Modal Analysis Total mass UZ issue

[federica.taglietti]

Hello all,

I have a model where I need to run a Modal Analysis for a footfall analysis check. 

I did a hand calculation check of the total mass UZ and it turns out that the value of the total mass in Robot is 15-20% smaller than the total mass in the hand calculation.

Please see attached an example models and the associated hand checks. 

Could anyone please advise if there is an issue with Robot total mass?

Thanks a lot,

Federica

@Rafal.Gaweda 

[Krzysztof_Wasik · Accepted solution]

Hi @federica.taglietti 

I have checked 201123 Plot C STEEL 9x12m grid OFFICE offset.rtd

In excel you have not taken into account facade load converted to masses

Robot does not consider in dynamic mass calculation masses assigned to nodes supported in Z direction. 

More supported nodes results in bigger difference between real and modal mass. 

To verify model 

1. I have created load combination including all loads converted to masses  1*1+2*1+3*1+4*0.1+22*1

2. Calculated sum of vertical reactions 9463 kN (converting to mass is 964628 kg). This is real structure mass when loads are converted to masses.

3. I have removed all supports and defined fixed support in four corner columns only to minimize difference in real and modal mass.

After calculations modal mass is 964593.68 kg. It is still smaller, but this small difference is caused by neglecting mass applied to four fixed nodes. In my opinion dynamic mass is calculated correctly.

[federica.taglietti]

Hi @Krzysztof_Wasik ,

Thank you for taking the time to check the model, I am pleased to know that the mass calculation runs correctly.

Correct, in my hand calcs I have not considered facade load as I assumed it would not be considered in mass calculation due to the presence of pinned supports at perimeter.

If I factor that in, the gap between hand calcs and modal mass is increased even further.

The pinned supports at facade line are an assumption we use as per SCI P354, section 6.1.1. "continuous cladding provided around facades may be assumed to provide full vertical restraint to perimeter beams. The edges of clad should therefore be modelled as pinned". Do you agree with this assumption?

As a result of what you highlighted below, do you advise to remove the pinned supports at perimeter?

"Robot does not consider in dynamic mass calculation masses assigned to nodes supported in Z direction. 

More supported nodes results in bigger difference between real and modal mass".

Many thanks you for any input,

Federica

[Krzysztof_Wasik · Accepted solution]

Hi @federica.taglietti 

You have to decide regarding cladding influence on structure (cladding type , type of connections between cladding and beams etc.). In my opinion "..may..." in code statement allows both solutions. If cladding will reduce possibility of vertical beam deflection/vibration they can be simulated as pinned support.

[federica.taglietti]

Thank you @Krzysztof_Wasik ,

I just have another final query regarding Footfall analysis assumptions. I noticed that the  walking path length Lp has an impact on the results, i.e. a shorter Lp reduces Response factor values. What does the Lp need to be compliant with? (i.e bay width, building width..).

Your advise is much appreciated,

Thank you

Federica

[Krzysztof_Wasik]

HI @federica.taglietti 

Walking path length influence is discussed in section  6.5.2. I would consider the longest distance on building story, single person can walk without stopping.