PEMB base plate with anchors in the bottom 3rd only 2

[canwesteng]

A PEMB vendor has supplied a base plate with anchors only in the bottom 3rd of the base plate - say it was an 18" depp BP, then you have 3"x3" bolt square 6" from one edge. The column is straight and doesn't taper, so surely there is some eccentricity in tension loads here that they aren't reporting? Has anyone else dealt with this?

 

[r13]

A sketch will help in understanding this problem.

 

[phamENG]

They did report it. They gave you a tension load, and they gave you a picture

Can't say I've seen that. It is interesting - could it be an access issue? They sequenced construction such that an iron worker won't be able to get to those anchors during erection? It's a stretch, but the only thing I can think of.

 

[Ron247]

Like retired13 said, give us a sketch but include the footprint of the column itself on the baseplate. If I am picturing this correctly, lateral load from the direction the bolts are shoved towards is also a concern. That is a fairly long moment arm if the column itself is 18" and straight.

As far as pure uplift, the load path tends to head towards whatever resists uplift and to me that would be the bolts, not the column profile.

 

[JLNJ]

Often, the anchors are in the same locations (relative to the Building Line) no matter what the column depth.

 

[canwesteng]

Here's a sketch - to me I think the bolts have to resist pure tension and create moment in the column for this to work. I somehow doubt there is much consideration for this in the design though.

 

[r13]

I guess you don't have significant lateral load, compared to the gravity load, to cause instability of this column. However, I've never seen it, but can work, I think.

 

[Ron247]

I assume their reactions show Horizontal and Vertical with no Moment since it appears they claim it is pinned. To me, you are designing the foundations for the forces their structure imposes on the foundations. They give you bolt quantity, diameter and pattern along with THEIR reactions. You come up with foundation size and bolt embedment based on their information.

Since there is no such thing as and ideal pin for simple PEMB foundations, to me, that detail is going to act a little more fixed than the more traditional PEMB tapered columns with 8" to 12" depth at the bottom and 36" to 50" depth at the top.

 

[human909]

If the bolt holes are located inward to the building then that would improve the pinned connection behavior under wind and gravitational loads. In fact it makes plenty of sense and seems quite a clever solution.

(Maximum tensile load is on the exterior of the columns both for gravitational loads and for wind loads. Thus by locating the bolts away from this point you are greatly reducing the stiffness of the connection.)

 

[r13]

Is this a crane runway column?

 

[jimstructures]

That column and base plate might be a standard straight column where the manufacturer wants to assume a pinned connection to the foundation but also needs to supply a straight column for whatever reason (i.e. light crane or mezzanine column). For a more accurate determination we would need 1. Loads applied to the building, 2. Load cases, 3. Load Combinations, 4. Reactions fore this building, 5. Anchor Bolt Layout. and Anchor Bolt Details for the entire building.

The manufacturer may take a tapered column design and just give it a straight column substitution the same depth at the top of the col as the tapered column with no attempt to optimize the column for depth.

"retired13" may be correct, this may be a light crane column


Jim

 

[Settingsun]

Human909, what is the problem that is solved by lower stiffness?

 

[human909]

Human909, what is the problem that is solved by lower stiffness?

Well the connection is being modeled as a pin connection. A less stiff connection gives you a reality that is closer to your model. AKA your foundation is likely to see less moment which is presumably what you want.

You can't just model a connection as a pin connection and then detail a stiff connection that will attract moment. You have the potential for failure of your column footing.

 

[JLNJ]

I think pushing the anchor bolts toward the outside is pretty typical for PEMBs.

If the column is part of a braced bay, the X-bracing is toward the girt side of the column and these bolts would be in line with that bracing.

This is not the detail I'd expect to see in a fixed-base application. If it's a pinned connection I don't see any issue with it.

 

[Settingsun]

You can't just model a connection as a pin connection and then detail a stiff connection that will attract moment. You have the potential for failure of your column footing.

Ok, thought there was some history. I've never heard of problems caused by wide-spaced anchors which are typical. Most guides suggest you get a free lunch with reduced deflection at service loads without having to design for moment at ultimate load.

 

[human909]

Ok, thought there was some history. I've never heard of problems caused by wide-spaced anchors which are typical. Most guides suggest you get a free lunch with reduced deflection at service loads without having to design for moment at ultimate load.

I've never seen that, though I can understand that approach.... But it is only a free lunch if your foundation doesn't fail at high loads. Start running the numbers of a high wind event and you could readily see footing failure and rotation. By failure I mean cracking and movement rather than structural collapse. By definition if your footing is failing due to moment you then do end up with a pinned conditions.

My point is that your connections should behave as closely as your ideal connection as possible or there could be unintended consequences. I've seen numerous issues arising from moment being imparted on members when the engineer modeled the connection as a pin but the connection used is too stiff.

 

[r13]

I think my guess was wrong. If this is a crane runway column, and longitudinally braced in the line of bolts, the base plate posses these problems:

1) As the crane beam is in the inner side of column that has no bolt below, the shear forces will not be felt by the bolts until the forces overcome the shear friction at the base.
2) The longitudinal thrust will introduce torsion, forcing the plate to rotate about the bolt group.

 

[RWW0002]

This looks very typical for pinned base PEMB columns based on what I see. The 3x3 bolt pattern is a bit tight, but pushing the anchors a fixed distance from the outside face of column is common practice in my area, even for straight columns.

 

[canwesteng]

A lot of speculation going on, and none of particularly germane to the question. I'm wondering about eccentricity caused by uplift on the column. Why they have used a straight column doesn't matter, but there is neither crane not mezzanine.

 

[r13]

Do you have uplift on the inner (building) side flange, or the entire column will subject to uplift with all available gravity load on it?