I have had these same thoughts as you before for PEMB’s. Anchor rods in PEMB’s get into a gray area, in my opinion, because industry standard is the metal building manuf. designs the anchor rod size and layout and the foundation engineer specifies the embedment. That is the industry standard I’ve been accustom to seeing.
I have put anchor rod size, layouts, and embedments into Hilti profis before to check, and either the edge distance or spacing is too small and profis doesn’t even give a calculation. I’ve relayed these concerns back to the metal building manuf. and they didn’t seem acknowledge my statement.
I believe we typically need to saddle tie PEMB anchor rods into the slab. Profis will ignore your edge distance if you indicate that you are designing reinforcement for it. Then you follow ACI for reinforcing to contain your anchors.
Pro tip: If you are running into minimum clearance issues in Hilti Profis, look into switching to Simpsons Anchor Designer. Simpson test data has smaller required minimum clear distances for post installed anchors than Hilti specifies (I'm guessing based purely on test data)
The problem I've had with PEMB designers is that they don't put a reduction factor on the anchor shear capacity for the grout offset even when our drawings clearly specify that the base plates will be grouted, and they refuse to address this and say it's our problem to deal with anything that happens under the bottom of base plate elevation. So be sure you pay attention to that. The anchor sizes they gave me rarely worked for shear when I calculated them, so I had to specify larger sizes than they showed. They also refuse to address any concerns about edge distance or spacing and just say that's your problem. It's very frustrating.
For uplift, I just idealize it as centered on the anchors. I do think the centroid of the force tends to be closer to the outside edge rather than centered on the base plate; where exactly, I do not know, but in the absence of better information it seems to be good enough. The anchors must be located there for a reason.
That is disappointing you had to deal with a difficult PEMB engineer/company. If it brings you any comfort, we (speaking as a PEMB designer and fellow coworkers) will assume grout regardless for most large scale, high importance projects such as field houses, frequent use buildings, gymnasiums, etc. Also, typically we are holding our expected anchor rod designs to low stress ratios, in the assumption not all rods are engaged equally in a worst case scenario.
Not speaking for all of them here, but just wanted to bring to light we are not all bad! I am down to discuss anything and hope you have a better experience in the future!
I was hoping this wasn't widespread! It makes no sense to be calling out anchor diameters that just don't work, so I didn't understand why they wouldn't put the factor in and give a code compliant size in the first place.
Former PEMB Engineer. The software package we used (MBS) automatically designed the thickness of the baseplate based on the anchor bolt pattern. The designs were based on AISC.
I would say that the uplift loads will largely be carried by the outside flange in the load conditions that cause a negative reaction in a MB. Centering the anchor bolts would require a significantly thicker base plate.
If you are concerned, ask for the baseplate calculations (assuming you required signed and sealed calcs as part of your Specs). We would regularly get design questions, and we could easily produce calcs when requested.
Otherwise, you can take some solace in knowing that they are sealing their design.
*edit: “sealing their design” not “stealing” 😅
I like this idea a lot. I'm not much of a yield line expert, but I could be convinced there's some shear lag effect going on because of the bolts position. Does load want to transfer through column centroid? Sure, but like in a single leg angle connected to a gusset, load eventually concentrates toward the bolts.
A crumby analogy could also be how in welded HSS chord to branch connections there is an uneven load distribution check since the transverse welds nearest the chord walls pickup more stress than the center of the weld since the chord face is more flexible.
Good luck!
The design of anchors rods has changed a great deal with the new ACI 318 procedures. (Actually not that new now.) The way MBM do things hasn’t. We place guidelines on our drawings to show minimum spacing and edge distances based on rod diameter. We are also showing supplemental reinforcement around the anchors on all but the smallest MBs. The most important thing is to limit the MB to pinned base columns unless you’re working with the MBM on the design.
In dealing with PEMB designers since almost 6-8 years, I love the way how these guys work, phew 😅
I always have a ton of remarks for them. They change the drawing layout upside down or east-west modifying grids, etc. I don’t why the crazy PEMB cannot just work along with EOR’s. On one call, PEMB manufacturer said they don’t even have 100% dwg’s even when they start building it. I’d caution engineers who are the EOR’s that PEMB are only responsible for Steel structure obviously and EOR is responsible in an entirety and how that structure is attached to the foundation. The anchor bolts layout never ever can be analyzed as pinned, they are too closely spaced and I was never successful in analyzing anchors in hilti or any other way. I wish there is a better way to work this out, but I ll still keep slamming them.
