An engineering problem

[sbosecker]

I am planning to build a mechanic's pit in my upcoming garage. Just to avoid the inevitable cries of horror regarding this, let it be known that I also plan to have a lift. Sometimes a lift isn't the proper tool for the job due to size of vehicle or the simplicity of the job.

I am aware that many people don't like pits. Let me request that this thread remains a discussion of the engineering problem I'm going to sketch out in the following paragraphs and leave the pit vs. no pit discussion for another thread.

I have some 11-W-4 grate (serrated - 1-1/4" tall) that I would like to use to span a distance of 2' 11-1/2". There will be at least 2-inches of the grate resting in a support "lip". A picture of a mock-up I made is attached as well as a load table for this grate.

I am comfortable that this grate - spanning less than 3 feet - will support foot traffic without issue. That would be the grate's primary function - to prevent personnel from injury.

I have spoken with a local building inspector regarding my plans and he has indicated that he would want the grate to be able to support a vehicle should a tire somehow wind up on top of it. I don't know why a vehicle needs to be protected from falling into this hole but not protected from running into the sides of the building - I suspect the resulting risk to life and limb would be the same - but that is beside the point. The inspector will probably want this addressed.

This grate is not stout enough by itself - at a 3 foot span - to support a vehicle should a vehicle tire cross it.

I am thinking some sort of temporary situation to reduce the span to a point that it could support a single SUV tire without failure would do the trick for the inspector.

Example: Two lengths of wood framing spaced 1-foot apart supporting the grate from below might do the trick.

Another possible solution might be to somehow spread the load over the grate to prevent the concentrated load from overwhelming the grate.

One last solution might be to replace the grate with some other material for the inspection process. I cannot imagine that this could be cost effective but I include it.

There may be other ways to solve this - the above was just to get the ball rolling.

Once the building was complete and all inspections were complete, I would probably eliminate the engineering solution to return the pit to a more useable state.

My Toyota Highlander is just under 6000 pounds. Let's call it 1500 pounds for each tire. I believe the tire "patch size" is about 8-1/4 inches long.

Thoughts regarding how to achieve the temporary support needed would be appreciated.

EDIT: It should be noted that the fine print in the Load Table indicates that one should use the 1-inch values when the grate in question is 1-1/4 inch serrated grate.

Scott

 

[MagKarl]

If I'm reading the chart correctly, it looks like the 1 1/4 grating at 3 foot span can support a concentrated load of 1705lbs per foot of width with a deflection of 0.107". I'd interpret the "width" as a foot in the length direction of your pit, perpendicular to the span.

How are you reading the table?

 

[theoldwizard1]

Most local codes prohibit pits or have very severe requirements (drains, electrical, etc.). Check before you start digging !

 

[sbosecker]

If I'm reading the chart correctly, it looks like the 1 1/4 grating at 3 foot span can support a concentrated load of 1705lbs per foot of width with a deflection of 0.107". I'd interpret the "width" as a foot in the length direction of your pit, perpendicular to the span.

How are you reading the table?

MagKarl,

This confused me for a long time as well but I think you are correct that the "lbs per foot" concentrated load is not cross span but perpendicular to the span.

There is a note at the bottom of the table that indicates for serrated grate (like mine) one should knock 1/4" off the depth. I believe one has to use the 1-inch values for the 1-1/4" serrated grate.

I think that takes us to 1091 lbs per foot concentrated load (3-foot span). 8-inches (tire footprint length) divided by 12-inches = .666. Multiply that by 1091 lbs gets us to a tire weight of about 727 lbs. So, if I'm doing this right, we're about half way to where I'd like to be at a 3-foot span.

Best regards,

Scott

 

[JWVan]

Size the pit to the size of the lift, then have the platform act as your support for both foot traffic as well as the occasional tire load. I have a motorcycle platform lift, when I mover I plan on this type of set up for it. May not met your needs, but this is my plan in the future.

 

[Leaky88]

Has the slab been poured yet?

