STAAD.Pro steel design parameters tell the program how a steel member should be evaluated during code checking or member selection. Parameters such as LY, LZ, KY, KZ, UNT, UNB, FYLD and TRACK can significantly affect the calculated capacity of a steel member.
The difficult part is that these parameters do not simply represent properties of the steel section. Many of them describe how the member is restrained in the actual structure.
For example, changing LY or LZ changes the effective buckling length used in the design, while changing UNT or UNB changes the assumed unsupported length of a flange for flexural design. Bentley's documentation specifically distinguishes LY/LZ from UNT/UNB because they are used for different aspects of steel-member design. (Bentley Systems)
This guide explains the most commonly encountered STAAD.Pro steel design parameters, what they mean, when they are required, and the mistakes engineers should avoid.
What Are STAAD.Pro Steel Design Parameters?
After performing structural analysis, STAAD.Pro needs additional information to perform steel design.
A simplified workflow is:
Structural model → Loads → Analysis → Steel design parameters → Code check/selection
For example:
PARAMETER
CODE AISC
FYLD 345000 MEMB 1 TO 20
LY 6.0 MEMB 1 TO 20
LZ 3.0 MEMB 1 TO 20
CHECK CODE MEMB 1 TO 20The exact parameters required depend on the design code, member type, structural system and design assumptions.
A critical point is that design parameters should represent the actual structural behavior. They should not be changed merely to make a failed member pass.
1. FYLD — Yield Strength
FYLD defines the yield strength used by the steel design procedure.
For example:
FYLD 345000 MEMB 1 TO 20The value must be consistent with the units being used in the STAAD.Pro model.
Bentley's technical support documentation confirms that FYLD is one of the parameters used to specify steel yield strength. (Bentley Systems)
Why is FYLD important?
Yield strength directly influences the resistance of steel members. If the wrong steel grade is specified, the calculated design capacity can be incorrect.
Therefore, always check:
Steel grade
Yield strength
Units
Applicable design code
Member material assignment
Do not simply enter a higher FYLD to improve the design result.
2. KY and KZ — Effective Length Factors
KY and KZ are among the most misunderstood STAAD.Pro steel design parameters.
They represent the effective length factors associated with buckling about the member's local axes.
Conceptually:
where:
= effective length
= effective length factor
= relevant reference/unbraced length
STAAD.Pro documentation identifies KY and KZ as effective-length factors for the local Y and Z axes. (www2.tecgraf.puc-rio.br)
For example:
KY 1.0 KZ 1.0 MEMB 10 TO 20does not mean the column is automatically correctly modeled. The appropriate values depend on the actual restraint and framing conditions.
Why KY and KZ can be different
A column may be restrained differently about its two principal axes.
For example:
Strong bracing in one direction
Flexible framing in the perpendicular direction
Therefore:
KY ≠ KZmay be perfectly reasonable.
The engineer must determine the appropriate effective-length assumptions from the actual structural system.
3. LY and LZ — Buckling Lengths
LY and LZ specify the lengths used for buckling calculations about the member's local Y and Z axes.
For example:
LY 6.0 MEMB 25
LZ 3.0 MEMB 25The STAAD.Pro technical reference describes LY and LZ as lengths used in calculating slenderness for buckling about the respective local axes. (www2.tecgraf.puc-rio.br)
These parameters are particularly important for compression members such as steel columns.
Why LY and LZ matter
Buckling resistance depends strongly on slenderness.
Conceptually:
where:
= effective buckling length
= radius of gyration
= slenderness ratio
A longer effective buckling length generally means a more slender member and therefore a lower buckling capacity.
4. KY/KZ vs LY/LZ — What's the Difference?
This is one of the most important distinctions to understand.
| Parameter | Meaning |
|---|---|
| KY | Effective length factor about local Y-axis |
| KZ | Effective length factor about local Z-axis |
| LY | Buckling/unbraced length associated with local Y-axis |
| LZ | Buckling/unbraced length associated with local Z-axis |
Do not treat them as four different ways of entering the same number.
Bentley's support documentation specifically states that LY and LZ are used for axial-compression buckling, while UNT and UNB are used for flexural design. (Bentley Systems)
A common mistake is to modify UNT/UNB when the actual issue is the column's axial buckling length.
5. UNT — Top Flange Unsupported Length
UNT represents the unsupported length of the top flange used in flexural design calculations.
Bentley's documentation states that STAAD.Pro uses UNT and UNB to specify unsupported lengths for flexural design. The program determines which flange is in compression and uses the applicable parameter. (Bentley Systems)
For example:
UNT 3.0 MEMB 35 TO 40The value should reflect the actual lateral restraint condition of the top flange.
Example
Consider a steel beam supporting a floor system.
If the compression flange is adequately restrained by the floor system at regular intervals, the relevant unsupported length may be significantly smaller than the full beam span.
Therefore, blindly assigning:
UNT = full beam lengthmay be inappropriate.
The actual restraint must be established from the structural arrangement and design assumptions.
6. UNB — Bottom Flange Unsupported Length
UNB is similar to UNT but applies to the bottom flange.
Bentley introduced UNT and UNB to replace the older single UNL approach because the top and bottom flange unsupported lengths may be different. (Bentley Systems)
For example:
UNT 3.0 MEMB 35 TO 40
UNB 6.0 MEMB 35 TO 40This could represent a situation where the top flange has more lateral restraint than the bottom flange.
The actual values must come from the structural configuration rather than being selected simply because they produce a better design ratio.
7. UNT/UNB vs LY/LZ
This distinction deserves its own section because it causes frequent STAAD.Pro design mistakes.
LY and LZ
Primarily relate to member buckling under axial compression.
