topic Re: Hollow slab and Formulas of matrix elements (material orthotropy) in Robot Structural Analysis Forum

Hollow slab and Formulas of matrix elements (material orthotropy)

rokas_varanauskas — Tue, 30 Apr 2024 13:54:52 GMT

I have to model steel frame with Hollow slab panels but also to consider the following:

Diaphragm action for horizontal load transfer into stability cores
Correct load distribution (one way spanning)
Thermal loads applied to panels and bar elements

Due to thermal load application, cladding panels with diaphragm actions have been eliminated from modelling consideration. It was down to two options - solid slab (equivalent thickness to hollow core) and hollow slab panels.

Hollow slab panel geometry shown above, 150mm hollow slab with 100mm structural topping.

Above geometry was approximated into below panel type in Robot:

In order to verify Robot calculated values of stiffness matrixes hand calculations were performed using RSA help files:

Membrane stiffness verification was relatively simple and hand calculations almost perfectly matched robot values. In addition, robot values were within 5% of simplified stiffness assessment from first principles. All good there.

When it comes to bending stiffness, I found that Robot calculated D_yyyy value is only about 62% of one calculated by hand using formulas. There also appears to be error in argument description for Iy calculation.

Iy = InertiaIsec(a/xv,a1,h,h1,h2) + 4*(0,00755*r*r*r*r + 0,2146*r*r*(c/2+0,7766*r)*(c/2+0,7766*r))

According to above image, b=a1, which is actually core width in the below description:

I only get correct Iy value if a1 argument is replaced with rib width (a-a1).

It's also worth noting, that arguments d=h1 and e=h2 contradict between two diagrams (top and bottom flange thickness mixed up).

Dyyyy= E*a/((a-a1)/(InertiaIsec(1/xv,0,h,h1,h2) + a1/InertiaIsec(1/xv,0,h,hz1,hz2))

Can Dyyyy Inertia calculation formulas be checked and confirmed as correct by Autodesk team @Krzysztof_Wasik

Re: Hollow slab and Formulas of matrix elements (material orthotropy)

rokas_varanauskas — Thu, 02 May 2024 14:48:30 GMT

@okapawal is this something you can advise please?

Re: Hollow slab and Formulas of matrix elements (material orthotropy)

Stephane.kapetanovic — Sat, 27 Sep 2025 08:28:22 GMT

hi @rokas_varanauskas

This doesn't answer your first question but note that you can edit values more efficiently through the API

Sub GetHollowCoreSlabsParameters() Dim ThkData As RobotThicknessData, ThkOrthoData As RobotThicknessOrthoData Dim RobApp As RobotApplication: Set RobApp = New RobotApplication Dim Labels As RobotLabelServer: Set Labels = RobApp.Project.Structure.Labels Dim RNA As RobotNamesArray: Set RNA = Labels.GetAvailableNames(I_LT_PANEL_THICKNESS) Cnv = 100 For i = 1 To RNA.Count LabelName = RNA.Get(i): If Labels.Exist(I_LT_PANEL_THICKNESS, LabelName) = 0 Then GoTo NextSlab Set ThkData = Labels.Get(I_LT_PANEL_THICKNESS, LabelName).Data With ThkData: If .ThicknessType <> I_TT_ORTHOTROPIC Then GoTo NextSlab Set ThkOrthoData = ThkData.Data With ThkOrthoData: If .Type <> I_TOT_HOLLOW_CORE_SLAB Then GoTo NextSlab H = .H * Cnv: HC = .HC * Cnv: A = .A * Cnv: A1 = .A1 * Cnv: H0 = .H0 * Cnv With .Matrix D_XXXX = .GetValue(I_TMV_D_XXXX): D_XXYY = .GetValue(I_TMV_D_XXYY): D_YYYY = .GetValue(I_TMV_D_YYYY): D_XYXY = .GetValue(I_TMV_D_XYXY) K_XXXX = .GetValue(I_TMV_K_XXXX): K_XXYY = .GetValue(I_TMV_K_XXYY): K_YYYY = .GetValue(I_TMV_K_YYYY): K_XYXY = .GetValue(I_TMV_K_XYXY) H_XX = .GetValue(I_TMV_H_XX): H_YY = .GetValue(I_TMV_H_YY) End With End With End With NextSlab: Next i Set RobApp = Nothing End Sub

Best Regards

Re: Hollow slab and Formulas of matrix elements (material orthotropy)

rokas_varanauskas — Mon, 20 May 2024 07:48:08 GMT

It was confirmed by Autodesk team that formulas in Robot help contain errors.

Specifically, error in help formulas for calculating Iy parameter:

Image above shows original help formulas

Image above shows formulas used within RSA provided by Autodesk.

a-a1 argument in Iy calculation is the key difference between RSA and help files.

I hope that this will get rectified in help files.