2 vs 4 anchor bolts 1

[WWTEng]

I understand that minimum 4 ABs for a column base plate is an OSHA requirement. I have an existing job from 1955 where we need to replace the base plate for an exterior column due to significant corrosion (column to be removed/shored etc). These existing baseplates were installed with 2 ABs. Can we replace the baseplate with just two ABs or do I have to use 4 per the new requirements? The only issue with using 4 ABs is that the existing pier would have to be enlarged to accommodate 4 ABs.

The column is a W10x49 with gravity load of 8 kips max.

 

[rb1957]

FWIW, yes ... i think you're doing more than simply repairing the existing column, you're rebuilding it.

if the loading on the strucutre increased, and you had to add a new column ... would you use 2 AB or 4 ?

Quando Omni Flunkus Moritati

 

[frv]

The requirement isn't there for the structural load requirements- it's there for the "stability" of the column during erection. 8 kips is a very small load, but it's not what's driving the quantity of anchor bolts.

 

[rb1957]

so why wouldn't you brace with temporary supports ?

Quando Omni Flunkus Moritati

 

[frv]

I believe OSHA's concern is the period between when the column is erected and it is connected to the rest of the structure. Normally, the columns are not braced during this period. Now, this situation is different, so I guess you could use some engineering judgement here. It would appear that the top of the column is going to be attached immediately upon erection of the column- the stability issues really wouldn't come into play.

You may have a problem with the inspector, but for an application like this, I wouldn't feel uncomfortable using only two anchor bolts (provided, of course, there isn't any sort of lateral/uplift loading on the column)

 

[bridgebuster]

I don't think the two bolt rule applies. The 4 bolt rule is to provide added stability against a column toppling over until it's sufficiently braced.

 

[Bagman2524]

Here's the section from OHSA (29CFR1926.755)

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=12746

I don't see any exceptions for the 4 anchor bolt rule.

 

[bridgebuster]

I think 755a(1) and 755b are describing two different conditions.

 

[Hokie93]

Are you removing the entire column to replace the baseplate or is the column remaining in place while only the baseplate is replaced? If the latter, then I don't see how section 1926.755(a) applies since the column has been erected and, presumably, stable since 1955. If you are temporarily removing the column and re-erecting it then the situation is a bit murkier but I would still argue that two anchor rods are sufficient since the column would immediately be connected to a complete, stable structural system upon re-erection. In my opinion, section 1926.755(a) is intended to apply to columns while they are being erected and not yet stabilized by horizontal diaphragms and permanent lateral bracing.

 

[bridgebuster]

I meant to say.
I don't think the two four bolt rule applies. The 4 bolt rule is to provide added stability against a column toppling over until it's sufficiently braced.

I must be getting senile.
The other day I got an e-mail about something I ordered and was wondering why they couldn't find my address; turns out I used my zip code from 20 years ago

 

[racookpe1978]

Since you are wedging it into place right on site between two floors ... Can't you count the 2x anchors bolts at the bottom, and the 2x AB's at the top against OSHA's 4x? 8<)

 

[WWTEng]

Thanks for all the input. I have decided to use 4 ABs. The anchor spacing is tight but there is no uplift or moment on the column.

 

[dik]

The anchor spacing is tight but there is no uplift or moment on the column.

All more the reason to use 2 anchor bolts...

Is there an OSHA inspector in the area and can you explain that there is no stability issue, the col is existing and is not going to topple...

Dik

 

[RFreund]

I think if the column weight is less than 200lb than the (4) bolt requirement does not apply. Just call it a post.

EIT

http://www.HowToEngineer.com

 

[nutte]

What is most frustrating about this problem is that a general erection requirement (properly brace columns during erection) has led to a prescriptive engineering requirement (provide 4 anchor bolts) that sometimes leads to ridiculous results, with no apparent way out for special cases. I've seen small miscellaneous posts detailed with 4 anchor bolts all on one side of the post. "OSHA wants 4, I gave 'em 4." I don't see any benefit here.

RFreund, the limit you refer to is 300 pounds. And at 49 pounds per foot, it won't take long for the W10x49 to exceed that.

 

[StrucPatholgst]

It's always good to check and see if your jurisdiction has a rehabilitation code, which could simply this down to replace-in-kind.

 

[RFreund]

Nutte - good catch, thanks.

EIT

http://www.HowToEngineer.com

 

[frogit22]

based on the intent of the OSHA rule (stability during erection), there is some room for judgment here.

 

[jlkells]

Can someone point me to where the OSHA req't for 4 anchor bolts does NOT apply for columns less than 300-lbs. I looked up the code section in OSHA and it seems pretty clear that 4 bolts are required period. It also says that they must be able to resist a 300-lb load 18" eccentric to the edge of the column, but not that a column weighing less than 300-lbs is excepted from the requirements.