If not, how about using channel tubing - inside of channel tubing - under the grate. One sleeve would have to be long enough to conceal the support when not in use. The other, enough to ensure adequate inside contact when in use. The sleeves would be flush on each side of the pit walls concealed under slab. When you need the barrier, retract and slide across the opening. Could even add a hole for safety pin. You would need to have the span piece in place prior to pour. I would also add a tracks for an adjustable pan on rollers to capture the oil. Leaky

 

[sbosecker]

Most local codes prohibit pits or have very severe requirements (drains, electrical, etc.). Check before you start digging !

TheOldWizard1,

Hence the discussion with the inspector.

Best regards,

Scott

 

[sbosecker]

Size the pit to the size of the lift, then have the platform act as your support for both foot traffic as well as the occasional tire load. I have a motorcycle platform lift, when I mover I plan on this type of set up for it. May not met your needs, but this is my plan in the future.

JWVan,

The lift will be located in a different part of the building.

Best regards,

Scott

 

[sbosecker]

Has the slab been poured yet?

If not, how about using channel tubing - inside of channel tubing - under the grate. One sleeve would have to be long enough to conceal the support when not in use. The other, enough to ensure adequate inside contact when in use. The sleeves would be flush on each side of the pit walls concealed under slab. When you need the barrier, retract and slide across the opening. Could even add a hole for safety pin. You would need to have the span piece in place prior to pour. I would also add a tracks for an adjustable pan on rollers to capture the oil. Leaky

Leaky,

Nothing has been poured yet.

I'm sorry but I'm not following you.

Best regards,

Scott

 

[MagKarl]

Ok, I didn't read the fine print about the serration reducing the height. You only get 1091lbs per foot. I'd do it right and get taller grating and skip the extra band aid structure.

 

[KenB]

You probably should run your final plans past your insurance agent, before you start construction.


Ken

 

[Leaky88]

Scott,
Steel "tubing" is term I should have used. Take maybe 2" tubing and use it as pockets on each side of the pit. (One side will be longer than the other to conceal support when not in use.) Inside the longer 2" pocket, you would have the next smaller size tubing/bar (the support) that could be pulled out and extended across the opening into the 2" pocket on the other side under the grate. If tires are a concern, put these every 18" or whatever inspector deems necessary. In the end, it would look like a ladder from top.

Leaky

 

[hoppsxc140]

Here is something that could work as a solution but might require a bit more cost to purchase, but should pass any requirements in a home.

Last summer on I-90 we had to retrofit the top of catch basin inlets. To achieve this we cut off the top 1' of the existing structure and had the new 1' section precast with a recess to hold new traffic rated, lockable lids. Attached are the plans for both the precast concrete section along with the new lids that were installed and a picture of one of the finished structures. Each frame and grate are just sitting on the lid so one or all can be removed to allow easy access.

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[readhead]

We supply grating for this kind of application quite often for trench drains, wash bays and pits. What we see speced most often is either 2 1/2" or 3" grating. It is usually in small enough sections so it is light enough to remove and replace. Serrated is generally not required. Galvanized is usually required but in your case I wouldn't think it would be necessary. We usually fab and supply some kind of frame to be embedded in the concrete to support the grate and protect the edge of the concrete.

Whenever there is even a hint of a possibility that a vehicle will place a tire on the grate the heavier grating is specified. End banding the pieces reduces the chance of "rolling " the grate and you may be able to convince someone to reduce the height.

 

[xtremek]

^ I like this suggestion. Something that is divided into sections so that it can be moved by 1 person. If it were me, I'd want something like this so I could keep it covered when it's not in use, people and animals don't fall in. Hinge it on one side, and if it's thin enough, you should be able to drive on it when it's open.

 

[sbosecker]

Ok, I didn't read the fine print about the serration reducing the height. You only get 1091lbs per foot. I'd do it right and get taller grating and skip the extra band aid structure.

MagKarl,

The fine print is very fine. Ha!

I understand your thinking. Normally I might take the approach you are advocating.

However, I am not worried about driving over it. It won't be impossible to run over it with a car tire but it will be very difficult due to walls on either side of the pit.

This "band aid" is only to placate the inspector.

