Seismic Force-Resisting System of Concrete Tilt up Panels

[sadienerd]

We are currently having a debate in our office regarding the applicability of using R=3 "Steel systems not specifically detailed for Seismic Resistance" for a concrete tilt up building we are doing in a seismic design category C site. The tilt up panels will be designed by others but we are the engineers of record and will be responsible for the design of the interior steel and diaphragm. The diaphragm will be metal deck on bar joist. The panels will be load bearing and will make up the vertical portion of our seismic force resisting system. Just wondering what the consensus is out there and if this is something that is being done in practice. We have only done a few tilt up buildings before and are relatively new to this type of construction. I feel that the fact this system is specifically called a Steel system excludes us because the bearing walls make up a portion of it.

 

[OHIOMatt]

Your system, as described should conform to the requirements of

Bearing Wall System
Intermediate Precast Shear Wall or Ordinary Reinforced Concrete Shear Wall - Both of these have an R of 4.

You could use the R = 3, but the walls will still have to conform to the detailing requirements of one of the above systems.

You mention that the project is tilt-up. Are the walls site cast or factory cast? True tilt-up involves site cast panels, that are lifted into place. I frequently run into people that call factory cast panels as tilt-up.

 

[sadienerd]

The walls will indeed be site cast. We refer to factory cast as pre-cast in this part of the world. Thanks for your response.

 

[sadienerd]

Also, If I stick with the R=3 for my steel but specify the R of 4 for the walls how do I convey the seismic loading info to the wall designers? The R=4 will affect the amount of shear distributed to the wall as well as the loading at the connections to the steel. I am assuming I would have to run the numbers for the shear per the R=4 but design my diaphragm and chords and collectors with the R=3

 

[TXStructural]

The answers you seek would probably be answered by the publication on this link, Engineering Tilt-Up from the Tilt-up Concrete Association:

http://www.tilt-up.org/resources/engineering_tiltup.php

The book is fairly comprehensive and is written for designers.

 

[JAE]

You might also possibly be a building frame system (vs. a bearing wall system as described by OHIOMatt above) if the building is significantly larger with lots of interior columns compared with the perimeter tilt walls.

ASCE 7 defines a building frame system this way: [blue]"A structural system with an essentially complete space frame providing support for vertical loads. Seismic force resistance is provided shear walls or braced frames."[/blue]

 

[SAIL3]

why not use R=3 for the tilt-up panels and the steel.Not sure how you would use R=3 for steel and R=4 for the tilt-up panels in the same analysis?

 

[Hokie93]

The R coefficient is selected based on the seismic force-resisting system(s) and only on the seismic force-resisting system(s). Given your seismic design category of C, that means intermediate precast concrete shear walls since ordinary precast concrete shear walls are not permitted in seismic design category C per ASCE 7-05 (assuming you are operating under ASCE 7-05). If your structure is classified as a bearing wall system, R = 4 for intermediate precast concrete shear walls. If the structure is best described as a building frame system, R = 5 for intermediate precast concrete shear walls. At any rate, you do not select one R coefficient for your seismic force-resisting system and a separate R coefficient for the diaphragm.