ANCHOR BOLT UPLIFT

[strguy11]

I have a 6 story building located in Nashville, TN high seismic area. The building is is about 125'x 250'. I have analyzed the building in RAMm with 3 braced frames in the long direction, and 2 in the short. I am getting what seem to be extremely high uplift loads 1700 kips at the frame reactions. All of the frame member sizes seem right, so what I am wondering is has anyone else ever had these high of forces, and what type of anchor rod configuration did you use. It seems like I may be using 2.5" rods, but I have never had something with this high of uplift loads before.

 

[msquared48]

I, too, am in a high seismic area, the Puget Sound area of the Northwest.

You are probably resisting 1500 to 2000 kips of lateral base shear to begin with, and with only 2 frames in one direction and 3 in the other, that level of uplift could be possible if there is very little resisting dead load attributed to the frames. If architecturally possible, look to more frames, longer frames, transfer beams, or adding more dead load (position of frames wrt framing) to lower the uplift.



Mike McCann
McCann Engineering

 

[Galambos]

ive had similar loads on a 12 story building. i would do the simple statics hand calc to see what your wind reaction should be and compare. your member sizes wont affect the uplift.

 

[strguy11]

Galambos -

Seismic is by far contolling over the wind, but I have check the story shears, and they seem right. What kind of anchor bolts/base plate connection did you have??

 

[JLNJ]

Your net uplift is going to control your column size?

If the numbers are right, maybe you need to have a talk with the architect about making a few changes...

 

[strguy11]

JLNJ -

No, tension does not control the column size. I am only asking if others have encountered uplift reaction this high, and what they did.

 

[strguy11]

I should say that these are the LRFD factored reactions.

 

[JLNJ]

I was being a bit facetious regarding uplift controlling.

You have a host of details to think about - from getting the loads into the anchor rods then into the foundation and into the rock or soil for the uplift. The stretch on the anchor bolts may even effect your sway.

 

[Galambos]

strguy,

do a statics check to see if ram is right. simple statics...sum the moments from the applied forces about the base of column and divide by your column spacing.

how far apart are your columns? that has alot to do with it. i would double check seismic shear controlling in the short direction in tennessee. what seismic design category is it?

 

[strguy11]

Seismic Design Category "D"

Ss = 0.346
S1 = 0.133g

Site Class "D".

I am going to see if more soils investigation can be done by the geotech to see if we can do better than a "D" classification. I am finding that too many geotechs are using this as a Default.

 

[haynewp]

I think you have to use special concentrically braced frames which will lower the Cs with the higher R value. Without knowing anything else about your building I think your uplift looks high.

 

[twinnell]

If your uplift isn't high, we have designed and are currently designing 3 265 feet tall steel structures supporting large ductwork. One column had a factored uplift reaction of 3500 kips. Only have 12 columns and had very little room to place the columns due to spagetti underground ductbank.

Anyway, we used uplift chairs on the columns. 8 3 inch diameter 105 ksi anchor rods. w shape shear posts and 8 inch thick base plates. uplift chair seat was 4 inches thick and a thick stiffener between each bolt welded to the column. The bolts are embedded 8 feet into the 9 foot thick mat foundation. The vertical brace gusset plate on this thing is 9 feet tall. huge.

 

[slickdeals]

@Strguy11
I have had situations where I have had very high tension loads at braced frames. Trying to design anchor rods for such a load is almost futile. The solution I have used to transfer the tension load into the foundation is using tension rebar (if conjestion is an issue use either 100 ksi rebar or DYWIDAG bars).

I have designed a concrete collar on top of my pile cap. The loads from the steel column are transferred by studs into the concrete collar, which are then transferred into the pile cap using rebar. The lateral forces are carried by a grade beam which carries the thrust from the brace. By providing a collar, you are really only designing the base plate for bearing. provide 4 anchor rods for stability. Check for shear around the base plate and provide rebar in both directions above the base plate to prevent a punching shear like failure.

HTH, let me know if you have additional questions.