grout omitted below baseplates 5

[samdamon]

Never seen this before. G.C. decided to leave out 3/4"+- thick non-shrink grout and leveling plates below wide flange column baseplates. Steel frame is fully erected and bearing on concrete supports directly. Sheesh.

Gravity forces aren't that substantial (its a one story superstructure) but moment frames were designed to be fixed at col bases so forces on the concrete under the baseplates can be large during a seismic event or wind storm.

On balance, the owner doesn't care if the building is 3/4" shorter and the concrete the baseplates are sitting on is level and generally smooth. But its not perfectly smooth, as you can probably imagine.

My opinion is that the columns cannot be left as they are, therefore need a remedy to get baseplates to bear uniformly over their entire area per the original design intent. Jacking the columns up and placing grout does not seem feasible (Many columns would have to be jacked up all at once, uniformly). Pressure-injecting a low viscocity epoxy under the baseplate might work but there would be no way to verify that the full bearing area was filled in. The cured epoxy also probably wont have as high a modulus as grout.

Its a quandary. Any suggestions?

 

[WillisV]

You could always just check to see if it works using only the anchor rods as supports (i.e. under moment the anchor bolts on one side take the compression with the other side taking the tension) like many utility pole applications that don't use grout. If it works this way then it is fine as whatever concrete is in bearing can locally crush under the force until it transfers to the anchor rods.

 

[DaveAtkins]

Preeengineered metal buildings are almost always built as you describe--the base plates sit directly on piers or footings without grout. There have not been problems with this method of construction, so I don't think you should be concerned about the way this building was put up.

DaveAtkins

 

[scottiesei]

Wow, that's a tough one. What about supporting the columns and cutting a bit off of the bottom. Then you could form up a concrete pedestal and dowel into the footing. Field weld a new base plate to the column and hold the pedestal short leaving more room for a high strength epoxy. Just throwing ideas out there....
How many columns are we talking about? Are they fixed in both x and y directions?
Or how about ignoring the existing base plate, and adding two more base plates on either side, grouted as required. Then come up with a gusset detail and weld steel to the side of the columns and then to the new base plates. That might be fun to design but prob look pretty ugly! Maybe these crazy ideas of mine will help someone brain storm something better!

 

[scottiesei]

But Dave has a point, what exactly are the requirements for bearing under the base plate in this situation? And to what Willis stated, see page 4-131 in the ASD. You could prob use something like that and design the bolts to take the forces. Adding angles and more bolts if required would be pretty easy.

 

[swearingen]

One problem I see with leaving it as it is: what's taking the shear? As the few points of concrete supporting the vertical load under the plate disintegrate during a major seismic or wind event, the only thing taking shear will be the anchor rods. Are you really comfortable with the rods that are in heavy tension also taking a major amount of shear? I wouldn't be. You're going to want the friction developed by the grout between the baseplate and the concrete. Run the numbers and it will probably scare you into fixing the problem...

 

[PEinc]

Even if the base plates had been grouted, the bolts would be taking the shear (according to the described situation). Also, it sounds to me like WillisV is assuming that the base plates aleady have leveling nuts below the plates and that the columns are bearing on the leveling nuts only. I don't think that samdamon's post indicated the presence of leveling nuts. It sounds like DaveAtkins may have best answered the question.

 

[SlideRuleEra]

In the past, I have "justified" the omission of grout, as described, by looking at the situation as self-correcting. You may not full contact, but as the loads increase the underlying concrete will "crush" to create sufficient bearing area. Even when this happens, the settlement is minimal (a few hundredths of an inch, maybe).

Even if you don't agree with this, the risks of doing "something" (and having unintended consequences) have to be weighed against doing "nothing".

In any event, IMHO, the Owner is due a negotiated financial settlement if "nothing" is done.

http://www.SlideRuleEra.net

 

[WillisV]

PEinc - yup I was assuming he had backwashers somewhere under there - not a good assumption. I agree with SlideRuleEra.

 

[TomBarsh]

I had a similar situation long ago for a large steel structure in a paper mill's wood yard. The contractor had omitted grouting the grout sleeves, water had collected in the sleeves, then froze, resulting in cracks and some huge spalled-out chunks of concrete (largest being around 100 lbs).

