STAAD.Pro Steel Design Parameters as per IS 800:2007 – Detailed Guide
STAAD.Pro is widely used for the analysis and design of structural steel members. For design as per IS 800:2007, appropriate steel design parameters must be assigned to the members before performing the code check or selecting the required steel section.
Parameters such as FYLD, FU, KY, KZ, LY, LZ, LX, UNT, UNB, MAIN, TMAIN, STP, TRACK, CHECK CODE, SELECT and RATIO control different aspects of steel design including material strength, effective length, slenderness, lateral-torsional buckling and design output.
This article explains these parameters in simple language and shows how they are used in practical STAAD.Pro steel design.
1. STAAD.Pro Steel Design as per IS 800:2007
IS 800:2007 is the Indian Standard code of practice for general construction in steel. STAAD.Pro uses the provisions of the selected steel design code to check whether a structural member is adequate for the applied forces and moments.
A typical steel member may be subjected to:
- Axial tension
- Axial compression
- Shear force
- Major-axis bending
- Minor-axis bending
- Torsion
- Combined axial force and bending
The design parameters tell STAAD.Pro how the member is restrained and what assumptions should be used while calculating its design strength.
The correct value of a design parameter depends on the actual structural arrangement, support conditions, bracing system, section orientation and loading. Parameters should not be entered simply to obtain a satisfactory design ratio.
2. Important STAAD.Pro Steel Design Parameters
| No. | Parameter | Meaning | Purpose |
|---|---|---|---|
| 1 | FYLD | Yield strength of steel | Used for calculation of design strength. |
| 2 | FU | Ultimate tensile strength of steel | Used for checks involving ultimate strength. |
| 3 | KY | Effective length factor about local Y-axis | Used to calculate effective buckling length. |
| 4 | KZ | Effective length factor about local Z-axis | Used to calculate effective buckling length. |
| 5 | LY | Buckling length about local Y-axis | Used in slenderness ratio calculation. |
| 6 | LZ | Buckling length about local Z-axis | Used in slenderness ratio calculation. |
| 7 | LX | Length associated with torsional/flexural-torsional buckling | Used for relevant buckling calculations. |
| 8 | UNT | Unsupported length of top flange | Important for lateral-torsional buckling when the top flange is in compression. |
| 9 | UNB | Unsupported length of bottom flange | Important when the bottom flange is in compression. |
| 10 | MAIN | Maximum allowable slenderness ratio for compression members | Controls slenderness limit for compression members. |
| 11 | TMAIN | Maximum allowable slenderness ratio for tension members | Controls slenderness limit for tension members. |
| 12 | STP | Section type | Identifies rolled or welded/built-up section for design. |
| 13 | TRACK | Design output/detail level | Controls the amount of design information reported. |
| 14 | CHECK CODE | Code check | Checks the adequacy of the assigned section. |
| 15 | SELECT | Automatic section selection | Selects an economical section satisfying the design requirements. |
| 16 | RATIO | Utilization/design ratio | Indicates whether the member satisfies the design requirements. |
3. FYLD – Yield Strength of Steel
FYLD represents the yield strength of the steel, normally expressed in MPa (N/mm2).
Yield strength is the stress at which the steel begins to undergo significant permanent deformation.
Example
For a steel grade having a yield strength of 250 MPa:
FYLD 250
For a steel grade having a yield strength of 345 MPa:
FYLD 345
The actual value must correspond to the steel grade specified for the project and the applicable material standard.
FYLD represents yield strength, whereas FU represents ultimate tensile strength.
4. FU – Ultimate Tensile Strength
FU represents the ultimate tensile strength of the steel in MPa. It is the maximum tensile stress associated with the ultimate strength of the material.
For example, if the selected steel grade has an ultimate tensile strength of 410 MPa, the corresponding value may be specified as:
FU 410
The value should always be taken from the applicable material specification rather than assumed.
5. KY – Effective Length Factor About Local Y-Axis
KY is the effective length factor used for buckling about the member's local Y-axis.
