Design of Anchor Bolts Embedded in Concrete Foundation 1

[DCX_2156]

Hello,

I am performing an analysis of a skid structure that is embedded in concrete foundation via cast-in place L-type anchor bolts.

I am considering various load combinations for the structure (e.g. dead loads, live, thermal and environmental loads). As part of the design, I need to assess the anchor bolts against the resulting reaction forces due to the aforementioned load types (and factored combinations). I am looking to assess the bolts under ACI 530-13 standard.

In my model, the anchor bolts are represented by their resulting connection stiffness as spring constraints (calculated based on published literature) with a stiffness magnitude of 1.0 E+09 N/m. When I apply thermal loads (thermal gradient of ~50 degC), I get very high reaction forces at the anchor bolt locations (>100 kN in shear). This reaction force already accounts for the relative expansion between steel and the concrete foundation and also the shear resistance at the connections due to friction between the steel base plate and concrete.

If I follow the design criteria for anchor bolts in ACI 530-13, the allowable for shear loads is calculated as: (0.6 * A_b * phi * f_y), where phi = 0.9 and f_y is the yield strength of the bolt material. The material for the bolt is A36 (yield strength of 250 MPa), therefore the bolts fail under the expected thermal loading from my analysis.

I am constrained in terms of changing the bolt material (and as far as I know this material is typical for cast-in anchor bolts). Is there a different design criteria that I can assess the bolts to such as displacement at the anchor bolts under free thermal growth? Considering that the anchor bolt mounting hole has some clearance, they as long as the base plate displacement does not exceed the hole clearance under thermal loading, then the anchor bolts should not see those very high shear loads calculated in my model.

Any advice would be appreciated. Thank you.

 

[dik]

Use headed bolts in lieu of 'L' or 'J' type bolts... they work better and are stronger. I often use ASTM F1554 Grade 55 S1. What is causing the very high thermal loading?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[DCX_2156]

Thanks for the response!

As of now, I am constrained to the ASTM A36 specification for their anchor bolts - the equivalent specification for anchor bolts would be ASTM F1554 Grade 36. Would headed bolts be assessed in a different way?

The high thermal loads occur at the anchor bolts at the four corners of the skid structure. The thermal gradient is 50 degC and because the structure is trying to expand out (but restrained by the anchor bolt springs), I get the high load concentration at those locations.

 

[dik]

Headed bolts can be designed using the same anchorage methods of the ACI but yield higher values for anchorage. Tests indicate they are generally 2 to 3 times greater than 'L' bolts. There are several good papers on the design of headed anchor rods. I usually use Grade 55 S1 because they are stronger than Grade 36 and only marginally more expensive. Get extra strength for nearly nothing...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SWComposites]

Your model is likely way too stiff, hence the calculated thermal loads are likely way conservative. You need to account for all of the flexibility in the joints.

 

[1503-44]

It is usually not cost effective to restrain thermal movement.
Why not let the skid expand unrestrained?
I've designed hundreds of skids for all types of equipment and never had a requirement to restrain a skid from thermal expansion at its four corners.
Normally only one end of a skid might be restrained with anchor bolts.
The other end has a slotted bolt hole to accommodate expansion.
What is so different with your skid that such an arrangement is not possible.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[DCX_2156]

Hello,

Thank you all for the responses!

The skid was not designed by me. I am just performing the analysis and providing recommendations but I want to avoid recommending bigger and stronger bolts if thermal loading on a structure is not usally an issue. The skid is a simple design with four I-beams at the perimeter then several rows of cross-members across the middle. The skid design is such that there are about 30 anchor bolts around the skid perimeter.

As you all rightly point out, the thermal reaction loads are mainly affected the following factors:

1. Stiffness at the member joints (assumed rigid as these are welded). There is also a checker plate on top which is assumed to be rigidly connected to the top flange of the beams which also affects the thermal loads that I get.
2. Thermal gradient (50 degC).
3. Stiffness at the anchor bolt connections(based on published literature and estimates from previous similar work).

I can perhaps tweak the connection at the checker plates so that its ot fully rigid. For Item 3) i would need a referenceable stiffness that I can assume at the bolted joints to reduce the stiffness I am using.

Slotted holes would also be an option - would this br assessed on a displacement based criteria (i.e. if the displacement at the anchor location under free thermal growth is less than the slot size, then the high thermal reaction from the model is neglected)? If so, does ACI or other codes have provision for this type of assessment? I also need to consider seismic hazards therefore thats an additional consideration for using slotted holes.

My main question I guess is if there is a procedure somewhere for assessment of thermal loads on anchor bolted structures. Which can give provisions on say, what stiffnesses to use in the model and also other criterias that I can assess the anchor bolts against.

Thank you all again for your inputs!

 

[dik]

Does whatever the skid is supporting require anchors? Can anchors be provided just for reatraint?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[DCX_2156]

The skid is being designed against seismic loads so it looks like it will need to be anchored down.

 

[1503-44]

Can you post a picture of this thing.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[CSTranEng]

DCX_2156 - Did you get a response yet outside of this forum? I am asking the same question at my work.