ACI 318 -- D.8.1 & D.8.4 requirements

[ChuckerD]

As a career metal building engineer I am fairly ignorant of the requirements of ACI. However, recently we have had a couple instances where our standard 4"x4" anchor bolt pattern has been rejected where we have specified 1 1/4" dia anchor bolts. We were referred to section D.8.1 of ACI 318 as the reason for the rejection.

A couple questions for those of you working in foundation design:

1) Are cast-in anchors the typical anchor used for metal building foundations?

2) If we were to turn a blind eye and insist on the 4"x4" pattern for the anchor bolts (meaning cast-in anchors cannot be used), how will the foundation and anchors have to be designed? What is a broad estimate of the cost impact to the foundation if this is required?

Thanks in advance for the feedback!

ChuckerD

 

[JedClampett]

As tempted as as I am to editorialize, I'll limit myself to just answering your questions;
1) If the contractor has his act together enough to remember, he'll put in cast in anchors. But if you're thinking that you can get out of these provisions by using post installed anchors, they have minimum spacing requirements, too.
2) In my end of the engineering world, I'm not allowed to "turn a blind eye" on code provisions. We respect them and don't take exception to them unless we have a pretty good reason and documentation. Seriously, how effective do you think bolts at that spacing are? And the cost impact is probably nothing. It doesn't cost any more to install four bolts at fourteen inch spacing than four inch spacing. If we need a little more concrete, the cost is negligible.

 

[ChuckerD]

Thanks for the reply Jed.

My phrasing of "turn a blind eye" was a poor choice - by no means did I intend to imply that we are going to knowingly violate any specification. However, our design begins and ends with the steel building (at the bottom of column baseplate). In other words we are specifying an anchor bolt size based on the shear & tension requirements according to the AISC spec. We don't do foundation designs and are not familiar with the ACI spec.

My questions are intended to help us determine whether alternate foundation design solutions exist when the 4 x do requirement is not satisfied, and whether those are an economical alternative for our customers. And if the answer is no - I'll push for us to change our standards so that this doesn't become an issue again in the future.

ChuckerD

 

[JedClampett]

You'll have to excuse my testy tone. I recently had to design a foundation for very high uplift (Florida) using anchor bolts and spacing specified by the Metal Building designer. In this case the spacing was 4 inches by 5 inches and it was still very difficult to get the anchors to work. I had to continue them into the spread footing to get sufficient edge distances. And I'm somewhat suspicious that the contractor actually did what I asked.
We all know that your responsibility ends at the foundation. But you need to familiarize yourselves with enough of ACI 318 Appendix D to appreciate the issues involved. Close spacing greatly reduces anchor capacity. Small edge distances (not really your problem) are even worse.

 

[civilperson]

Confinement of the anchor bolts within ties or within a grid of reinforcement or hairpin bars around the anchor bolts are ways to achieve the strength of concrete in breakout due to shear and tension. ACI D 5.2.9 uses the strength of the reinforcement with a phi of 0.75 for the strength formula. This can allow the use of 4" spacing for anchor bolts.

 

[haynewp]

It is hard to get 1 1/4" diameter cast in bolts to work by ACI at that close of a spacing. I have tried to do similar size and pattern and ended up extending them 2'-6" into the footing. They are being rejected because they are too close together.

 

[sdz]

Try referring to clause D.8.4.