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Length of Rebar in Grid

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Feb 11, 2014, 12:53:29 PM
LengthRebar=f(length,width,18,3,1)LengthRebar=f(length,width,18,3,1)
(l)Slab Length(l)Slab Length
(w)Slab Width(w)Slab Width
(oC)Grid Spacing(oC)Grid Spacing
(i)Rebar Inset from edge of slab(i)Rebar Inset from edge of slab
(rS)Rebar Size(rS)Rebar Size
(rL)Uncut rebar length on site(rL)Uncut rebar length on site
(m)Number of rebar mats(m)Number of rebar mats
(LF)Lapping Factor(LF)Lapping Factor
Tags
Masonry Length Slab Concrete Rebar Steel
UUID
e6cffcd1-da27-11e2-8e97-bc764e04d25f

The Length of Rebar in Grid calculator computes the total length of rebar needed for one or more mats of rebar in a concrete slab.

INSTRUCTIONS: Choose units and enter the following:

  • (L) Length of Slab
  • (W) Width of Slab 
  • (oC) On-center spacing of rebar  
  • (i) Inset distance from edge of slab to rebar grid
  • (m) Number of Mats. 
  • (rL) Length of the pre-cut rebar
  • (rS) Size of Rebar
  • (lF) Lapping Factor.  40 (default) and 60 are common.

Length of Rebar (LR): The calculator returns the length of rebar in feet.   However, this can be automatically converted into other length units (e.g. meters or yards) via the pull-down menu.  

The Math / Science

The Total Grid Rebar Length calculator computes the total length of rebar in a rectangular grid, useful in both calculating the weight of rebar and the cost of rebar. The .  It employs rebar lapping for dimensions (length or width) in excess of the user specified rebar lengths (rL: see Lapping below).  A tolerance for determining the need for an extra rebar is set to less than one inch  for a spacing fraction above the tolerance.  Lapping is a function of the lapping factor and the diameter of the rebar size.

rebar lapping length diagram

The algorithm calculates the area of the slab, and calculates the length of rebar needed for a rectangular grid within the slab.  The grid is specified by the inset from the edge of the slab and the nominal separation between horizontal and vertical rows of rebar (onCenter - see diagram).  Once the algorithm calculates the length of rebar, the length is used with the user specified rebar size for the rebar lapping length (see below).

The user specified parameters are:

  • (L) Length - length of the slab
  • (W) width - width of the slab
  • (rS) size - this is the size of the rebar.  The standard sizes include: 3-11,14,18
  • (oC) OnCenter - the on-center spacing between rebar, which defaults to 18"
  • (i) inset - an offset distance from the side of the slab to the first row the rebar grid,  which has a default of 3"
  • (m) mats - this is the number of mats (layers of grids) of rebar.
  • (OC) Output choice - this lets the user choose between the weight of the rebar in the grid or the length of rebar in the grid.
  • (rL) Rebar Length - this is the length of the rebar sticks used in constructing the rebar grid.
  • (LF) Lapping Factor - this the multiple of the rebar diameter used to define the overlap length at rebar joints (extension).

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".  

General Rebar Information

Rebar is short for reinforcing bar.  Rebar is a roughly circular steel bar with ribs used to provide added tensile strength to concrete structures.  Rebar is put in place before concrete is poured.  When the concrete has hardened, the concrete around the rebar ribs keep the rebar in place.  Rebar and concrete expand similarly with temperature variations.  This all has the net result of substantially added tensile strength when rebar is part of the concrete form.  Carbon steel is the most commonly used material for rebar, which may also be coated with zinc or epoxy resin.

Rebar is laid out in grids, crisscrossed patterns of rebar, tied at the intersections where runs of rebar touch.  The grids have spacing between the rebar rows, and they are placed within the concrete form by a specified inset from the edge of the concrete.  Multiple parallel grids, at uniform space intervals, are referred to as rebar mats.  

Rebar Terms

  • Rebar - reinforcing steel bar.
  • Stick - one length of rebar.  In the U.S., the most common lengths of rebar sticks are 20', 40' and 60'.
  • Lapping - when two sticks of rebar are overlapped and bound together.
  • Lapping Factor - the multiple of a rebar diameter used to specify appropriate rebar lapping length.
  • Mat - a crisscross grid of rebar sticks.  There may be more than one mat with space in between mats.
  • Size - the indicator of the diameter of rebar sticks.  Note: guage is not a correct term for rebar.

Rebar Size

In the United States, rebar sizes are in increments of 1/8th inches in diameter.  Therefore, size 4 is 4/8th of an inch, which is 1/2", and size 8 is a full inch in diameter.  Based on this and the density of steel used in rebar, the Rebar Size Table contains reasonably accurate specifications of rebar linear weight and lateral (face) area based on rebar size.

Rebar Lapping

/attachments/7862ef61-5bd2-11e5-a3bb-bc764e2038f2/Rebar Lapping.png

The most common lengths of pre-cut rebar in the United States are 20`, 40' and 60'.  These are known as rebar sticks.  When the dimensions of a slab, wall or other form exceed the length of a single stick of rebar, it is required to overlap and tie rebar pieces to create the added length.  This process is called lapping, and the length of the overlapping rebar is the rebar lapping length.  The length of the lap is specified by a "Lapping Factor (LF)" which is often 40 or 60 times the diameter of the rebar. Engineering specifications of a lapping factor should always be applied.

Rebar Tools

A class of rebar tools, both powered and manual, have been developed to aid construction workers in working with rebar.  These include the following:

  • Rebar Cutters are used to cleanly and safely cut sections of rebar.
  • Rebar Benders are used to bend rebar sticks precisely to fit into concrete forms.
  • Rebar Tiers are used to tie rebar grid intersections and for rebar lapping. 

Rebar and Slab Calculators:
This equation, Length of Rebar in Grid, is used in 6 pages
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