As you can see from the picture, I have the grate in hand (acquired at a very reasonable price) and it is more than adequate to keep someone from falling into the pit. It is also light enough to move without heavy equipment so that I can actually use the pit as desired.

Best regards,

Scott

 

[finn]

There is a floor drain in the shop I bought last year, about 16"X36" (from memory), which drains to an oil separator.

The grating is about 1 1/2" tall, and banded with a ~3" wide steel flange. It is severely distorted, presumably because it was driven on.

It's on my list of things to redo, as it created a trip hazard when it permanently distorted.

I wouldn't recommend that you procede.

 

[sbosecker]

Scott,
Steel "tubing" is term I should have used. Take maybe 2" tubing and use it as pockets on each side of the pit. (One side will be longer than the other to conceal support when not in use.) Inside the longer 2" pocket, you would have the next smaller size tubing/bar (the support) that could be pulled out and extended across the opening into the 2" pocket on the other side under the grate. If tires are a concern, put these every 18" or whatever inspector deems necessary. In the end, it would look like a ladder from top.

Leaky

Leaky,

I don't think that would solve the problem. The bars crossing the pit basically carry the load. I don't think running a tube parallel to the bars (running a tube across the span) wouldn't change that materially unless you sowed them in there very thick. It would probably help some but the calculations would be complex and I wouldn't be surprised if when you got enough tubes to do the job, you wouldn't need the grate anymore.

I could be wrong about this but I'd have to see some calculations to convince me otherwise.

Best regards,

Scott

 

[sbosecker]

Here is something that could work as a solution but might require a bit more cost to purchase, but should pass any requirements in a home.

Hoppsxc140,

I hesitate to ask but how much was one frame (roughly 4' x 2') and two grate inserts?

I would need 9 frames & 18 grates for my project.

Best regards,

Scott

 

[readhead]

Load requirements change when you start talking dynamic vs static loads. Of course the chart usually only gives static loads. Most grating is used for stairs and walkways which is still static in grating speak. As soon as rolling heavy loads are considered then the engineer gets involved.

I can tell you from personal experiance that 1 1/4" grating will not hold up to driving over. I know you said that it is unlikley that anything will drive over it. The inspector is looking at it a different way. He is considering the use if something changes and there is a possibility that it will be driven on. What if you decide to move in a year and the next guy parks his dually truck on it.

 

[sbosecker]

While I appreciate the suggestions to use other products that are designed to support a vehicle, that goes beyond the scope of this thread.

I am aware that there is grating available that is designed to drive vehicles over.

I am interested in solutions - if available - that would allow me to use my existing grate to pass an inspection.

1. My grate is adequate to prevent bodily harm resulting from someone falling into the pit.

2. I accept the small risk that a vehicle tire will overload the grate with out additional structure.

My father has had a pit in his farm shop for over 40 years and no one has ever driven over the 2-by-wide planks he has covering it. No one has fallen into the pit either. My brother also has a shop 1/2 mile away from my parents. Similar pit - same outcome.

My architect - after a lovely rant about the lack of a need for a more robust grating system - helpfully suggested that a bumper around the perimeter of the pit to prevent a tire from driving over the grating might placate the inspector. I pointed out that I was more likely to trip over this "safety feature" and break an aging hip than I was to drive into the pit with my car. He laughed and agreed.

Best regards,

Scott

 

[sbosecker]

Load requirements change when you start talking dynamic vs static loads. Of course the chart usually only gives static loads. Most grating is used for stairs and walkways which is still static in grating speak. As soon as rolling heavy loads are considered then the engineer gets involved.

I can tell you from personal experiance that 1 1/4" grating will not hold up to driving over. I know you said that it is unlikley that anything will drive over it. The inspector is looking at it a different way. He is considering the use if something changes and there is a possibility that it will be driven on. What if you decide to move in a year and the next guy parks his dually truck on it.

Readhead,

Thanks.

Can you tell me more about the personal experience of driving over the 1-1/4" grate? That might persuade me to do something different than I'm thinking.

Much of the following is tongue-in-cheek.

Regarding the guy parking his dually truck... I understand the thinking but, again, it would be very difficult to do this. Not impossible but very difficult. When he does somehow manage to drop a tire into the pit, I suspect the truck will fall about the radius of a tire and then stop.

