Roof Collapses 1

[RHTPE]

Just investigated a PEMB this morning. The frame's columns are bearing on a 10" thick foundation whose top is 48" above the floor slab. The load on an interior frame pushed the top of wall outward about 3/4". No connection between the slab on grade and the wall. Nothing to tie the bottoms of the columns together. Go figure. I'm sure the PEMB supplier dumped the responsibility for foundation design elsewhere.

Checked the density of the snow cover in my corner of CT today. Current cover is about 20-21". Current ground snow load of that cover is 32 psf - determined by actually weighing it. With the drifts seen on some of the roofs around here it's not surprising that even newer structures are straining.


Ralph
Structures Consulting
Northeast USA

 

[hawkaz]

RHTPE
Our of curiosity, what is the code required ground snow load in your area?

 

[RHTPE]

hawkaz

CT's code ground snow load varies - 30, 35, or 40 psf depending on the town. Our NW & some NE towns at higher elevations see substantially more snow than the CT River valley & shoreline towns.


Ralph
Structures Consulting
Northeast USA

 

[SteelPE]

RHTPE

The column bearing is 48" above SOG? From my experience, that is not a wise decision for a PEMB. Especially if the PEMB does not have a center line of columns.

I find that some PEMB are designed/specified by contractors. They seem to think that since they have some computer program provided by the mfr that they will be alright. This is a detail that is very dangerous in the design of the structure and should be eliminated where ever possible.

 

[EdR]

Just curious?? Could part of the problem also be that a combination of things (such as decreased snow loads, more "precise" structures solutions, LFRD.....etc.) has decreased the overall factor of safety that used to built into designs to a point where we are getting these failures.....

I haven't been involved in structural design for many years so have not kept up with code changes and am just wondering if we have gotten "too" smart.....i.e. we are computing things (or think we know things) to a degree that really is not safe when all of these things are combined.

Ed.R.

 

[SteelPE]

EdR,

That may be a possibility, however, you may also include the effects of energy codes. As you increase the insulation required inside of a building, you will end up decreasing the heat loss. I haven't brushed up recently where the snow load formulas come from so I don't know if heat loss is considered in the formula already (other than the Ct factor).

Also, in my area, the snow loads have been increased in recent codes. Some areas (though rare) have seen a 30% increase in the amount of roof snow load required by the code. Owners have complained about it.... but I hope they know why now.

 

[SteelPE]

An article dealing with all of the problems mentioned above. It's interesting to read some of the comments attached with the article.

http://www.boston.com/news/weather/...collapses_prompt_hard_look_at_building_codes/

 

[a2mfk]

Not to reiterate too much that has been said many times about PEMB, but in theory these structures could be some of the better engineered and designed structures around. In theory. But enter a "budget minded" owner and a wham-bam GC who treats the EOR of the foundations as a necessary pain in the butt, and who is left in charge of these projects?

Also, PEMB have evolved from warehouses and shop buildings with a few overhead doors and that is about it, into retail shopping centers, churches and sometimes even office buildings... Have the contractors, PEMB, A and E involved in these types of more complex projects evolved with these buildings or are they still being treated like a warehouse with a slab?

Now I have also had some very good experiences with PEMB where the architect, contractor and ourselves were all experienced with PEMB, and got together on the phone or in person during design and lots of communication. We all knew who was designing what and we were all together on the same team. The contractor also used the same PEMB which is a big national company and we actually met with a structural engineer that designed the building!

There was a great article a few years ago in one of the structural rags about failures of PEMB. Examples of failures included no site inspection or observations from the PEMB engineer (since if there is an EOR he is not responsible for the building structure, just the foundations). Many PEMB companies offer little to no support during erection, just here is your erector set, here are the instructions, go put it together. And not to be too harsh on the PEMB, the owners don't want to pay for any additional services either so they get what they pay for. In some cases, it is a pre-fab kit building, even if too complex to be treated as such...

I don't want to hijack this thread about snow on roof collapses, since I live in Florida anyway. But knowing also that many PEMB seem to be designed within a hair on a gnat's @$$ of code requirements, there probably is not much fluff in any of their new designs, its no surprise that there are these snow collapses. These often get built in more rural areas where the building department, engineering, and contracting services may leave something to be desired. That is without mentioning the older PEMB buildings and the effects of energy requirements on snow and ice dams and all that good stuff I don't deal with

A PEMB in general seems to have a lot less redundancy and due to the light gage members, bracing becomes very critical. Miss a couple of braces and your roof could be in trouble.

 

[LSPSCAT]

I will add "rough" construction practices that go along with PEMBS. As stated there is not much "fluff" in these designs; add in some rough handling with lulls and you have pre-bent roof joists!

