2 versus 4 nominal anchor bolts in column bases 3

[IJR]

Imagine a heavy wide flange column (depth over 20 inch). Statics shows you can use 2 anchor bolts as nominal.

Some literature and experienced friends suggest a minimum of 4 anchor bolts for the following reasons:

1) It makes it easier to erect a column with 4 anchor bolts.

HOW?

2) The four anchor bolts will induce some fixity at the base which will be useful in case of fire.

HOW?

I request a detailed discussion on this subject, on US practice and on the explanations above.

respects
IJR

 

[fkd]

IJR,

First of all, Happy New Year and Merry Christmas (if it's the case)for you and all the collegues around here.

Now, as a steel structures constructor and designer I agree with your frinds concerning that four bolts makes easier the assembly because I very oten use to place nuts under the base plate and use them for leveling the base plate (already welded to the collum) so we can easily put the collum standing in the right position (I meam vertical axis realy vertical) and them we install the top nuts and washer and do the grouting. Another side effect is that the structure during the assembly get's more stable due to the parcial fixity at the base with whatever kind of foundation solution you have.

The second point is also true because due to this "fixity" at the base your collums will have a smaller effective buckling lengh.

Best,

Fred

 

[Taro]

In the U.S., we are now required to provide a minimum of 4 anchor bolts for column bases in order to provide for stability and ironworker safety during erection. More information on this topic and other new requirements for steel construction safety can be found at:

http://www.aisc.org/documents.asp?mode=docdetail&doc=246

 

[Ron]

IJR..Happy Holidays
The base partial fixity is important for several reasons. In as much as our analyses are often done with almost arbitrary assignment of constraints (pinned conditions at baseplates), the structure reacts as it is constructed...with some fixity at the base, varied by the attachment conditions. When we check this in the primary plane of stress action, we decide the bolts and patterns we need. In the secondary plane of action, we usually do a cursory check to validate our assumptions. One of the things we rarely do is check the thermal effects during a fire event.

Consider a building with four 40-foot column bays. Lets say at the perimeter, a fire breaks out in bay 1. Since the perimeter beam is restrained in one direction and unrestrained in the other, it pushes outward on the corner column. If the fire induces even a 500 degree F temperature change (it will likely be somewhat higher than that), then the expansion of the beam will induce a moment in the column base equivalent to a 1.5-inch deflection. If you only have two bolts in plane to resist that, they end up on the line of action of the moment, offering little resistance. Four bolts would offer the resistance of the couple distance, thus placing the inboard bolts in direct tension as they were designed.

 

[jrmack]

I agree with Taro, OSHA as of 18 January 2002 requires that you use 4 bolts for a column (not a post). Use Google for OSHA structural steel. There are other requirements there that may surprise you like shared bolts thru web connections, no longer a good idea if each beam shares the same bolts.