UNT and UNB
Relate to unsupported flange lengths for flexural design.
Bentley's support documentation explicitly warns against using UNT/UNB to define the column unbraced length for axial compression; LY and LZ should be used for that purpose. (Bentley Systems)
In simple terms:
Column buckling → think LY/LZ
Beam flange restraint → think UNT/UNB
This is a useful rule of thumb, although the exact design behavior depends on the selected steel design code.
8. UNL — Older Parameter
You may encounter UNL in older STAAD.Pro tutorials and models.
For current AISC steel design workflows, Bentley explains that UNT and UNB replaced UNL because separate top- and bottom-flange unsupported lengths can be specified. (Bentley Systems)
Older files may still contain UNL for compatibility.
If UNT/UNB are specified together with UNL, Bentley states that UNL is ignored. If neither UNT nor UNB is specified, an older UNL value can continue to be used for compatibility. (Bentley Systems)
Therefore, when following an old tutorial, don't blindly copy its UNL parameter into a new model without checking the design-code documentation for your STAAD.Pro version.
9. TRACK — Design Output
TRACK controls the amount of information reported in the steel design output.
For example:
TRACK 2.0 MEMB 1 TO 20can be used when more detailed design output is required, depending on the applicable design code.
The parameter is particularly useful when troubleshooting a failed member because the detailed output can help identify whether the controlling condition is related to:
Axial compression
Tension
Bending
Shear
Slenderness
Interaction
Buckling
Don't automatically use the highest level of output for every member in a large project. It can produce very large reports.
10. CODE — Selecting the Design Standard
The CODE command tells STAAD.Pro which steel design code to use.
For example:
PARAMETER
CODE AISCor an applicable national design code supported by your STAAD.Pro version.
The design parameters available and their interpretation can vary between design codes.
This is important because a parameter used in one code may not have exactly the same meaning or applicability in another.
Therefore:
Always interpret a STAAD.Pro design parameter in the context of the selected design code.
Don't assume that an AISC parameter behaves identically under IS 800, Eurocode or another steel design standard.
11. CHECK CODE vs SELECT
Two commands frequently appear alongside steel design parameters:
CHECK CODE
Checks the existing assigned section.
CHECK CODE MEMB 1 TO 20The question is essentially:
"Does this section satisfy the design requirements?"
SELECT
Attempts to select a suitable section from the available database.
SELECT MEMBER 1 TO 20The conceptual difference is:
CHECK CODE → check my existing section
SELECT → find a suitable section
This distinction is particularly important when developing automated steel-design workflows.
12. Example of STAAD.Pro Steel Design Parameters
A simplified design block might look like:
PARAMETER 1
CODE AISC
FYLD 345000 MEMB 1 TO 20
KY 1.0 MEMB 1 TO 20
KZ 1.0 MEMB 1 TO 20
LY 6.0 MEMB 1 TO 20
LZ 3.0 MEMB 1 TO 20
UNT 3.0 MEMB 1 TO 20
UNB 6.0 MEMB 1 TO 20
TRACK 2.0 MEMB 1 TO 20
CHECK CODE MEMB 1 TO 20This is only an illustrative example. The values should not be copied into a real project without establishing the actual structural restraint conditions and applicable design-code requirements.
13. Common Mistakes in STAAD.Pro Steel Design Parameters
Mistake 1 — Setting KY and KZ to 1.0 everywhere
A value of 1.0 is not automatically correct for every structural system.
The effective length factor should represent the assumed restraint.
Mistake 2 — Using beam span as UNT automatically
UNT should reflect the relevant unsupported length of the flange, not simply the overall member length.
Bentley notes that when UNT and UNB are not specified, STAAD.Pro assumes their values are equal to the member length for the relevant AISC design procedures. (Bentley Systems)
Mistake 3 — Using UNT to solve a column buckling problem
If you're trying to define the unbraced length for axial compression, examine LY and LZ, not just UNT/UNB. (Bentley Systems)
Mistake 4 — Changing parameters until the member passes
This is the worst approach.
If a member fails, investigate why it fails.
Then determine whether the assumed:
Bracing
Effective length
Unsupported length
Material grade
Section
Loading
is correct.
Only then should the design parameters be changed.
STAAD.Pro Steel Design Parameters Quick Reference
| Parameter | Basic purpose |
|---|---|
| FYLD | Yield strength of steel |
| KY | Effective length factor, local Y |
| KZ | Effective length factor, local Z |
| LY | Buckling/unbraced length, local Y |
| LZ | Buckling/unbraced length, local Z |
| UNT | Unsupported length of top flange |
| UNB | Unsupported length of bottom flange |
| UNL | Older unsupported-length parameter |
| TRACK | Design-output/detail level |
| CODE | Design code |
| CHECK CODE | Checks existing member section |
| SELECT | Selects a suitable member section |
Final Thoughts
Understanding STAAD.Pro steel design parameters is much more important than simply knowing their syntax.
The software can perform the calculations extremely quickly, but it cannot automatically know every detail of the real structure. The engineer must tell STAAD.Pro about the relevant restraint, bracing, unsupported lengths, material properties and design assumptions.
The parameters that deserve particular attention are:
KY/KZ for effective-length factors
LY/LZ for buckling lengths
UNT/UNB for flange restraint in flexural design
FYLD for material strength
TRACK for understanding design output
The most common conceptual mistake is confusing LY/LZ with UNT/UNB. They address different design behaviors, and Bentley's documentation explicitly distinguishes them. (Bentley Systems)
Finally, never change a parameter simply to make a member pass. A passing STAAD.Pro ratio is useful only when the assumptions behind that ratio represent the actual structural system.
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