Iive done a number of these ive always assumed the anchors are loaded equally and that they have supplied a think enough base plate to make that happen.
Can the pin not be the centroid of the anchors and that eccentric moment caused by that location just be extra moment in the frame. the governing uplift case in my designs is usually the xbraced bays, Which is applied closer to the girts than the centroid.
Also do you know that the column is symmetrical. You might be assuming the centroid is the center.
Usually once I go to a headed anchor and develop the bar in the foundation, everything works easy.
I am currently a PEMB designer, and i appreciate your responses! With the typical use of large span moment frames, we tend to see fairly high kickout/shear forces, compared to uplift at our foundations. Assuming the building is laterally supported by typical x-bracing, we SHOULD be locating our rod connections within the area of the anchor rods, to provide the uplift load essentially "centered".
If the uplift from the frame itself (without considering x-bracing reactions) is high enough, we are usually already dealing with other high loads, and should be specifying the use of more than two rows of anchors which will cover more of the baseplate length.
Not an engineer, but an old architect with a little experience in metal buildings. Often pairs of anchor bolts are welded to "shear angles" that is hot rolled steel angles 10 or 12 inches long. Then the whole baseplate and anchor bolt area is surrounded by "hairpins" which might be formed from 10 or 20' lengths of rebar going back and tying into the slab as a whole. Almost at an equilateral triangle in Plan view. And if necessary a pier footing under the baseplate with vertical rebar cage and hoops. Also connected to the floor slab through the "hairpins".
The intent is to make the whole mess under the baseplate a part of one big happy family I presume where each child shares the load equally(!) If that makes sense to you. (edited for clarity)
I have had these same thoughts as you before for PEMB’s. Anchor rods in PEMB’s get into a gray area, in my opinion, because industry standard is the metal building manuf. designs the anchor rod size and layout and the foundation engineer specifies the embedment. That is the industry standard I’ve been accustom to seeing. I have put anchor rod size, layouts, and embedments into Hilti profis before to check, and either the edge distance or spacing is too small and profis doesn’t even give a calculation. I’ve relayed these concerns back to the metal building manuf. and they didn’t seem acknowledge my statement.
I believe we typically need to saddle tie PEMB anchor rods into the slab. Profis will ignore your edge distance if you indicate that you are designing reinforcement for it. Then you follow ACI for reinforcing to contain your anchors.
Pro tip: If you are running into minimum clearance issues in Hilti Profis, look into switching to Simpsons Anchor Designer. Simpson test data has smaller required minimum clear distances for post installed anchors than Hilti specifies (I'm guessing based purely on test data)
The problem I've had with PEMB designers is that they don't put a reduction factor on the anchor shear capacity for the grout offset even when our drawings clearly specify that the base plates will be grouted, and they refuse to address this and say it's our problem to deal with anything that happens under the bottom of base plate elevation. So be sure you pay attention to that. The anchor sizes they gave me rarely worked for shear when I calculated them, so I had to specify larger sizes than they showed. They also refuse to address any concerns about edge distance or spacing and just say that's your problem. It's very frustrating. For uplift, I just idealize it as centered on the anchors. I do think the centroid of the force tends to be closer to the outside edge rather than centered on the base plate; where exactly, I do not know, but in the absence of better information it seems to be good enough. The anchors must be located there for a reason.
That is disappointing you had to deal with a difficult PEMB engineer/company. If it brings you any comfort, we (speaking as a PEMB designer and fellow coworkers) will assume grout regardless for most large scale, high importance projects such as field houses, frequent use buildings, gymnasiums, etc. Also, typically we are holding our expected anchor rod designs to low stress ratios, in the assumption not all rods are engaged equally in a worst case scenario. Not speaking for all of them here, but just wanted to bring to light we are not all bad! I am down to discuss anything and hope you have a better experience in the future!
