Residential: reducing baseplate thickness

[shacked]

Preface: The majority of my projects are residential, therefore the loading that I encounter for design purposes is relatively low.

I'm designing an attached 250sf open patio roof and I am using 2 steel cantilever columns to resist lateral seismic loading at the free edge, furthest from the house.

Knowns: HSS4x4x5/16 columns, Axial load = DL+Lr = 3.6kips, Design seismic moment at base of column = 11.0kip-ft.

Designing the column as fixed, therefore the baseplate design is to resist bending. Using AISC 360 requirements the required thickness is 1 inch. This seems excessive for a lightweight patio roof. I see that size of baseplate being used for traffic lights approximatelly 15ft tall that cantilever 10-15ft over the street width.

Is there a way, other than fully embedding the steel column in the footing, that could be used to reduce the bending in the baseplate?

Tie the column into the concrete slab?

Thanks.

 

[dauwerda]

You could use stiffeners, but that would cost more than the thicker base plate. 1" thick sounds very reasonable to me for an 11 k-ft overturning moment. Those traffic light supports probably have around the same base moment as you do, one signal typically only weighs 30-50 pounds.

 

[r13]

Enlarge the base plate size.

 

[BAretired]

What is the size of your base plate? Where are the bolts? Are the loads and moments unfactored?

BA

 

[Guest090822]

The seismic force seems unreasonable.

 

[Tomfh]

1” seems fairly heavy.

How did you get that seismic load?

 

[dik]

Something in the order of 5/8" Bolt force is about 15.5K for a 10x10 BP and e= 8.5"
bolt moment about 23.3 "K, and plate moment about 46.6"K

Z for 5/8 plate = 0.98 in^3

Mr = .98 x .9 x 50 = 43.9 "K.... time to fine tune...

Dik

 

[dold]

1" BP isnt going to break the bank. 40lbs vs ~30lbs. Embedding the column/tying into the slab/etc/etc/etc - all going to cost more than the plate. Just my $0.02

250ft^2 / 2sides / 2columns x 15psf DL x (Cs= say...Sds=.5 guess / R=1.25 = 0.4) = 375lbs ultimate seis load. 375# x ~10ft = 3.75k-ft ultimate x overstrength=1.25 = 4.7k-ft ultimate. Depends on where OP's project is.

 

[shacked]

Thanks guys. In california so Sds=0.93, & DL=16psf, So seismic load per column = 15x(250/2)*(0.93/1.25)x1.3=1815Lb and design moment = 1815x8.5ft=15.4k-ft. The 11.0k-ft overturning moment was ASD.

I used allowable stress design originally.

 

[dik]

try 3/4"x10x10 BP... Mf=15.4 'K
Sorry, I've been using limit states for 50 years and only work in it... Class of '69 was the first class to use limit states for concrete and steel.

Bolt Force= 15.4*12/8.5= 21.7K
BP moment= 21.7x3"= 65.1"K

Z = 10x.75^2/4= 1.406 in^3

Mr= 1.406x.9x50= 63.3 "K Close... I'd use it... maybe 7/8" plate.

Dik

 

[dold]

OP
Is it 250ft^2 trib to the two columns or to the entire porch? If entire porch, then your moment can be cut in half since you've got 2 cols?
Might not need rho=1.3 but overstength is still 1.25 for ordinary canti columns (but you dont need to use both). Dont forget you need to be using strength level forces for your appendix D anchorage calc. Also dont forget your foundation overturning has to be designed for overstength for a cantilevered column. Cali plan checkers have a keen eye for that one.

Dik
I'm guessing OP wants to use A36 for his BP?

 

[dik]

50 ksi stuff is common here... A36 is obtainable... no real reason to use it... similar cost and BP just gets thicker... but, same approach...


Dik

 

[CANPRO]

If the fabricator has 1" plate in stock, it'd be cheaper and easier to just use a heavy baseplate vs stiffeners. These kinds of a jobs tend to go to smaller fabricators, a thinner plate + stiffeners might be better for some...but 1" plate is fairly common.

Conversation above has been focused on the strength of the plate, but since this is a moment connection which is providing lateral stability to the structure, the stiffness of the connection comes into play as well.

To use dik's example above - looks like a 5/8" is close to working...but how much does that plate have to deform to resist the applied moment, and what does that do to the structure as a whole?

 

[dik]

A 3/8 plate, I'd be concerned about... 5/8 or 3/4... less so. I've often encountered fabricators that have substituted plate for something they have in stock... also... almost never use stiffeners... a lot cheaper to thicken the plate... recent job used 1-3/4" plates for transferring moments to small HSS columns.

 

[XR250]

Seems like you need to divide by 2 again as dold stated..
1" does seem excessive

 

[shacked]

Thanks for all the input. Actually, this load case is not for loading parallel to both columns, rather it is towards the house. I can't drag the load into the roof diaphragm since the beam supporting the patio roof is dropped below the double top plate of the house.

I assumed plate was A36, didn't realize that 50ksi plate was that common.

 

[Guest090822]

As others have stated, you should be able to divide the force by 2 again since it's attached to the house.
Your tributary area becomes 250/4 = 62.5 SF. If DL is 16 psf then the total dead load to a single column = 1,000 pounds.

Seismic load = 1,000 (0.93) = 930 pounds.

Moment = 930 (8.5') = 7,905 ft-lbs.

ASCE 7-10 maximum earthquake load for ASD = 0.7E so moment = 5,534 ft-lbs (without any other adjustments if required)

I don't have the most recent ASCE code, but for older folks, recall that they changed the factors in the load combinations in the last 10 years. Especially for wind and seismic.

Take a look at the AISC base plate and anchor design guide:

https://www.construccionenacero.com...uide_1_-_column_base_plates_-_2nd_edition.pdf

 

[dauwerda]

I can't drag the load into the roof diaphragm since the beam supporting the patio roof is dropped below the double top plate of the house.

Keep in mind stiffness attracts load. If the patio roof is attached to the wall of the house (at any point on the wall), it will probably result in a much stiffer connection than a HSS4x4 that is cantilevered up 8'-6". You either need to isolate the roof from the house or ensure it can handle the additional load it will see.

I assumed plate was A36, didn't realize that 50ksi plate was that common.

The fabrication shop I work with doesn't stock anything but A572 Grade 50, it is typically dual certified as A36.

 

[phamENG]

AISC updated the material spec tables in the new SCM. They updated their MSC "Are You Properly Specifying Materials?" article accordingly.

[URL unfurl="true"]https://www.aisc.org/globalassets/modern-steel/archives/2018/04/areyouproperlyspecifyingmaterials.pdf[/url]

 

[TheDW]

Unrelated to the OP, but that document says 50ksi is the new preferred material for rectangular HSS...that's news to me. Good to know.