shallow#Foundations#
§ Slab#on#Grade#with#Thickened#Edge#
§ Raised#Foundations#with#a#Spread#Footing#
§ Pad#Foundations#
§ Grade#Beams#
q Deep#Foundations#
§ Piles#
§ Caissons#
#

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