Structural Damping - Logarithmic decrement (d)

[Shannon Hall]

Hi all,

I am trying to get my head around the BS EN 1991-2:2003 - Part 2: Traffic Loads on Bridges.

I am struggling on within being able to find the structural damping (δ) needed in figure NA.9.

I have found that it is safe to assume 2% for a fully welded bridge/truss.

What I cannot find is what percentage is safe to assume if the structure features pinned connections or how to calculate it.

Any direction on where or how to solve this would be much appreciated.

Thanks!!

 

[JoshPlumSE]

Well, logarithmic decrement is a really easy way to determine damping when you have a measured response over time.

 

[HTURKAK]

I could not see why you are struggling.. Figure NA.9 is for Reduction factor, δ , to allow for the unsynchronized
combination of pedestrian actions within groups and crowds . You are expected to look for Structural damping – logarithmic decrement, δ to Table 6.6 , Values of damping to be assumed for design purposes.

FYI Damping ratio values for elastic analysis of bridges for seismic ;

- Welded steel: ξ = 0,02
- Bolted steel: ξ = 0,04 ( in general ) but you may use for high resistance bolts 0,03 and for ordinary bolts 0,05

- Prestressed concrete: ξ = (no cracks) 0,04
- Reinforced concrete : ξ = 0,05 and with cracks 0,10.

I hope my respond answers to your struggle .

Good Luck..

 

[Agent666]

Regarding the pinned connection questions:-

Usually for vibration, nominal pinned bolted connections (shear tabs and so forth) are treated as fixed. The reason being that the strains involved are very small and any normal snug tight bolting creates sufficient friction to achieve this apparent continuity.

 

[HTURKAK]

Just for clarification i added the excerpt from Eurocode 1: Actions on structures ,Part 2: Traffic loads on bridges

and the excerpt from Eurocode 8 — Design of structures for earthquake resistance — Part 2: Bridges

Notice that the damping for traffic loading is in general related with superstructure while for seismic damping, the pier damping governs ( but second order effects may occur in arch and cable stayed bridges )
0

In this case , if the span is greater than 20 m, ξ =0,5 % =0.005.


I hope ,this clarification will be answer to your thread.

 

[JoshPlumSE]

Note:
Question for the vibration experts on this topic. Can you confirm my understanding that estimating structural damping from measured logarithmic decrement tends to OVER-estimate the actual structural damping?

How is that possible when you're using the actual response and measuring the difference in magnitude between peaks? Well, if we're not talking about closed systems. Some of the energy radiates out to other bays and such.

In my academic life, I used Log Decrement to estimate the damping in golf club shafts. But, that was a mostly closed system. One single cantilevered golf club shaft that was fixed at the base. Some energy may have transferred into the base. However, that was probably trivial. A foot drop on the deck of a real bridge or building floor is a different situation.

 

[HTURKAK]

IMO, sometimes underestimate.. depends on the structural system and loading. The thread was for pedestrian loading and the applicable code Eurocode 1: Actions on structures ,Part 2: Traffic loads on bridges. The logarithmic decrement method is based on experimental SDOF vibration decay and the term logarithmic decrement is defined as the natural logarithm of the ratio of any two successive peak amplitudes.


Yes.. some of the energy dissipates with soil structure interaction , seismic dampers ( when used )..etc.
I will suggest you to look the following doc.

https://wbdg.org/FFC/DHS/fema451b.pdf

But mr. Shannon Hall (OP) is too busy to read the posts and hardly ever responds..