Development Length Calculation as per Indian Code

Development length is the extra length of bar provided beyond the required section in order to ensure the following;

• To develop a safe bond between the bar surface & the concrete so that no failure due to slippage of bar occurs during the ultimate load conditions.
• Also, the extra length of the bar provided as development length is responsible for transferring the stresses developed in any section to the adjoining sections (such as at column beam junction the extra length of bars provided from beam to column).

Importance: Provision of appropriate development is an important aspect of safe construction practices, proper development length in reinf. bars shall be provided as per the steel grade considered in design otherwise in scenarios where less development length against the required is provided the structures will be prone to encounter  failure due to slippage of joints, bonds, anchors & Laps, in such cases the bars will not yield first but the failure will happen at joints & laps prior to yielding of reinforcement bars.

Calculation of Development length:

As per the Indian Standard – IS 456: 2000, clause 26.2.1 the development length Ld is given the following expression;

 Where,             Ø            = nominal dia of reinforcement bar

 σ_s               = Stress in bar at the section considered at design load

 τ_bd           = Design bond stress

The above given formula is used to calculate the required development length in mm for any given dia of bar, same formula is used for limit state method as well as working stress method. The only change in calculation in both methods is due to the different value of design bond stress; the values of design bond for Limit State & working stress are as follows;

Design Bond Stress in Limit State Method
Concrete Grade	M20	M25	M30	M35	M40 & Above
Design Bond Stress (τbd, N/mm2)	1.2	1.4	1.5	1.7	1.9	For Plain Bars in Tension
	1.92	2.24	2.4	2.72	3.04	For deformed bars in tension

**Note: For bars in compression 1.25 times the above-given values shall be used.
              

Design Bond Stress in Working Stress Method
Concrete Grade	M20	M25	M30	M35	M40	M45	M50
Design Bond Stress (τbd, N/mm2)	0.8	0.9	1.0	1.1	1.2	1.3	1.4	For Plain Bars in tension
	1.28	1.44	1.6	1.76	1.92	2.08	2.24	For deformed bars in tension

**Note: For bars in compression 1.25 times the above given values shall be used.
            

Generally, in practice, the development length requirement is expressed as ‘41 times Ø’ or ‘41 Ø’ where 41 is the factor calculated using the above formula & Ø is the dia of the bar.

Example: In the following images, calculation for development length in limit state method (Fig.1), as well as working stress method (Fig.2) for M25 concrete grade & 415 steel grade, are illustrated;

Fig-1 (For Limit State Method)

Fig-2 (For Working Stress Method)

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Foundation Settlement Analysis in STAAD Pro

Footing Type support under foundation support in STAAD Pro V8i:

Almost all the times the STAAD Pro users preferably use fixed or pinned type of supports for frame analysis. However, there are times when the soil below the foundation is elastic & the joint displacements due to settlement in soil have to be taken into consideration in analysis, at such times one can use the foundation support feature provided in the Supports page of STAAD Pro.

Here below I am going to brief about creation of footing type support for a column;

• First create a column member of height 3.5 m & cross-section 0.30 m x 0.30 m.
• In the next step go to General page>>Load & Definition subpage & create a primary load case. In the created primary load case define a nodal load of Fy = -50 Kn & assign it on the top node of the column.

Figure-1

• Now, go to General page>> Supports sub-page & from data sheet area click on create button in Supports-Whole Structure window.

Figure-2

• By doing so the Create Support window will be prompted on screen, now choose foundation tab from above.
• Under the foundation tab three options viz. footing support, elastic mat & plastic mat are available. In this post we will discuss about how to model a footing type of support using Length (L) & Width (W) values.

Figure-3

• Define the values for ‘L’ & ‘W’ as 1.5 m, choose Y direction for spring generation & define the value of subgrade modulus as 2000 kn/m2/m (for SBC of 100 Kn/m2 & 50 mm allowable settlement) & click ADD. After adding the support to Supports-Whole Structure window, assign the support to the bottom node of the column.
• Now, in this example we have modeled a column of 3.5 m high 0.30 m x 0.30 m section size supported on 1.5 m x 1.5 m size footing loaded with an axial load of 50 kn.