One option suggested was to cut off the lower end of columns and place elastomeric bearings as we used on other equipment in the mill. Very expensive. But once it became apparent as to the real cause of the cracking the contractor was brought back, they chipped out the grout under base plates (still had steel shims), put forms around the base plate, drilled into the grout sleeves from side of piers, then pressure grouted with an epoxy grout until pure grout (no air bubbles) came out around the forms, making certain visually that there was full contact with bottom of the base plate. This worked very well as there was 1.5" allowed for grout.

It would be more difficult in your case since there is no grout space under the base plates. But if you are concerned this may be a method to consider.

 

[samdamon]

Thanks for all the comments, they are valuable. Fyi there are no leveling nuts under the baseplate. Also, I normally don't count on friction under the baseplate to resist shear, just use the anchors.

No decision yet. Sure wish I got to the site earlier!

 

[Ron]

samdamon...even though PEMB's are usually constructed as noted without much attention to grouting the baseplates, you can still inject epoxy under the baseplates to provide better load distribution. SIKA makes a high modulus epoxy that will work. I would drill through the baseplate as near as practicable to its center, create and injection port (while the injector guys frown on it, this is a perfect application for an alemite fitting and epoxy injected with a standard automotive grease gun)..you might need to pop the little check valve out of the fitting before injecting.

Put a good quality sealant/adhesive around the baseplate on all sides to provide a small amount of backpressure confinement. Drill a small (1/8" dia.) hole in each corner of the baseplate to see that the epoxy is getting all the way out and to allow air to escape as you inject.

This will increase your bearing contact area so that the issues you noted will not be as pronounced.

 

[swearingen]

Most of the information below is taken from AISC's Steel Design Guide Series No. 1, Column Base Plates.

The four ways to resist shear in base plates and some comments on each:

1. Friction development: LRFD Spec. states that the coefficient of friction should be 0.9 for plates embedded one full plate thickness into the surface, 0.7 for concrete or grout with the contact plane coincident with the surface, and 0.55 for grouted conditions with the contact plane above the surface. For ASD, use a safety factor of 2.0. Note that compressive axial load should not be overestimated; it is recommended that a 0.75 factor on dead load be used.

2. Anchor rod shear friction or bearing: anchor rod shear friction has been discounted because of the uncertainty of concrete creep releasing the preload on the rods. Because of the oversized holes in baseplates, it is recommended to use not more than 2 rods to resist shear in bearing. Research has suggested that anchor rods in bearing shear should only be used for small loads - no greater than 2 kips per rod. One way to help is to use tight fitting washers and weld the washers to the baseplate, providing a direct load path. The bolts are still in bending, however, because of the distance between the resistance in the concrete and the washer.

3. Shear lugs: welded plates protruding from the bottom of the baseplate and cast into a pocket in the foundation with grout are called shear lugs. They are commonly used, but are obviously no good in your case.

4. Embedding the column base into the foundation: self-explanatory, but again not applicable in your case.

Note that even for moment resisting baseplates, as they are in your case, friction is developed on the compressive side of the plate. Hopefully this plus some help from your anchor rods can provide the resistance you need.

 

[IFRs]

If your concrete is wide (or narrow) enough, you could anchor plate or angles into the top (or sides) of the pier to take care of the shear loads. How about field weld a collar around the pier say 1/2" steel plate extending below the column top, also extending up above the base plate with grout between the collar and base plate. This would take the shear, no? Have you calculated your shear loads? How high are the rest of the loads? Are we talking about minimal factors of safety or is the pier way over sized, etc?

 

[AggieYank]

DaveAtkins mentioned PEMB columns are often this way. That is correct, but typically PEMB columns are designed as fixed at the base.

If they had put the balancing nut on the anchor rod (under the column plate) you could check the anchors only for capacity. Because these nuts were left off, you'd have to rely on the concrete bearing strength, as there is nothing to stop the column plate from moving down the rod. As JAE mentioned, the concrete will crush to a small degree before uniform bearing happens, and even when it is uniform bearing, a small amount may still occur. The problem with this probably won't be strength, but the deflection which may occur as the column base rotates a small degree.

Tell the owner that you base isn't truly fixed, and so your H/300 or H/400 deflection is now more like H/250? or H/350?.

Like someone else mentioned, you could epoxy under the column base. Something I haven't tried (has anybody) is to take the top nut off and weld the rod all the way around to the plate, so you could rely on the rod in both compression and tension. You'd have to check the rod for weldability, as well as the rod weld for the tension / compression force.