grout omitted below baseplates
grout omitted below baseplates
(OP)
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?
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?






RE: grout omitted below baseplates
RE: grout omitted below baseplates
DaveAtkins
RE: grout omitted below baseplates
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!
RE: grout omitted below baseplates
RE: grout omitted below baseplates
RE: grout omitted below baseplates
RE: grout omitted below baseplates
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.
www.SlideRuleEra.net
RE: grout omitted below baseplates
RE: grout omitted below baseplates
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.
RE: grout omitted below baseplates
No decision yet. Sure wish I got to the site earlier!
RE: grout omitted below baseplates
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.
RE: grout omitted below baseplates
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.
RE: grout omitted below baseplates
RE: grout omitted below baseplates
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.