Figure-4

• Finally go to Analysis & Print Page & assign No Print command to the structure, afterwards give Run Analysis (Ctrl+F5) command from Analyze menu.

Figure-5

• Once the analysis is complete, close the STAAD Analysis engine window & go to Post Processing mode from the Mode bar.

Figure-6

• In the Post-Processing Mode go to Node Page>>Displacement Subpage>>Node Displacements Table (on the right side of the screen). Here in the Node Displacements table check for the displacement of support node in Y direction, the value provided generated here is in mm & this displacement value is calculated on the basis of Area of Footing (L x W), Soil Subgrade Modulus, Spring Constant ‘K’& Load on Column.

Figure-7

Figure-8

• Kindly, refer the attached image for the manual calculation of support displacement for verification & for any query or suggestion kindly post your comment in the comment box.

DXF Import in STAAD Pro From AutoCAD

STAAD Pro software has the ability to import drawing files from AutoCAD for which can save the user a lot of time of geometry creation, for a successful and error free import of AutoCAD file into STAAD Pro a user shall first understand the correct procedure for importing DXF and the following given example will help you in understanding the procedure.

• Step-1: Creating a DXF: AutoCAD DXF (Drawing Interchange Format, or Drawing Exchange Format) is a CAD data file format developed by Autodesk for enabling data interoperability between AutoCAD and other programs.

• A user can create a DXF by using a write block command or by using save as function from file menu, in our example we have created a 2d frame in X-Y plane using line command.

• When imported into STAAD all the lines will be treated as members thus a user must import only center line plans using only single lines for all the beam members.

Figure-1

• In fig-1 you can see we have created a dwg file in X-Y plane using line command, the unit settings which I have used for my dwg is given in the fig-2.

Figure-2

• Now select your created geometry & use write block command ‘ w ‘, the following window will appear on the screen.

Figure-3

• Now define a file location & save your file as DXF & keep the insert units same as your default drawing units.

• Step-2: Importing DXF: Now go to staad pro welcome screen & create a new file and make sure that you set your length units as inches same as that of your dxf units (fig-4).

Figure-4

• Now after successfully creating a new staad pro file go to File>>Import the following window (fig.5) will open.

Figure-5

• Choose 3D DXF & click import, choose the file location where you have saved your DXF & click ok. The following window (fig. 6) will appear on the screen.

Figure-6

• This option is given to the user to change the orientation of coordinate axis of the DXF file, as we know the coordinate axis directions are different in AutoCAD as comparison to STAAD. In Auto CAD Z axis is UP while in STAAD Pro Y axis is UP. Suppose for example if you create a 2d floor plan in autocad you will draw it in X-Y plane with the length span in X & width span in Y but when you will import this floor beam plan in staad pro you will have to choose Y-Up so that STAAD Pro will make all the adjustments & replace the coordinate values of Y in Z, thus by doing this it will import the plan created in X-Y plane into X-Z plane.

• In our example we have created a frame in X-Y & we don’t need to change the structure convention so we will select No Change & Click Ok. Set current input units dialogue box will appear.

Figure-7

• Now, choose Inch from this dialogue box & click ok. You will find that your geometry has been imported in STAAD Pro. You can use display node to node distance command available in Tools menu to check the dimensions of the imported members.

• Note: If the imported dxf contains any of other things such as dimension lines or other lines they also will be imported. So make sure that the created DXF doesn’t contain any unnecessary objects.

Figure-8

• If the user wishes to change the current display units he/she can change the units by using current display units available in quick access toolbar (fig-9).

Figure-9

• set current input units as Meter & you can now check the structure again, this time the dimensions will be displayed in meters (fig-10).

Figure-10

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