Oh, the humanity!

It is more likely that the next guy living in my homestead would try to drive his oversize vehicle through one of the 7x9 garage doors that I have on my current house garage. In doing this - and failing to clear the door - he could destroy a load-bearing structural member and the entire north end of the house would collapse on him and his vehicle.

For some reason, this is OK. Ha!

Best regards,

Scott

 

[hoppsxc140]

I can't dig up this invoice but they were right under $6,000.00 for 11 units. 3 of these were standard items. The cost in all of these included the 1ft precast concrete section, engineering and construction on the 8 non standard sizes. The company that sold me the structures doesn't manufacture the steel so the price was not split out. I would suggest contacting someone local to you that deals in water works/precast concrete. They would be able to help point you in the direction of someone local.

Here is another option that would work, same principle but looks like they make them out of steel angle and cast iron.

http://www.usfoundry.com/usfoundry/wp-content/uploads/2012/10/TRENCH-DRAINAGE-187-1901.pdf

 

[Keel]

I know you might not want to hear it, but let me ask, WHAT INSPECTOR have you been talking to..
My guess is it hasn't been the fire inspector.. good luck.. but I'd go to your fire dept. and talk to them about pits in garages..

 

[sbosecker]

I know you might not want to hear it, but let me ask, WHAT INSPECTOR have you been talking to..
My guess is it hasn't been the fire inspector.. good luck.. but I'd go to your fire dept. and talk to them about pits in garages..

Keel,

Labels are everything.

http://www.survivorsheltersllc.com/sentinel/

This may not work out but we'll see.

Best regards,

Scott

 

[readhead]

Clearly you have already made up your mind about this. $150 worth of grating is driving the whole program.

 

[sbosecker]

I can't dig up this invoice but they were right under $6,000.00 for 11 units. 3 of these were standard items. The cost in all of these included the 1ft precast concrete section, engineering and construction on the 8 non standard sizes. The company that sold me the structures doesn't manufacture the steel so the price was not split out. I would suggest contacting someone local to you that deals in water works/precast concrete. They would be able to help point you in the direction of someone local.

Here is another option that would work, same principle but looks like they make them out of steel angle and cast iron.

http://www.usfoundry.com/usfoundry/wp-content/uploads/2012/10/TRENCH-DRAINAGE-187-1901.pdf

Hoppsxc140,

Thanks!

Scott

 

[sbosecker]

Clearly you have already made up your mind about this. $150 worth of grating is driving the whole program.

Readhead,

I really would like to hear the story about the grate. If I have offended you in some way I apologize. That was not my intent.

Best regards,

Scott

 

[gearhead1]

My cousin built a pit years ago and you can drive on it.

He used standard cinder block, but the top is two rows of red brick with angle iron on top of that. The angle iron had studs welded on the back so it's supported by the concrete. The concrete is flush with the top of the angle iron. He uses 2 x 10s, but actual 2 inches thick. I've seen him drive on them without issue.

If wood is an option, it can be calculated how thick it needs to be given your span.

 

[Nick Danger]

While I appreciate the suggestions to use other products that are designed to support a vehicle, that goes beyond the scope of this thread.

I am aware that there is grating available that is designed to drive vehicles over.

I am interested in solutions - if available - that would allow me to use my existing grate to pass an inspection.

1. My grate is adequate to prevent bodily harm resulting from someone falling into the pit.

2. I accept the small risk that a vehicle tire will overload the grate with out additional structure.

My father has had a pit in his farm shop for over 40 years and no one has ever driven over the 2-by-wide planks he has covering it. No one has fallen into the pit either. My brother also has a shop 1/2 mile away from my parents. Similar pit - same outcome.

My architect - after a lovely rant about the lack of a need for a more robust grating system - helpfully suggested that a bumper around the perimeter of the pit to prevent a tire from driving over the grating might placate the inspector. I pointed out that I was more likely to trip over this "safety feature" and break an aging hip than I was to drive into the pit with my car. He laughed and agreed.

