SIG Virtual Workshop: STAAD – Design Structures With Custom, Built-up Structural Sections in STAAD.Pro

Hi,

I missed the live version of this Workshop, but I have just viewed it on demand.
I have some comments, I am not sure if this is the right place to ask questions. If it is not, could you please advise, where I can send my question (together with an input/output file and a report of my comparison - see below)

1. It is not possible to copy-paste the whole table of polygon points from the section wizard to the User Table, when you define a section with its profile polygon. You can copy-paste single cells, but for this you have to open two Staad windows (your main Staad window is not active when the Section Wizard is open, therefore you cannot switch between them). Another option is to type the values by hand (take a screenshot of the table in the Section Wizard and see the values there), I found that this is the most convenient.
2. It is indeed a more sophisticated way to define the General sections by their profile polygon points. By doing this, the profile is visible in the 3d representation of the program. However, I found that Staad doesn't use correct section classification (acc. to EuroCode) for the parts in compression. Staad uses Class 1 sections no matter what the profile is.

I did a comparison of two types of sections using two different methods in the User Table:

• Define the sections in the User Table as General Sections with their polygon points
• Define the sections in the User Table as Wide Flange sections

The sections I used are:

1.) Stiffened deck plate: PL10x225+HP300x11 bulb section, and

2.) Built-up welded H-section (symmetrical): H=300mm, B=225mm, tw=10mm, tf=10mm.

The model is a simple supported beam with a span of 10m and a distributed live load of 10kN/m, S355 steel, no load factors and no partial safety facors were used. Analysis according to EC.

When defined as a General section, Staad uses Class 1 section (which is wrong for both cases) and the moment resistance of the section is not in correspondence with the section modulus written in the output file (how does Staad calculate the moment resistance?) The utilization is significantly lower, than the actual utilization calculated by hand).

When defined as a Wide Flange section, the classification for the stiffened plate profile is ok, Staad uses Class 3 and the result is ok. However, since this section is unsymmetrical, I did  a test of applying the distributed load from below, which causes compression in the lower flange and tension in the upper flange. The lower flange is Class 1 and therefore plastic moment capacity can be used here, but Staad still uses Class3 and doesn’t differentiate the projection of the load (and thus gives a higher utilization).

For the built-up member (no.2) there was not such a big difference in the utilizations, but the section classification is wrong in both methods (this section is Class 3):

• As a General section, Staad uses Class 1 and again, the bending capacity is not in correspondence with the section modulus (neither elastic nor plastic)
• As a Wide Flange section, Staad uses Class 2 and uses plastic section modulus for the calculation of the bending capacity – this is wrong.

In both cases, the calculated utilization by Staad is lower, than the actual utilization calculated by hand.

My conclusion is that as per now, Staad can be a good tool to check Custom Built-Up sections, but the results after the analysis must be verified by a hand calculation when using other than Class 1 or 2 sections. In many cases, these user defined profiles are Class 3 or Class 4. Also, maybe it is just me, but I don’t understand how Staad calculates the bending resistance when defined as General section (it doesn’t match the section modulus).

In the output file there is mentioned a “Squash Load”, which is also different for the exact same section defined by the two different methods (Wide Flange vs General).

According to this page in the Staad.Pro help: https://docs.bentley.com/LiveContent/web/STAAD.Pro%20Help-v7/en/GUID-DA7D2181-78EF-406C-8503-5894D58FDC6A.html, the Squash Load is the max. cross section capacity. If I understand well, since the sectional area of the profile is the same, no matter how it is defined (Wide flange vs General), the Squash Load should be the same too.

Could you please have a look at this matter and suggest which method the users should use when checking non-standard built-up sections?

I appreciate you help!

Hi Beth,

I am forwarding your questions/comments to Karimu for attention. You should be hearing from him within the next few days.

Have a good evening,

Patsy