The effective length is generally expressed as:
Therefore:
The value of K depends on the actual end restraint and framing conditions. It should not automatically be taken as 1.0 for every column.
Typical conceptual values
| End condition | Approximate conceptual K |
|---|---|
| Pinned – Pinned | About 1.0 |
| Fixed – Fixed | Less than 1.0, depending on restraint |
| Fixed – Pinned | Intermediate value |
| Cantilever | About 2.0 for idealized end conditions |
Actual effective length should be established from the structural system and the assumptions permitted by the design standard.
6. KZ – Effective Length Factor About Local Z-Axis
KZ is similar to KY, but it applies to buckling about the local Z-axis.
This parameter is particularly important for columns because the major and minor axes of an I-section have very different stiffnesses.
For example, a steel column may be adequately restrained in one direction but poorly restrained in the perpendicular direction. In such a case, KY and KZ may not be equal.
7. LY – Buckling Length About Local Y-Axis
LY represents the effective buckling length used for slenderness calculations about the local Y-axis.
Slenderness is related to:
where:
- Le = effective length
- r = appropriate radius of gyration
- λ = slenderness ratio
The buckling length is therefore a very important parameter in compression member design.
8. LZ – Buckling Length About Local Z-Axis
LZ defines the buckling length about the local Z-axis.
A column can have different buckling lengths about its two principal axes. For example, a column braced strongly in one direction may have a much smaller effective buckling length in that direction.
Correct specification of LY and LZ is therefore essential for realistic column design.
9. Difference Between KY/KZ and LY/LZ
This is one of the most commonly misunderstood aspects of STAAD.Pro steel design.
| Parameter | Meaning |
|---|---|
| KY | Effective length factor about local Y-axis |
| KZ | Effective length factor about local Z-axis |
| LY | Buckling length used about local Y-axis |
| LZ | Buckling length used about local Z-axis |
Conceptually:
In practical STAAD.Pro modelling, the exact interaction of these parameters should be checked against the STAAD.Pro design documentation and the actual member modelling assumptions.
10. LX – Length for Torsional/Flexural-Torsional Buckling
LX is associated with the length used in relevant torsional or flexural-torsional buckling calculations.
This parameter can become important for compression members where torsional restraint and the warping characteristics of the section influence the buckling behaviour.
LX should therefore be based on the actual restraint conditions of the member rather than simply using the physical member length in every case.
11. UNT – Unsupported Length of Top Flange
UNT represents the unsupported length of the top flange when the top flange is relevant to compression and lateral-torsional buckling.
Consider a steel beam supporting a concrete slab. If the slab provides adequate lateral restraint to the compression flange, the effective unsupported length may be significantly different from the unbraced beam length.
Why is UNT important?
Lateral-torsional buckling can significantly reduce the bending capacity of a steel beam. Therefore, the actual lateral restraint provided to the compression flange must be considered.
Do not automatically enter the complete beam span as UNT. Determine the actual unbraced length based on the structural arrangement and available lateral restraint.
12. UNB – Unsupported Length of Bottom Flange
UNB represents the unsupported length of the bottom flange when the bottom flange is the compression flange.
This becomes particularly relevant where the bending moment changes sign or where loading produces compression in the bottom flange.
For beams subjected to both positive and negative bending, the compression flange can change along the member. Consequently, the designer must understand the actual restraint conditions before assigning UNT and UNB.
13. MAIN – Maximum Slenderness Ratio for Compression Members
MAIN controls the maximum allowable slenderness ratio considered for compression members.
The image associated with this article shows a commonly used/default value of:
The applicable limit, however, must be checked against the governing provisions of IS 800:2007 and the actual type and use of the member.
Slenderness is important because a compression member may fail by buckling before the yield strength of the steel is reached.
14. TMAIN – Maximum Slenderness Ratio for Tension Members
TMAIN controls the maximum allowable slenderness ratio for tension members.
