Any opinion on seismic design of a 470ft(156m) stadium roof truss? 3

[IJR]

Pals,

We will soon be designing a truss to support membrane-clad stadium roof spanning 470ft(156m) supported only at ends by R.C. box towers at elevation 70ft (23m) above ground.


The top chords are arch-formed and clears away 55ft(18m) from the lower chords, at midspan.

We have done gravity and temperature loading already, and we have allowed movement along the longitudinal axes at one end to accomodate expansion.

The structure is however in zone of high seismicity(eq. to UBC Zone 4, ground peak acceleration 0.4g).

1). Any idea on what R-FACTOR(UBC 1997 and/or 1994) to use for this structure?. Any other comments will also do.

2) What should the R and design procedure be, if we employ seismic base isolators.

Thanks in advance

IJR

 

[JAE]

IJR

The R factors are different for the 97 vs. 94 codes and there is quite a difference between the two codes.

Since your structure appears to transfer lateral loads through cantilever action of one concrete tower (the other tower is disconnected due to your allowance for lateral expansion at one end of the truss), I would initially think that you would use an R of 2.2 based upon UBC 97 Table 16-N, item 5. Cantilevered column building systems. One problem with this, though, is the height limit of 35 feet.

If your boxed towers really act like shearwalls, perhaps a concrete shearwall with R = 4.5.

But I would still be nervous about that as well, since it appears you have a very unique structure that doesn't really fall into the "traditional" categories.

What I'm getting at is you may have to perform a dynamic analysis (UBC Section 1629.8 outlines when a static procedure is allowed and when a dynamic procedure is mandated).

 

[IJR]

Thanks JAE,

Your fast response is highly appreciated. We are searching in the direction of bridge on piers kind of structures too. If you have something to say, please dont hesitate.

IJR

 

[Qshake]

My response to you, IJR, is to do the dynamic analysis first and look at the response of the structure. Review the demand and make some comparisons to the capacity. Then take a look at the code. Doesn't have to be a sophisticated modeling effort. With only one fixed connection you could use a simple single degree of freedom model.

There are no specific reduction factors, R, for box towers in the AASHTO code. This is probably due to the lack of understanding for structures such as these than any other factor. Single column bents have a response modification factor of 3. However, there is much difference between the performance and redundancy of those two (tower vs single column) to warrant further investigation.

In terms of finding a modification factor I would investigate some of the work by others on bridges with box towers. A good database is Quakeline at MCEER at University of New York - Buffalo. Also, EERI's Earthquake Spectra or the UC- Berkely database as well. The latter may be found on the EERI website. Check out "bridge towers".

 

[IJR]

JAE, Qshake

Thanks for the great inputs. We are searching on bridge towers.

How do you feel about modelling the towers as frames(Area,stiffnesses,masses etc only, no FE) and placing the trusses on top of these frame elements, then excite the whole stuff with an acceleration spectrum using R=3? (Modal Response Method).

How bad would that be.

 

[Qshake]

You could model the tower (preliminarily, of course) as a single element. The truss could also be modeled as a single element and the response characterized. However, a long truss such as this will not provide much lateral stiffness and thus could be neglected. So the result is a single column with a very large load on top: a sinlge degree of freedom. If on the other hand, you are modelling more than one bay of the structure, yes by all means include the truss (again as only a beam element). Be wary of the direction, if the towers are rectangular then the dominant mode may be in another direction. Also, depending on the mass at each tower, they could act out-of-phase of one another.

 

[IJR]

Qshake.

Thanks for the discussion above.

I agree with you in every single point you have made. We have thought of the phase-shift problem.And us too have a feeling the truss will not have that much stiffness to affect response of towers.

If you would like to go further with this discussion, then please consider the next point, which is: the behavior of the truss itself. I have a feeling the lowest R =2 will not matter much for the truss if we model it along with the towers.

My next point is: if we agree that modelling the whole system as such then use of seismic isolators between the truss and the top of the tower will not be efficient, because the truss is not stiff enough to allow full deformation of the isolator(hence low energy absorption).

What is your opinion here.

IJR

 

[carlorom]

Hi

This is indeed intriguing. However after having done all this modeling what interests me is how do you know that the designed beam(s) perform as expected. How do you monitor its safety and deflections. I am a geomaticist interested in the actual measurement of such deformations. Often find I get asked to do such work but find it difficult to get information about which crucial points need monitoring. Am prepared to help if anyone has problems along these lines but realise I need to learn from structural guys where the problem areas in ssuch structures lie.

 

[IJR]

Carlorom

Thanks for joining in. If I got you right, you are interested in how the performance of these structures is monitored.

When we design structures we survey critical aspects of design and proportion /detail critical elements. Even in a very complicated structure there are such elements usually in a reasonable number. For this truss say, connections, midspan deflection, temperature etc. will receive our utmost attention. But we will follow only guidelines,codes and recommendations.

The task of monitoring I believe is a specialized subject. We will leave that to specialized staff. Firms like Degenkolb Engineers got ways to do this. You may want to check out their works.

Regs.

IJR

 

[carlorom]

thanks for response will be interested in seeing how your project works out good luck. Yes I do measure such structures will check out the suggested site.