# Construction Stage Analysis of Composite Arch

The arch has 96m in span and 19.2m in rise. It has pin support at both ends and is made of steel and concrete.
It has 3 Construction Stages

1. field installation of steel tube
2. put concrete in the bottom steel tube
3. put concrete in the top steel tube

We use Midas Civil, RM Bridge (TDV) and Sofistik to do the construction stage analysis
We will consider time effects, such as creep, shrinkage and E module increasing with time in the furture. But for now, the model is under self-weight only, no time effects.
The boundary conditions are the same for all 3 softwares in all 3 stages, the support is at geometry center of the whole steel tube as shown in the stage 1.

For Midas, we use NonLinear analysis considering Accumulated Stage

Geometric nonlinear analysis is carried out with accumulated effects of the models of each construction stage. This option is used for the forward analysis of a cable stayed bridge considering large displacement.

For RM Bridge, we use “Accumulate Stiffness (Stage) Analysis”

The internal force state (normal forces) of previous construction stages is considered in the p-delta effect calculation. The relevant internal force state must be stored in the specified summation load case.

For Sofistik, we use CSM → PROB → Line and CSM → PROB → TH3 (3rd order geometry nonlinear), both gives very similar results

RM Bridge and Sofistik show very similar moments of the arch in all three stages,
while Midas Civil shows similar moments of the arch in the 1st and 2nd stages as those in RM Bridge and Sofistik, but very different moment in the 3rd stage.

Stage 1 (The moment diagram due to self-weight is symmetrical, so only half of it is hown here)
Midas Civil

RM Bridge

Sofistik

Stage 2 The moment near pin support is due to the rigid link between support and neutral axis

Midas Civil

RM Bridge

Sofistik

Stage 3
Midas Civil

RM Bridge

Sofistik

I think the moment at pin supports at CS2 should exist in CS3 if accumulated effect are considered as shown in Midas Civil.
But why it disappear in Sofistk (RM Bridge)?

Hello

Could you please provide a simplified example of your problem? To understand the problem, I need to review the boundary conditions and the construction stages. If the results are not as expected, I recommend checking the settings for the CSM. You can also look at the TEDDY files “\$name\$_csm.dat” and “\$name\$_csmlf.dat”. These files should give you insight into the calculations going on in the background
As for the differences between the models, I can’t judge because I don’t know how Midas Civil works in this specific case.

Best regards
Frederik Höller

Thanks Höller

From the results of internal force, you can find that

1. Sofistik and RM Bridge have very similar results in all 3 stages,
2. Midas Civil has very similar results in previous 2 stages but very different results in the last stage.

I also compared the stress results from Sofistik and RM Bridge, they’re also very similar.
Unfortunately, Midas can’t give me stress results.

My conlusion in the 1st post is wrong

I think the moment at pin supports at CS2 should exist in CS3 if accumulated effect are considered as shown in Midas Civil.

The momnet at pin support at stage 2 is due to the rigid link between support and neutral axis, because the neutral axis moves downwards.

BUT at stage 3, the neutral axis moves back to where it was in stage 1, there is NO rigid link between support and neutral axis. Therefore there should be no moment at pin support.
The Sofistik and RM Bridge results are correct by the above qualitative analysis, while the non-zero moment at pin support at stage 3 shown in Midas Civil is wrong.

--------------------------------------Below is the description of simplfied model--------------------------------------

A simply supported beam structure.

The section has 3 parts, the 1st part (middle part) is activated at construction stage 1 (CS1),

the 2nd part (bottom part) is activated at CS2,

the 3rd part (top part) is activated at CS3.

Both pin supports are at the center of the middle part in all 3 stages.

An uniform line load = 1kN/m are aplied at each stage.

CS1

Midas

Sofistik

CS2

Midas

Sofistik

CS3

Midas

Sofistik

At CS1, there is no moment in the section at pin supports since the pin support doesn’t allow moment.
At CS2, there is a moment in the section at pin supports, because the neutral axis of section moves downward, there is a rigid link between the support and neutral axis. The simply-supported beam becomes a portal frame.
At CS3, the neutral axis of section moves back, so the portal frame becomes a simply-supported beam again, there should be no moment in the section at pin supports.

I use two softwares Midas Civil and Sofistik to model the structure

At CS1, the results are the same.
At CS2, the results are the same too, both softwares show moment (-3.57 kN*m) in the section at pin supports
At CS3, Midas Civil still shows moment (-3.57 kN-m) in the section at pin supports, while Sofistik shows 0.

Which one is correct?

Please remove “.dat” and unzip the “Sofistik.zip”
Sofistik.zip.dat (1.4 KB)

Best regards
Sean