I also looked at the interpretations of code section 1926.755(a)(1) but didn't see any exceptions.

Basically what I am wondering is, why are pre-engineered metal building manufacturers allowed to get away with 2-bolts at all of their pinned base connections...

Please let me know if I am missing something... Thanks!!
John

 

[msquared48]

If you take a look at one of the clarification attempts at the 300# requirement, it appears to me that the interpretation is that a minimum 300# X 1.5' resisting moment (not just the 300# here) is required to be verified for the connection, in addition to furnishing (4) anchor bolts.

May 27, 2004

Mr. L. Carlos Garcia
Project Manager
Urban Associates LP
1400 Geronimo
San Antonio, TX 79925

Re: Under Part 1926 Subpart R, is it permissible to field-weld a column to a base plate that has four anchor rods welded to its bottom side and is already embedded in the footing?

Dear Mr. Garcia:

This is in response to your telephone call of March 24, 2004, to the Occupational Safety and Health Administration (OSHA) in which you ask about the requirements for securing a column to a foundation under 29 CFR Part 1926 Subpart R (Steel Erection). We have paraphrased your questions below:

Question (1): This question has to do with a procedure for erecting steel columns on concrete footings. The scenario is as follows: A steel base plate with four ½" diameter headed studs welded to its bottom side is embedded in a concrete footing. Once the concrete meets the strength specified in Part 1926 Subpart R, the steel column is lowered by crane onto the base plate. While still attached to the crane, the column is braced in two directions (i.e., east-west and north-south). The column is then attached to the plate with a continuous ¼" weld. After the column is field welded, employees in aerial lifts will remove the crane hook and guy wires.

Photograph indicating procedure for erecting steel column using four pre-attached anchor rods attached to the column, and that assembly is embedded into the concrete footing.
Section 1926.755(a)(1) requires four anchor rods. Is it permissible under §1926.755 to use this installation procedure?

Answer:

Section 1926.755 "Column Anchorage" states:
(a) General requirements for erection stability. (1) All columns shall be anchored by a minimum of 4 anchor rods (anchor bolts).

(2) Each column anchor rod (anchor bolt) assembly, including the column-to-base plate weld and the column foundation, shall be designed to resist a minimum eccentric gravity load of 300 pounds (136.2 kg) located 18 inches (.46 m) from the extreme outer face of the column in each direction at the top of the column shaft. * * *
The intent of §1926.755(a)(1) and (2) is to ensure the stability of a column during the erection process. These hazards are addressed in part by the requirement that the column be "anchored by a minimum of 4 anchor rods."

However, §1926.755(a)(1) does not specify the manner in which the four anchor rods must be attached to the column. So long as the column anchoring system meets the strength criteria in (a)(2) and has four anchor rods, the assembly meets the requirements of §1926.755(a)(1). (emphasis added)

In your scenario, the four anchor rods are pre-attached to the base plate, and that assembly is embedded in the concrete footing. Once the concrete has reached the strength required under §1926.752, the column is placed on the steel plate, and pre-attached guy wires are used to brace it. The column is then field welded with continuous weld to the embedded base plate. At that point the column is anchored to the footing with four anchor rods. The requirement in §1926.755(a)(1) is thus met once the column is welded to the base plate. So long as the column in your scenario, after the anchorage assembly is completed, can resist a 300-lb. eccentric gravity load in all directions, the column anchorage will satisfy the stability requirements of §1926.755(a)(1) and (2).

If you need additional information, please do not hesitate to contact us by fax at: U.S. Department of Labor, OSHA, Directorate of Construction, Office of Construction Standards and Guidance, fax # 202-693-1689. You can also contact us by mail at the above office, Room N3468, 200 Constitution Avenue, N.W., Washington, D.C. 20210, although there will be a delay in our receiving correspondence by mail.

Sincerely,


Russell B. Swanson, Director
Directorate of Construction

Personally, I think the site needs to be updated with most of the clarification answers in the order of 7 years old or older.

In metal buildings, some of the columns, particularly in end wall (wind) columns under an end wall mainframe, do not see vertical loads greater than 300#, if only this vertical load this is in fact a definition of a "post". In this circumstance, (2) bolts could be used.

Personally, I think greater latitude needs to be made here if it can be shown that the column base arrangement can handle the eccentricity required. The intent of the code should be to use 4 bolts if that eccentricity criteria cannot be met or shown. Otherwise two bolts are OK.

That being said, I think that small required eccentricity for a typical mainframe column is very, very low.

Mike McCann
MMC Engineering