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Concrete Lecture

Footings Foundation | Slabs | Tilt ups

Footings and Foundations

CSI Codes

03 20 00 Concrete Reinforcing
03 30 00 Cast in place Concrete

Even though the CSI codes do not breakdown concrete work any further into footing/foundation/slabs etc, we will break it down further for the assignement.

Rebar

Rebar Diameter in inches = Rebar Size /8 (For bar sizes 8 and smaller. For larger bars
reference a table for the exact diameter).

For Example: #4 Bar / 8 = 1/2" diameter.

Lap

Suppose that you have 100' of footings with 2 runs of #4 rebar. The overlap is 30 bar diameter and the rebar comes in 20'pieces. What is the total LBs of rebar for this footing?

First draw a picture of what the problem looks like that you are trying to solve.

The general formula to calculate lbs of rebar is :

LBS of Rebar = (Total LF + Laps LF) * Number of Runs of Rebar * Conversion Factor to lbs

Total LF: 100'

Laps LF: To find out how much the laps add, take the Number of Laps * Length of a Lap.

  • The Number of Laps= total LF / length of the rebar pieces = 100'/20' = 5 laps
  • The Length of the Lap = Bar Diameter lap * (Rebar Size/8) /12 = 30 * (4/8)/12 = 1.25'
  • Laps LF = 5 laps * 1.25' = 6.25'

Number of Runs of Rebar: 2

Conversion Factor to lbs: .668 lb/ft

LBS of Rebar = (100' + 6.25') * 2 * .668 lb/ft = 141.95 lbs

The length of the lap is often given in the Structural Notes. Sometimes, you may need to dig for the lap length. The following is an example of how to find the lap length when it is not given easily in the structural notes.

First, in the Structual Notes on S-01 the following is given under Rebar Lap:

Now the Typical Reinforcing Bar Splice Detail needs to be found. It is found on Sheet S-03, which is still part of the Structural Notes pages:

From the above tables, first analyze are the bars in Tension or Compression. Footings are in Tension. Then, what is the PSI of concrete being used.

To find the PSI of concrete, refer back to sheet S-01, and the following is found:

All footings are 3000 PSI. Now refer back to the Typical Reinforcing Bar Splice Detail. Now the grade of rebar needs to be found. Referring back to S-01 the following is found.

Since the rebar is larger than #4, grade 60 rebar is to be used. Now refer back to the Typical Reinforcing Bar Splice Detail and use the Notes in the detail to determine the length of the lap.

Footings

Types of footings

Forming (SFCA)

Forming materials
  • 2 x Lumber
  • Plywood
  • Symons
  • Gang
  • Slip
For this class, both sides of all footings and foundations must be formed.

To calculate the SFCA of the continuous footing forms = footing length' * footing height' * 2 sides. Assume the ends of the continuous footings continue on in a footing of a different size.
100' * 12"/12 * 2 = 200 SFCA

To calculate the SFCA of the spread footing forms = ((footing length' + footing width') * 2) footing height'.
((5' + 5') *2) * 12"/12 = 20 SFCA

Placing

Placing costs cover the labor cost of getting the concrete from the truck to the location of placing the concrete. Many times the abbreviation of P/P/F will be used for the description of placing the concrete. P/P/F means prep/pour/finish.

Concrete Material (CY)

To calcuate the CY of Concrete = (footing length' * footing height' * footing width' / 27 CF/CY) CY of Concrete = (100' * (12"/12("/ft)) * 3') /27 = 11.11 CY

Pumping

When the concrete cannot be placed directly from the truck chute, a concrete pump or some other means may be needed to place the concrete. Other methods of transporting the concrete from the concrete truck would include, a crane and a bucket, or a conveyor systems.

Foundations

Calculating the Dowel Lengths (Vertical rebar) given the section below:

To calculate the length of the Dowel, find the footing/wall height, then add half the width of the footing. This provides the approximate length of the dowel.

Step 1
Reference the footing schedule to find the width and height of the footing shown in the detail. For this example assume the width to be 30" and the height of 12" for the footing.

Step 2
Calculate the height of the foundation wall. The section states that the footing must be a minimum of 2'6" below exterior finish grade. For this example, assume that the exterior grade is 6" below the finish slab elevation.
Therefore the height of the wall is:
    2' 6"
-  1' 0"   Height of the footing
+  0' 6" difference between exterior grade and Top of slab elevation
   2' 0"   Wall Height.

Notice in the wall section that the dowel does not go to the exterior surface of the concrete, therefore the concrete coverage needs to be subtracted from the height of the wall. Three varying conditons for concrete coverage exist in this wall. Each condition is labeled with a red number above. The following table taken from the structural notes describes the differing conditions.


OR


Condition 1:
The top condition vertical component of the dowel is essentially a formed surface Exposed to outside weather requiring 1 1/2" of concrete coverage.

Condition 2:
The bottom condition of the vertical component of the dowelis an unformed surface in contact with the earth so it requiring 3" of concrete coverage.

Condition 3:
The horizontal end component of the dowel is a formed surface in contact with the earth requiring 2" of concrete coverage.

Step 4
The vertical length of the dowel becomes:
    24"  Wall height coverted to inches
-    1.5" Top concrete coverage
+  12"  Depth of footing
-    3"   Bottom concrete coverage
    31.5 "

Step 5
The horizontal length of the dowel becomes
30" / 2 = 15" because the dowel leg is only through half the footing.
Then
    15"
-     2" of concrete coverage
    13"

Step 6
31.5" + 13" = 44.5" or 3.708' Vertical + Horizontal components

Cost Adders

Anything not listed above will add cost to the work. Items to look for are: Waterstop, Keyways, Expoxy Rebar, Chamfer Strips, Blockouts, Haunches, Architectural Finishes, Concrete Additives, Weather Conditions, Access Issues, ETC.

Remember, the prices give above are standard prices that I use. The pricing may go up or down depending on job conditions.
PICTURES

Slabs

The slab takeoff will use crews to determine the pricing of the labor.

Slab Types

Slab on Grade (SOG)
Slab on Metal Deck (SOMD)
Suspended Slabs (SS)

Reinforcing

Rebar [All]
Mesh [All]
Fiber Mesh [more typical SOG]
PT Cable [SS]

Forming

Not all slabs will need to be formed. Often walls are inplace before the slab is poured and can be used to form the perimeter of the slabs.

2x Lumber [SOG, SS, occasionally SOMD]
Angle Iron [SOMD]

Shoring [SS, SOMD]

Placing & Finishing

The size of the slab will affect the pricing. Also, super flat floors will be more expensive.

Concrete Material

Watch for additives into the concrete. Most additives will add to the cost of the material.

Thickened Edges
Thickened Slabs
Pour Strips

Dowels

Column Diamonds

The column diamonds occur at each column in the slab. This allows for the column to be placed after the the slab is poured and also guides cracking to the joints instead of going randomly throughout the slab.

Waste

SOG & SS 3%,  SOMD 7%

Pumping

Typically SOMD, SS, often SOG

Costs

Fiber Mesh

Call vendors for pricing information if you need it.

PT Cable

Call vendors for pricing information if you need it.

Shoring

Depending on requirements pricing will vary, starting range would be $3.00/sf to $6.00/sf.

Other Costs, see the Pricing Link.


Pictures

Tilt Up

Process

Casting Process

Hoisting

Pour Strips



Modified 02-14-05