Braced Frames on Piers in SDC D

[Galambos]

My structure is a small, two story school 160'x80' in the Southeast with SDC D.

I have a mat foundation covering the whole building at el -3'-0" and a slab on grade at 0'-0". My steel braced frame (SCBF) is bearing on two rectangular piers at el -8".

ACI 318-05 21.2.2.3 states "Structural members below base of structure that are required to transmit to the foundation forces resulting from earthquake effects shall also comply with the requirements of Chapter 21."

im not sure i can get a cantilevered pier to work, but if i did, would cantilevered columns be in compliance with ch 21?

it sounds as if my two piers at the base of my braced frame should be connected by an element such as a shearwall that is 2'-4" tall.

does anyone agree or have any thoughts or recommendations?

 

[Gumpmaster]

I think you'll have a hard time getting the concrete anchors to work in an isolated pier. Do you have an SCBF or an OCBF?

My recommendation would be to bear the column at EL -3'-0", and pour a pier around the column/ brace (provide rigid insulation around the brace and gusset plate to allow for a yielding zone in the gusset). You can then develop the uplift force into the pier through the reinforcing. For the shear force, you may be able to get it to work through the pier, if it's big enough. If that doesn't work, join the two sides of the brace together with a short wall and drag the shear into the wall. There are a couple of Steel Tips papers that show a similar configuration. Maybe the one on base plates is the best.

Braced frames get to be difficult in SDC D.