Tag Archives: live load

What Size Truss Carriers?

What Size Truss Carriers?

It seems every day I am asked to do structural design of post frame buildings – for free. Today’s request comes from BOB in ARKDALE who writes:

“Yesterday I asked a question about a double header and single trusses being spaced every 4 feet with 8 foot spacing on posts. I don’t use the internet much but a reply to my son’s email address would be great. The question was what is a proper double header? We thought one underneath the other off entered or sandwiched off enter.”


Bob’s earlier request was somehow spun off into an internet abyss, as it did not make it successfully to us.

nailing trussesIn my humble opinion, an ideal design solution eliminates need for a header (aka truss carrier) entirely, by having trusses bear directly upon columns. Why would this be ideal? Trusses (in my ideal dream world) are placed into a field cut notch in each column. This transmits all roof loads directly onto posts, without reliance upon beams typically scabbed onto each side. This eliminates trusses being driven to earth in a catastrophic snowfall event.

Back to your question – size and number of required members for your headers, as well as their orientation) should be clearly denoted upon plans provided by RDP (Registered Design Professional – architect or engineer) who produced them. Headers and their connections need to be able to withstand all imposed loads – live (snow), dead (weight of headers themselves along with trusses, purlins, insulation, roof sheathing (if used), roofing, any ceiling, lighting, etc.), as well as wind loads (uplift being a factor). These headers must be adequate to support one-half of clearspan width of your building, plus any overhangs beyond sidewall.

All of this takes an involved series of calculations best performed by an experienced RDP. If you somehow do not have one involved in your project – go hire one right now. This small investment into correct structural design becomes inconsequential compared to pain of building loss should it fail, damaging or destroying valuables your building was meant to protect, as well as injuring or killing yourself or your loved ones who may be inside when the roof caves in.

 

 

 

Under-Designed Ag Buildings

Does Anyone Else See How This Could Be a Problem?

Eric, one of the owners of Hansen Pole Buildings, had me check out a website today for a pole building supplier who is extolling the virtues of a particular “nailed up” laminated column, which has been the subject of some discussion in my articles. https://www.hansenpolebuildings.com/blog/2014/04/titan-timbers/

This particular supplier took verbatim the information provided by the nailed up column suppliers, without questioning the validity of the data supplied.

Me, being the curious sort, took a cruise around the pole building supplier’s website.

WHAT I SAW MADE BLOOD SQUIRT OUT OF MY EYEBALLS!!

“Snow Loading                                                                                   

Xxxxx Buildings commitment to quality is second to none. This is amplified by the fact that all buildings meet or exceed the MN State Building Code. Xxxxx Buildings provides all customers ‘peace of mind’ by making sure the roof system loading for your building will keep you protected from natures elements. The roof system loading includes the trusses and the roof purlins.”

Now I am all over this! I appreciate people with a commitment to high quality and excellence in pole buildings. “…all buildings meet or exceed the MN State Building Code” is way cool….

Hay Storage BuildingUntil I read their next paragraph:

“Ag Buildings
There is no code regulation of Ag buildings, (these buildings are exempt from the code) but suggested minimum loading would be 25 psf or 30 psf live load on the roof system. The definition of an Ag building would be a structure on agricultural land designed, constructed, and used to house farm implements, hay, grain, poultry, livestock, or other horticultural products. This structure shall not be a place of human habitation or a place of employment where agricultural products are processed, treated or packaged, nor shall it be a place used by the public.”

From one side of their mouths is “all buildings” meet Code, and out of the other – they are providing “ag buildings” with loads below Code!!

Here is the Minnesota State Snow Load map: https://www.dli.mn.gov/CCLD/PDF/bc_map_snowload.pdf

To get from a Pg (ground snow load), to a roof snow load, involves the multiplication by several factors. Learn more than you ever wanted to know here: https://www.hansenpolebuildings.com/blog/2012/02/snow-loads/

For discussion’s sake, we will assume these Ag buildings are unheated (most unoccupied buildings are) with the most common 4/12 roof slopes and steel roofing. The roof truss top chord live load under this combination should be 34.7 psf with 50 psf for Pg.

This provider’s, “suggested minimum loading would be 25 psf or 30 psf of live load on the roof system” is under designing these roofs to support snow by at least 13% and as much as 40%!!

You don’t own a farm, so what do you care?

When those under designed roofs collapse and the insurance companies pay to rebuild – it is YOUR insurance rates which are going to increase!

And if you do own a farm, I’d hate to be the one cleaning up the mess when your roof caves in…and hoping you are not in it when it does!