rebar - irregular shaped slab

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Equation / Last modified by AndrewBudd on 2019/07/23 15:48
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The Rebar for an Irregular Shaped Slab calculator estimates the total length or weight Irregular Slab with one Rebar Grid mat of reinforcement bars (rebar) needed for an irregular shaped concrete slab.

INSTRUCTIONS:Choose your preferred units and enter the following:

• (A) This is the approximate area of the irregular shaped slab (e.g. 72 ft2)
• (rP) This is the rough estimate of the perimeter of the slab (e.g. 178 ft)
• (S) This is the size of the rebar. Choose from the pull-down list size 2 to size 18.
• (oC) This is the on-center spacing of the rebar.
• (M) This is the number of rebar mats
• (WL) Output choice:  Choose rebar weight or length

Weight or Length of Rebar: If weight is chosen, the weight of the rebar is returned in pounds.  However, this can be automatically converted to other weight units (e.g. kilograms or tons) via the pull-down menu.  If length is chosen, the length returned in feet. However, this can be automatically converted to other length units (e.g. meters) via the pull-down menu.

Related Calculators:See the Rebar Calculator for more concrete and rebar functions and features.

• To compute the Length of Rebar needed for a slab, CLICK HERE.
• To compute the Weight of Rebar in a slab, CLICK HERE.
• To compute the Surface Area of a slab, CLICK HERE.
• To compute the Surface Area of Concrete Forms, CLICK HERE.
• To compute the Volume of Concrete in a slab, CLICK HERE.
• To compute the Weight of Concrete in a slab or wall, CLICK HERE.
• To compute the Total Weight of a Slab with Rebar, [Could not locate page KurtHeckman.slab – total weight estimate].

Reducing Rebar Waste

A major challenge with rebar in an irregular slab is minimizing wasted rebar.  The general guidance is this.  When possible, start with the longest cross-section of of your slab and work outwards from there. These will be the longest stretches of rebar in your slab.  When you cut off the ends, line them up in a row from shortest to longest.  When you get to shorter runs, first see if there is a cut-off section of rebar from your row of cut-offs.  This is only a little extra work and will save in both materials and labor removing cut-offs at the end of the project.

General Information

Note1: the default units are feet and inches.  However, you can change the input units to metric (SI) or others by clicking on the  units selection button (to the right of the entry fields).   You can also change the output units to metric (SI) units by clicking on the red output units selection button (to the right of the answer box).  In both cases, vCalc will make the automatic conversions.

Note2: The lapping portion of the rebar length calculation assumes that the uncut rebar on site is 40' (feet) in length and that lapping is not necessary under that length.  Furthermore, this calculator assumes that rebar is lapped by a factor of 40 times the diameter of the rebar chosen. For the same calculations with the ability to add a different "uncut" length and/or a different lapping factor CLICK HERE.

Description Rebar Specifications

The rebar weight algorithm calculates the weight of reinforcement steel bars in an irregular shaped concrete slab.   The algorithm uses the Rough Area, Perimeter and the onCenter spacing to estimate the total length of rebar in the slab.  Once the algorithm calculates the length of rebar, the length is used with the user specified rebar gauge, and density constants from the vCalc library to calculate the total weight of the reinforcement steel bars.

The user specified parameters are:

• Rough Area - area of the slab
• Rough Perimeter -  rough perimeter of the slab
• OnCenter - the on-center spacing between rebar, which defaults to 18"
• size - this is the size of the rebar.  The standard size include: 3-11,14,18
• mats - (m) this is the number of mats of rebar.
• Output choice - this lets the user choose between the weight of the rebar in the grid or the length of rebar in the grid.

The calculation determines the need for an extra rebar using an internal tolerance for for a spacing fraction that exceeds the tolerance with a default of 1".  It also assumes that the uncut rebar is in 40' lengths and that lapping is needed for dimensions in excess of 40`.  Furthermore, the lapping factor, when lapping is needed, is 40.  For the same calculations with the ability to add a different "uncut" length and/or a different lapping factor CLICK HERE.

Usage

Reinforcement bars are often used in concrete including common slabs.  This formula provides a length and weigh calculation that is useful in understanding the additional load of the slab added by the rebar steel.  It is also useful for calculating the weight of rebar when considering transport.  Steel is dense and heavy.  Most vehicles would be considerably overloaded in weight of rebar long before their potential volume is full, which poses a significant safety issue.  In the U.S., most pickup trucks are rated at a half or three quarters ton load rating.  This rating indicates the safe weight of a load that can be carried.  The rebar weight formula can help determine how many trips are required to transport the load of reinforcement steel safely.

Lapping

When the dimensions of your slab or wall exceed the length of a single piece of rebar, it is required to lap and tie the rebar to create the added length.  There are a few considerations.  First, the length of the lap is often specified as 40 times the diameter of the rebar.  In this case, 40 is lapping factor.  60 is also a common factor, but the engineering specifications should always be applied.  See these YouTube videos to better understand rebar lap:

Second, the typical length of pre-cut rebar is 40' and 60' in the United States.