The reference sheet shows:
The designer should verify the applicable slenderness limitation from the governing provisions of IS 800:2007 for the particular member and application.
15. STP – Section Type
STP identifies the type of steel section being designed.
The reference sheet identifies two common categories:
- STP 1 – Rolled section / hot-rolled steel section
- STP 2 – Welded or built-up section
This distinction can be important because the design provisions and section properties applicable to rolled and built-up members can differ.
Example
An ISMB or ISMC section manufactured as a standard rolled section would generally be treated differently from a fabricated plate girder or built-up welded column.
16. TRACK – Design Output Level
TRACK controls the amount of information generated in the steel design output.
| TRACK | Purpose |
|---|---|
| TRACK 0 | Minimum/basic output |
| TRACK 1 | Standard design output |
| TRACK 2 | Detailed design output |
During troubleshooting or detailed design review, TRACK 2 can be particularly useful because it provides more information about the calculations and governing checks.
17. CHECK CODE – Checking an Existing Steel Section
The CHECK CODE command is used when the designer wants STAAD.Pro to check whether the assigned steel section is adequate for the applied design forces.
In simple terms, the question being asked is:
If the member passes the applicable checks, the design ratio should satisfy the acceptance criterion.
18. SELECT – Automatic Steel Section Selection
SELECT allows STAAD.Pro to automatically select a suitable steel section from the specified steel table/database.
The program evaluates candidate sections and selects one satisfying the applicable design requirements.
Automatic selection can be very useful when designing a large number of beams and columns.
The lightest mathematically acceptable section is not necessarily the best practical section. Fabrication, availability, connection requirements, deflection, detailing, corrosion protection and constructability should also be considered.
19. RATIO – Steel Design Utilization Ratio
RATIO is one of the most important values in the STAAD.Pro steel design output.
In general terms:
A ratio less than or equal to 1.0 generally indicates that the member satisfies the corresponding strength requirement, subject to all other applicable checks.
| Ratio | General interpretation |
|---|---|
| 0.50 | Member has substantial unused capacity for that governing check |
| 0.85 | Generally within the strength limit |
| 1.00 | At the design limit |
| > 1.00 | Design check is not satisfied |
A ratio below 1.0 should not be interpreted as meaning that every aspect of the member is automatically satisfactory. Deflection, serviceability, connections, detailing, vibration, fatigue and other project-specific requirements may still govern.
20. Practical Example – Steel Column
Consider a steel column in a building frame. Assume the following conceptual conditions:
- Steel grade with FYLD = 250 MPa
- Ultimate strength as specified for the selected steel grade
- Column physical length = 3.0 m
- Different restraint conditions about the two principal axes
- Standard rolled I-section
The design process may involve determining:
- Steel material properties
- Effective length about local Y-axis
- Effective length about local Z-axis
- Slenderness ratios
- Compression buckling strength
- Combined axial and bending effects where applicable
- Final design ratio
The important point is that the effective length cannot be decided merely from the physical member length. The actual bracing and end restraint must be considered.
21. Practical Example – Steel Beam
Consider a steel beam spanning between two supports.
If the beam is laterally restrained at regular intervals, the unsupported compression flange length can be much smaller than the full span.
For example, if a 12 m beam is laterally restrained at every 3 m, the relevant unbraced length may be based on the actual restraint arrangement rather than automatically using 12 m.
This is why UNT and UNB are important in steel beam design.
The value must represent the actual lateral restraint available to the compression flange.
22. Difference Between Physical Length and Effective Length
A common mistake in STAAD.Pro modelling is assuming:
This is not universally true.
The effective buckling length depends on:
- End restraints
- Bracing
- Frame action
- Adjacent members
- Rotational restraint
- Lateral restraint
- Actual structural configuration
Therefore, KY, KZ, LY and LZ should be selected based on the actual structural behaviour.
23. Why Lateral-Torsional Buckling is Important
A steel beam subjected to bending can fail by lateral-torsional buckling before reaching its full plastic or yield capacity if the compression flange is insufficiently restrained.
