Anchor bolt configuration on a base plate

[sutki1]

Hi,

The connection of a silo leg to a foundation has been designed. There are 4 anchor bolts on a rectangular the base plate. The shear and tension capacity of 2 bolts is sufficient. If I replace the 4 bolts by 2 bolts would the configuration matter. What is the minimum number of bolts required?

Thanks

 

[aiden]

The configuration does matter by a greater degree. changing from 4 bolts to 2 bolts would demand a redesign of silo leg and the foundation pedestal/footing.
While 4 bolts can be assumed as a FR base, 2 bolts would mean a low PR base (depending on how you arrange the bolt configuration). the K factor for the column leg would be double (at least in one axis) which would result to unusually large leg for a silo. The pedestal/footing size would be substantially reduced due to the absence of FR moments.

my opinion was based on my previous designs of silos. hope to hear comments from anyone.

 

[JAE]

OSHA requires 4 bolts for building columns - the concept being that during erection the columns are put in place without any lateral bracing and until the rest of the structure is installed this presents a hazard.

For your silo, if its part of a complete component that would stand on its own, you probably don't have to worry about it - but if the silo leg is initially placed by itself, there would be a concern.

As far as structural design goes, two bolt columns have been used in the past. However, if one bolt gets damaged (which does happen quite often) then you'd be in trouble with only one bolt left.

 

[aiden]

i forgot to mention this;
i was referring to silo legs with no braces, case where bulk carriers are loading underneath and other equipment installations that prohibits braces. one of my previous design (1990) was a 450 MT fly ash silo on 15m legs, no braces, 2-75mm dia. x 1.4m anchor bolts (2 bolts - specifically requested by client engineer, my objections then).
if your silo legs are fully braced, then 2 bolts would not be a problem, it only boils down if there are codes that prohibit it, on the design point of view, i cant remember any.

 

[sutki1]

Thank you all for the messages.
The silo is seismic rated and the legs are braced. There are 6 legs placed in a circular fashion. Meaning 24 bolts to be placed in precast. Due to the constrution issues if even one bolt is off by an inch, can the precast anchors be replaced by drilled in anchors?

Silo's are usually pre engineered. So, is it the suppliers responsibility to determine whether the silo needed is seismic rated or non seismic?

 

[mrMikee]

sutki1,

I've worked in the silo and agg bin industry in the US for about 12 years. Concerning your question about "the suppliers responsibility to determine whether the silo needed is seismic rated or non seismic" I'd have to say based on my experience that sometimes they do design per the appropriate requirements, but many times they don't. If you are concerned (and you probably should be) ask someone in a position of responsibility to get the design specs and if necessary the calcs. Is there a PE involved? Who is the engineer of record?

Regards,
-Mike

 

[civilperson]

OSHA requires all column base plates must be designed and fabricated with a minimum of four anchor rods.
"OSHA Safety and Health Standards for the Construction Industry, 29 CFR 1926 Part R Safety Standards for Steel Erection" (can be found at

http://www.osha.gov)

 

[DaveAtkins]

OSHA allows two anchor rods at columns which either: weigh less than 300 pounds (that is the column itself weighs less than 300 pounds); or are not axially loaded and are laterally restrained at the top.

DaveAtkins

 

[civilperson]

Reread 29 CFR Column Anchorage 1926.755 and saw only four bolts allowed plus design for 300 pound eccentric load 18" from face of column. Where is the exception of column weight under 300 pounds and not axially loaded found?

 

[aiden]

hi all, just a thought;

i am not in the US, so i am not concerned with OSHA. but as a thought, ive been working on many projects (all outside US) and 2-bolt configuration is very common, which in design can be modelled as PR base or even conservatively as pin base (at least in one plane). one obvious advantage with 2-bolt configuration is the simplicity in construction for cast-in-place anchor bolts. during construction, i have not observed damage in erections of 2-bolt config. either, as long as the column is properly erected with temporary braces/guy wires.

 

[DaveAtkins]

I don't know where it is documented, but OSHA calls the column types I described "posts."

DaveAtkins

 

[civilperson]

"Column means a load-carrying vertical member that is part of the primary skeletal framing system. Columns do not include posts."
"Post means a structural member with a longitudinal axis that is essentially vertical, that (1) weighs 300 pounds or less and is axially loaded (a load presses down on the top end), or (2) is not axially loaded, but is laterally restained by the above member. Posts typically support stair landings, wall framing, mezzanines and other structures."

 

[DaveAtkins]

Well, I have always suspected that I am not following the intent of OSHA by allowing 2 bolt base plates at exterior columns on small one story buildings. But it seems excessive to build a concrete pier to allow for a 4 bolt base plate at an exterior column--back in the "old days," we allowed columns to set on the exterior foundation wall, with a rectangular base plate with 2 anchor bolts.

DaveAtkins