Hilti Profis vs. Simpson

[SJBombero]

I trying to get to the bottom of why I am getting disparate results for the same design in Hilti and Simpson. Concrete edge breakout in shear is the controlling failure mode. Although the capacities in both programs are identical for all shear failure modes, Simpson is combining the breakout and prying effects and applying reduced demands to each failure mode. Because Hilti is applying the full shear as Vua to the breakout capacity, it is failing. Has anyone seen this phenomenon before. I tend to think Hilti is correct since the capacity is based on a ratio of areas of the 4 anchor bolts and not just half of them like the Simpson results seem to assume.

https://files.engineering.com/getfi...-f050-4af5-b90e-a3f5c41808fa&file=SIMPSON.pdf

https://files.engineering.com/getfi...05-7bb3-4f0d-945e-74fbafceb9a8&file=HILTI.pdf

 

[ProgrammingPE]

Take a look at ACI 318-14, Fig. R17.5.2.1b. There's 3 different cases that can be considered for concrete shear breakout. It looks like both calculations are using the area from just the two anchors closest to the edge (Avc = (1.5*6") * (2*1.5*6"+10") = 252in²), but Hilti is using the full shear load (Case 3) and Simpson is using half of the shear load (Case 1). Since s >= ca1,1, I don't see why Case 3 needs to be checked.

Structural Engineering Software:

http://www.structuralcentral.com

Structural Engineering Videos:

http://www.youtube.com/structuralcentral

 

[phamENG]

I haven't looked at the files in detail, but based on ProgrammingPE's statements, look for an option about shear transfer. If you're transferring shear through the anchors, DG1 suggests only applying it to 2 of the anchors as it's not uncommon for construction tolerances and oversized base plate holes to result in an unequal shear resistance in the anchors. I believe Hilti does this by default, but in Simpson you have to choose it explicitly.

 

[SJBombero]

Thank you both. ACI recommends that Case 3 is inapplicable only for anchors welded to baseplate. Unfortunately, Simpson does not capture Case 3 by default and so is significantly unconservative compared to Hilti.

 

[phamENG]

Simpson does not capture Case 3 by default and so is significantly unconservative compared to Hilti

I disagree there. There's nothing unconservative about requiring a user to select the appropriate design criteria.

 

[SJBombero]

Simpson's default is to assume that least conservative design choice for that section of the ACI while Hilti's method (no option allowed) is the most conservative design choice. Whether or not a particular design professional agrees with that portion of the ACI Commentary is a separate issue.

 

[phamENG]

Removing that choice must have been part of their transition to a paid platform. Do you have the paid version or the free? I'm pretty sure there used to be a set of options for shear transfer mechanism.

It has nothing to do with agreeing with the code or not - it has to do with detailing and selecting a rational model that matches it. AISC ranks shear transfer through anchors as the least preferable method. This is one of those reasons. But you can detail your base plate to balance the forces and apply them to all of the anchors. That's how I do it, so I'm thankful for the ability to choose as it makes my designs more economical.

This is a bit cynical of me, but I think it's also important to understand the underlying purpose of the software. It's to get you to specify their products. So if the Simpson program gives a careless designer a cheaper design than Hilti, Simpson wins. If the connection fails, it's not Simpsons fault - it's the careless designer who didn't verify their design properly.

 

[SJBombero]

We have the paid version of Hilti. I can't find any option to eliminate Case 3.

 

[SandwichEngine]

I don't have any input on the above shear checks but I'll contribute that Hilti Profis does grouted anchor bolts very wrong. The code says to take a 20% reduction in the anchor bolt's shear capacity when grout is present. Simpson does that. Hilti Profis checks the anchors in double curvature bending as if there were no grout and shows extremely conservative anchors to be failing.

 

[Aesur]

Here is a good article about this: Structure Mag Link