Anchor Bolts - Ductile Design to ACI 318

[Egnorant]

I wonder if somebody could help me to understand the design of anchor bolts, in tension, in concrete and provisions of ACI318-11. I am trying to understand Clause D3.3.4.3 for lets say the following case:-

Design of base plate and anchor bolts for a simple goal post (Steel Frame) supporting a load at mid span. I have carried out the following steps:-

i. Calculate the seismic load and multiply the seismic loads by 2, where 2 is an overstrength factor.

ii. Use ASCE07-10 load combinations to analyse the goal post. Worst case loading, resulting in worst uplift/tension is for load case 5b (1.2D + E + L + 0.2S)

iii. Design base plate using reactions from the load case mentioned in item ii above.

iv. Base plate with four bolts will be required. Aim to is to have a ductile design conforming to ACI318. ACI318 Clause gives 4 cases viz D3.3.4.3a, D3.3.4.3b, D3.3.4.3c or D3.3.4.3d. I am not sure which one is the more appropriate in my case. Could somebody help to understand/decide.

I think that clause D3.3.4.3b is more appropriate for my case. But I am not sure how to calculate the anchor design forces to satisfy requirements of this clause. Clause is copied below:-

“The anchor or group of anchors shall be designed for the maximum tension that can be transmitted to the anchor or group of anchors based on the development of a ductile yield mechanism in the attachment in flexure, shear, or bearing, or a combination of those conditions, and considering both material overstrength and strain hardening effects for the attachment.”

I have the following questions:
1. Does step i mentioned above satisfy requirement “considering both material overstrength and …”?

2. How can strain hardening be considered in calculations, is there any recommended method that could be used?

3. In the light of above clause is it right to consider that base plate should fail/yield prior to the yielding of the anchor bolts (Please read clause "The anchor …… shall be designed ….. based on the development of a ductile yield mechanism in the attachment")

4. What forces shall I use for the design of anchor bolts? Can I use reactions as obtained in step ii above

5. if reactions from step ii are used, then it means that “attachment” (which in this case is base plate) and the anchor bolt are designed for the same loadings/reactions. I think in this case I’ll not be able to satisfying the requirement “development of a ductile yield mechanism in the attachment”. Should the force for the design of anchor be greater that used for the design of base plate, thus ensuring that base plate yields first, but how would one decide on the design force for anchor bolts.

 

[KootK]

Is this a cantilevered goal post or something else?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[wannabeSE]

I am assuming the overstrength, Ω₀, is for the lateral system per ASCE 7. For this overstrength factors, use the strength load combinations from ASCE 7-10 §12.4.3.2. The tensile force from these combinations can be used to design the anchors without any additional ductility requirements because it meets ACI 318-11 §D.3.3.4.3.d requirements.

In §D.3.3.4.3.b, an element (e.g. baseplate) is designed to yield. The material overstrength refers to the fact that the specified yield strength used for design is less than actual yield strength of the material. For instance A36 plate often times will yield at 50 ksi. This higher, expected yield strength is used to determine the forces Read ACI's commentary. You need to fully understand the meaning and reasoning before using this section. §D.3.3.4.3.b does not have an easy cookbook style solution.

 

[Egnorant]

KootK, Yes this is a cantilevered goal post

WannabeSE. Thanks for the reply. Please note that I am new to American codes, normally I use British or Euro codes. At the moment I am really struggling to understand the concept/philosophy of the overstrength factor. Added to that confusion is that the steel codes (ASCE 341) required the columns to yield (to be ductile) prior to it connection under seismic loading, which in my case is the base plate/anchor bolt. Then there is requirement of ACI 318 for the base plate to fail/yield prior to the failure of concrete (thus concrete -governed strength to be greater than the attachments). This is where I need help. I can't understand what forces/loads and loads factors to be used for each element viz. column, base plate, anchor bolts and concrete foundation so that requirements of both, steel and concrete, codes are satisfied. Thanks for the help

 

[KootK]

To me, this sounds like an inverted pendulum structure Egnorant. That simplifies some things I think. I would expect your design to proceed like this:

1) Work out your forces based on a code appropriate, low level of ductility.
2) Design your columns.
3) Calculate the plastic hinging capacity at the base of your columns. Amplify that by the 1.25 over strength factor to account for the fact that the column section will strain harden and generally be stronger than you think.
4) Design your connection for the lower of #3 and the elastic EQ design load.

The intent of having ductile failure modes govern in connections is to shield the brittle failure modes from load. That's why it's necessary to use a high side estimate of the yield strength of the ductile elements. In this particular case, I consider the base of your column to be part of the connection.

All of this stuff falls into the general philosophical framework of "capacity seismic design". If you're not familiar with capacity design concepts, you're in luck: there's information plastered all over the web for the clever googler.




The greatest trick that bond stress ever pulled was convincing the world it didn't exist.