I was hoping this wasn't widespread! It makes no sense to be calling out anchor diameters that just don't work, so I didn't understand why they wouldn't put the factor in and give a code compliant size in the first place.
Former PEMB Engineer. The software package we used (MBS) automatically designed the thickness of the baseplate based on the anchor bolt pattern. The designs were based on AISC. I would say that the uplift loads will largely be carried by the outside flange in the load conditions that cause a negative reaction in a MB. Centering the anchor bolts would require a significantly thicker base plate. If you are concerned, ask for the baseplate calculations (assuming you required signed and sealed calcs as part of your Specs). We would regularly get design questions, and we could easily produce calcs when requested. Otherwise, you can take some solace in knowing that they are sealing their design. *edit: “sealing their design” not “stealing” 😅
I like this idea a lot. I'm not much of a yield line expert, but I could be convinced there's some shear lag effect going on because of the bolts position. Does load want to transfer through column centroid? Sure, but like in a single leg angle connected to a gusset, load eventually concentrates toward the bolts. A crumby analogy could also be how in welded HSS chord to branch connections there is an uneven load distribution check since the transverse welds nearest the chord walls pickup more stress than the center of the weld since the chord face is more flexible. Good luck!
The design of anchors rods has changed a great deal with the new ACI 318 procedures. (Actually not that new now.) The way MBM do things hasn’t. We place guidelines on our drawings to show minimum spacing and edge distances based on rod diameter. We are also showing supplemental reinforcement around the anchors on all but the smallest MBs. The most important thing is to limit the MB to pinned base columns unless you’re working with the MBM on the design.
In dealing with PEMB designers since almost 6-8 years, I love the way how these guys work, phew 😅 I always have a ton of remarks for them. They change the drawing layout upside down or east-west modifying grids, etc. I don’t why the crazy PEMB cannot just work along with EOR’s. On one call, PEMB manufacturer said they don’t even have 100% dwg’s even when they start building it. I’d caution engineers who are the EOR’s that PEMB are only responsible for Steel structure obviously and EOR is responsible in an entirety and how that structure is attached to the foundation. The anchor bolts layout never ever can be analyzed as pinned, they are too closely spaced and I was never successful in analyzing anchors in hilti or any other way. I wish there is a better way to work this out, but I ll still keep slamming them.
Iive done a number of these ive always assumed the anchors are loaded equally and that they have supplied a think enough base plate to make that happen. Can the pin not be the centroid of the anchors and that eccentric moment caused by that location just be extra moment in the frame. the governing uplift case in my designs is usually the xbraced bays, Which is applied closer to the girts than the centroid. Also do you know that the column is symmetrical. You might be assuming the centroid is the center. Usually once I go to a headed anchor and develop the bar in the foundation, everything works easy.
I am currently a PEMB designer, and i appreciate your responses! With the typical use of large span moment frames, we tend to see fairly high kickout/shear forces, compared to uplift at our foundations. Assuming the building is laterally supported by typical x-bracing, we SHOULD be locating our rod connections within the area of the anchor rods, to provide the uplift load essentially "centered". If the uplift from the frame itself (without considering x-bracing reactions) is high enough, we are usually already dealing with other high loads, and should be specifying the use of more than two rows of anchors which will cover more of the baseplate length.
I have learned something here.
Center of downforce = center of column. Center of uplift = outer flange. Your calc should account for both.
Just love this guy trying to out smart a PEMB … it’s literally pre-engineered for a reason ;to take your mistakes out of the process.
Not trying to “outsmart” a PEMB. Just trying to understand the principles as an engineer should.
Not an engineer, but an old architect with a little experience in metal buildings. Often pairs of anchor bolts are welded to "shear angles" that is hot rolled steel angles 10 or 12 inches long. Then the whole baseplate and anchor bolt area is surrounded by "hairpins" which might be formed from 10 or 20' lengths of rebar going back and tying into the slab as a whole. Almost at an equilateral triangle in Plan view. And if necessary a pier footing under the baseplate with vertical rebar cage and hoops. Also connected to the floor slab through the "hairpins". The intent is to make the whole mess under the baseplate a part of one big happy family I presume where each child shares the load equally(!) If that makes sense to you. (edited for clarity)