Two qualifiers before we place everyone under the bus:

Being in the Northeast, we do have an anomalously high snowpack. Snow has been higher than average in most areas of CT, MA, and RI. The compounding factor is that most expereienced there entire seasonal snowfall in approximately 5 weeks time with out any warmup. Therefore some roofs had the 5-6 inch water equivalent that brought them to or above their design snow loads. This is not justification for collapse or failure.

Second, we have also seen the collapse of wood structures (residential, mill roofs, agricultural) that have withstood 80-100+ years of New England winters, these were not the majority but offered as to the frequency of roofs seeing similiar conditions.

 

[RHTPE]

Attached is a photo of the building I looked at last Friday. A huge PEMB in an "L" shape, full clear-span at the "L" with some sort of transfer beam to pick up the perpendicular frames. Needless to say, my role ended after determining the snow load on the adjacent building's roof. I found the snow load to be very close to the code value (30 psf), and the circumstances surrounding the collapse to be too vague to readily determine what triggered the collapse (it happened well after the last snaowfall while they were clearing the roof). The property & building ownership is such that I had to back away. As a sole practioner I just do not have the resources to do a proper investigation for little chance of getting paid in my remaining lifetime.


Ralph
Structures Consulting
Northeast USA

 

[chicopee]

These roof collapses and questionable roof structural integrities throughout the country are the best thing for the US econnomy. Another construction boom is imminent.
What is interesting is that most roof collapses that are in the news are with relatively flat steel roof supported by steel joists. Don't hear much about residential wood roof collapses particularly with ranch styles homes.

 

[BAretired]

RHTPE,

(it happened well after the last snowfall while they were clearing the roof)

This is not uncommon. The workers pile up snow in some areas causing local concentrations which can trigger collapse.

BA

 

[SteelPE]

chicopee

I hope you are correct about another construction boom.

I have heard of a few residential roof collapses. They are just not broadcast on the regional news at night. A neighbor of my parents had their whole 3 car garage collapse last week (conventional "stick" framed). So it isn't unheard of.

 

[RHTPE]

I agree with SteelPE - I am aware of as many wood-framed structure collapses as steel framed. Some new, most much, much older.

It's my contention that many wood-framed structures have had connections damaged in prior years with little obvious indication. Then we get walloped with 2 massive storm accumulations and guess what - they fall down.

The collapse of a private home or residential/farm structure gets only a brief media blurb - unless people and/or farm animals are injured/killed.


Ralph
Structures Consulting
Northeast USA

 

[a2mfk]

Ralph,
If you want to get paid well and do investigations of these structures, get in with the insurance companies. They will flip the bill for all kinds of investigative work. Or, and it pains me to say it, attorneys for either side...

Insurance companies will sometimes even pay the bill and then subrogate against the original contractor and/or engineer. Especially if the value of the claim is relatively high, like with that big commercial building you were looking at.

 

[RHTPE]

a2mfk - My initial investigation was for the "owner's" attorney. I put 'owner' in quotes because some financial glitches exist between this owner and the financing bank, and apparently the bank has taken over the insurance on the property. Knowing this, and knowing that the attorney's firm is probably too small to deal the magnitude of the investigation and its duration, as well as not knowing the preceding conditions of snow load and the sequence of snow clearing operations, I decided the most prudent thing for me is to back away.

I understand and agree with your point. I simply do not want to devote the rest of my career to this one investigation. I am not in a position to hire and pay a small staff to handle it. This will likely be a 7-digit claim involving many parties.


Ralph
Structures Consulting
Northeast USA

 

[miecz]

Ralph-

If this is the Motocross building on Day Hill Road, I saw it during construction. I believe it was designed and built by a company in New Hampshire. The builder was looking for a P.E. to perform a threshold review or to sign and seal the drawings, I forget which. After spending some time talking to the builder and looking at the unusual roof framing, I got cold feet. I told my boss I thought it was a job for a prefab specialist, so we passed on the opportunity. Sure would be interested in who signed and sealed the structurals. Was it a Connecticut based firm? Was it a prefab specialist?

 

[RHTPE]

miecz - I only had an opportunity to quickly review the foundation drawings and the superstructure of the more conventionally built portion. The folks that I met with had not yet obtained the PEMB drawings. The PEMB erector or fabricator was there but not yet allowed on site. I believe they were from out West somewhere - didn't really pay attention because I knew where I was going with project after checking snow load on the building that was still standing. The owner's attorney is from my home town so I'm sure that I'll get regular updates.

Do you recall how the "L" was handled with the main frames? There had to be some sort of transfer beam utilized.


Ralph
Structures Consulting
Northeast USA

 

[miecz]

I remember a deep steel girder carrying the frames at the "L", which struck me as so large that it must have been a plate girder. So I think that the frames parallel to the Girder were conventional rigid frames, while those carried by the girder were like 7s. The short leg of the "L", then, had conventional rigid frames. Don't know if you can trust my memory though, as it insists that the erector was from northern New England. Also, it seemed like a small outfit to be erecting such a large building.