Best regards,

Scott

Service garages had 3" tall plate steel bumpers around the pits where I grew up. I thought they were a trip hazard. But in your case, you can unbolt them from the floor as soon as the final inspection is complete.

 

[sbosecker]

Service garages had 3" tall plate steel bumpers around the pits where I grew up. I thought they were a trip hazard. But in your case, you can unbolt them from the floor as soon as the final inspection is complete.

Nick,

I might check on this option. It would be a simple solution if the inspector would buy off on it.

...and oh, by the way

No anchovies? You've got the wrong man! I spell my name "Danger"!

Thanks for giving me flashbacks to stuff that happened a very long time ago!

Best regards,

Scott

 

[bczygan]

Too bad you talked to the inspector.

I would have excavated for, and installed the pit, then formed and placed the concrete slab, with a 1" thick form and pour in the pit area. Chip it out later.

Bill

 

[sbosecker]

Too bad you talked to the inspector.

I would have excavated for, and installed the pit, then formed and placed the concrete slab, with a 1" thick form and pour in the pit area. Chip it out later.

Bill

Bill,

This, or some variant thereof, is by far the most common suggestion when I talk to my friends about this.

Best regards,

Scott

 

[srmofo]

Wouldn't a 4 post drive on lift be cheaper? You could even sink it into the slab so ramps would not be required

 

[bczygan]

Bill,

This, or some variant thereof, is by far the most common suggestion when I talk to my friends about this.

Best regards,

Scott

One good thing though........

When you build it to satisfy their requirements, it is sure to be substantial!

The problem as I see it, is to meet the load requirements of the vehicle while making the materials light enough to remove.

I would do that with enough cross beams, set into pockets, so that they cut the span and allowed higher loads because of less contributing areas.

The beams would be removable and the grid would be cut in smaller pieces, so it too could be easily removed, in part or in whole. More closely spaced and numerous beams also means lighter ones, which are more easily removable.

You could also cut the span in the width of the pit by installing a longitudinal beam (Also in pockets). This might be enough by itself to allow use of the grid, or the even smaller cross beams could bear on this.

Cutting spans with members makes the members more numerous and smaller, by each degree of increase. Find the balance you need between quantity and size/weight.

Bill

 

[HoosierMark]

So why not get yourself some oak boards and use them to span the opening. Your Dad did it for years, use them for the inspection. Or notch in a place for an I beam to split the distance length wise. The right size beam should give you plenty of support for the grate. Then when the inspection is done use it for something else, keep it to insert if you ever sell the place or sell the beam.

 

[hoppsxc140]

Here is another option, especially if driving into it is extremely unlikely.

http://www.incord.com/baynets/truck-service-bay-covers.htm

 

[rsanter]

How about a rail bolted to the ground on each side to guide the tire to keep the tire off the grating. Kind of like a car wash uses. See if that would make him happy

Bob

 

[sbosecker]

So why not get yourself some oak boards and use them to span the opening. Your Dad did it for years, use them for the inspection. Or notch in a place for an I beam to split the distance length wise. The right size beam should give you plenty of support for the grate. Then when the inspection is done use it for something else, keep it to insert if you ever sell the place or sell the beam.

HoosierMark,

I think oak boards would likely suffer the same fate as my grate. They would fail if a vehicle was driven over them. Dad used lumber for years for the same reason I'm using the grate. To keep people from falling into the pit. He never drove a car tire over the planks.

If 2 x wide oak planks will support a 1500 pound load in the middle of a 3 foot span, that would be an easy fix to my dilemma.

Best regards,

Scott

 

[sbosecker]

My cousin built a pit years ago and you can drive on it.

He used standard cinder block, but the top is two rows of red brick with angle iron on top of that. The angle iron had studs welded on the back so it's supported by the concrete. The concrete is flush with the top of the angle iron. He uses 2 x 10s, but actual 2 inches thick. I've seen him drive on them without issue.

If wood is an option, it can be calculated how thick it needs to be given your span.

Gearhead1.

That is intriguing.
Do you know how wide the pit is?
What kind of vehicle did he drive onto these planks?
Type of wood?

Thanks for any further information that can be provided.

Best regards,

Scott