The beam may move laterally and twist simultaneously.
Important factors include:
- Unbraced length
- Section properties
- Loading arrangement
- Moment distribution
- Lateral restraint
- Torsional restraint
Therefore, simply checking the bending stress without considering lateral-torsional buckling may lead to an unsafe assessment.
24. Common Mistakes in STAAD.Pro Steel Design Parameters
Mistake 1 – Using FYLD incorrectly
The yield strength should correspond to the actual steel grade specified for the project.
Mistake 2 – Assuming KY = KZ = 1.0 for every column
Effective length factors depend on restraint and structural behaviour.
Mistake 3 – Using the complete beam span as UNT
If the compression flange is laterally restrained at intermediate locations, the unbraced length should reflect the actual restraint arrangement.
Mistake 4 – Ignoring negative bending
The compression flange can change from top to bottom depending on the bending moment. Both UNT and UNB may therefore become relevant.
Mistake 5 – Looking only at RATIO
A satisfactory ratio does not replace engineering review of the entire design.
Mistake 6 – Selecting an uneconomical section
A section that passes the design check may still be unnecessarily heavy or impractical for fabrication.
25. Recommended Workflow for STAAD.Pro Steel Design
- Create the structural model correctly.
- Assign appropriate member releases and supports.
- Assign the correct steel material/grade.
- Define the appropriate design code.
- Review local member axes.
- Determine the actual buckling restraints.
- Define appropriate KY and KZ values where required.
- Define LY and LZ based on the applicable buckling lengths.
- Review LX where torsional/flexural-torsional buckling is relevant.
- Determine actual top and bottom flange unbraced lengths.
- Assign UNT and UNB where required.
- Run the steel design.
- Review the governing design ratio.
- Review detailed design output.
- Check serviceability and other project requirements.
- Review connections and constructability separately.
26. Quick Memory Table
| Parameter | Easy way to remember |
|---|---|
| FYLD | Yield Strength |
| FU | Ultimate Strength |
| KY / KZ | Effective Length Factors |
| LY / LZ | Buckling Lengths |
| LX | Torsional/Flexural-Torsional Buckling Length Parameter |
| UNT / UNB | Unsupported Top/Bottom Flange Length |
| MAIN | Compression Slenderness Limit |
| TMAIN | Tension Slenderness Limit |
| STP | Section Type |
| TRACK | Design Output Detail |
| CHECK CODE | Check Existing Section |
| SELECT | Automatically Select Section |
| RATIO | Design/Utilization Ratio |
27. Important Engineering Note
STAAD.Pro design parameters should always represent the real structural behaviour. Changing KY, KZ, LY, LZ, UNT or UNB can have a significant effect on calculated design strength.
Therefore, these parameters should not be changed merely to make a member pass the design check.
The designer should establish the appropriate assumptions from the structural drawings, framing arrangement, bracing system, connection details and applicable provisions of IS 800:2007.
STAAD.Pro calculates what you tell it to calculate. Therefore, correct modelling and correct design parameters are as important as the final design ratio.
28. Conclusion
Understanding STAAD.Pro steel design parameters is essential for obtaining reliable design results as per IS 800:2007.
The most important parameters to understand are:
- FYLD and FU – material strengths
- KY and KZ – effective length factors
- LY and LZ – buckling lengths
- LX – torsional/flexural-torsional buckling related length
- UNT and UNB – flange unbraced lengths
- MAIN and TMAIN – slenderness limits
- STP – section type
- TRACK – output detail
- CHECK CODE – adequacy check
- SELECT – automatic section selection
- RATIO – design utilization indicator
A good STAAD.Pro model is not simply one that produces a ratio below 1.0. It is a model in which the geometry, loading, supports, releases, member orientation, material, effective lengths and restraint conditions correctly represent the actual structure.
For important structural works, the final design should always be reviewed by a qualified structural engineer with reference to the current applicable codes, project specifications and STAAD.Pro design documentation.
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