# Construction stage analysis

**Question:**I would be grateful for support with a construction stage analysis I am undertaking.

**1.**

**Bending moments change slightly between construction stages 2 and 3, despite no additional load added. In construction stage 3 I have deactivated the wet concrete load, which should counterbalance the self weight of the slab portion which is activated at this stage. Transverse slab is weightless in model.**Moments in Construction stage 2 – moment at mid-span of edge beam is 6536 kNmMoments in Construction stage 3 – moment at mid-span of edge beam is 6541 kNm

**2.**

**In construction stage 3, I cannot understand why the bending moments change depending on the component part selected. For example please see below screenshots.**

Moments in Construction stage 3 with total component part selected – moment at mid-span of edge beam is 6541 kNmMoments in Construction stage 3 with component part 1 selected – moment at mid-span of edge beam is 6538 kNm

As all load is carried by the beam only section, I would expect the bending moments in the total component part to be the same as in part 1.

**3.**

**I cannot understand why the hogging bending moment at the beam ends varies between the final construction stage, and the surfacing load case adopted in the post construction stage. Surfacing is the only load applied after the model becomes integral, therefore I would expect these bending moments to be the same. (please note ‘diaphragm upper’ is weightless).**

**4.**

**I do not understand the difference between ‘total’, ‘part 1’ and ‘part 2’ in the components part of the forces tab in the tree menu. I thought part 1 represented the beam and part 2 the slab in my case. However, bending moments for total are not the sum of parts 1 and parts 2.**

**Answer:**

Hello,

Following are the response to each issue:

**1. Bending moments change slightly between construction stages 2 and 3, despite no additional load added (please see below screenshots). In construction stage 3 I have deactivated the wet concrete load, which should counterbalance the self weight of the slab portion which is activated at this stage. Transverse slab is weightless in model.**

The UDL applied is 20.8kN/m for wet concrete load, while judging from the are for slab, this UDL should be 20.8345 kN/m. The accuracy considered while considering the slab are is the reason for difference in the bending moments due to slab load.

**2. In construction stage 3, I cannot understand why the bending moments change depending on the component part selected:**

Since the self weight of the slab part is activated with the slab itself, there will be minor bending moment in part 2 and hence the result of part 1 bending moment won't exactly match with the total bending moment. If the slab, i.e., part 2 is kept as weightless and the UDL isn't deactivated, then the bending moment of part 1 and total would stay the same.

**3. I cannot understand why the hogging bending moment at the beam ends varies between the final construction stage, and the surfacing load case adopted in the post construction stage. Surfacing is the only load applied after the model becomes integral, therefore I would expect these bending moments to be the same. (please note ‘diaphragm upper’ is weightless):**

Kindly note that CS: Summation shows the results for all the static load cases activated in the construction stage, including the time dependent effects if defined. In the final stage, it was noted that the the elastic links were deactivated and the plate elements were activated. The force in the links is redistributed to the adjoining structures in such cases. Now, to clarify on the results obtained, the Surfacing load results were separated from the CS: Dead load results by using the analysis control option as shown below.

Once this is done, the results for DL and surfacing could be checked separately. The results for element no. 11 are shown below:

It could be noted that results of CS: Surfacing_CS and ST: Surfacing load cases are exactly matching.

**4. I do not understand the difference between ‘total’, ‘part 1’ and ‘part 2’ in the components part of the forces tab in the tree menu. I thought part 1 represented the beam and part 2 the slab in my case. However, bending moments for total are not the sum of parts 1 and parts 2:**

The forces for parts 1 and 2 are calculated on strain compatibility for each part and hence, the summation for this isn't essentially the bending moment for the total part. Also, the forces of part 1 and part 2 are of no engineering significance. This bifurcation of parts is mainly for stress results. Hence, as a general guideline, when checking the force results, it is always advisable to check for Total and when checking the stresses, it is always advisable to check for individual parts.

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