PEMB FOUNDATION

[LOKSTR]

Hi Everybody,

I am looking into a foundation drawing for a PEMB( pre-engg. building). The building is not having any concrete slab. To tackle the horz. forces(50k factored) I need to have a 1-1/2" dia solid tie rod from one column to another.

The foundation will be of steel screw piles with steel beam cap on top of them.

Will much appreciate if someone can shed any light on followings:

1. Do the screw piles need to be designed for 50k horz. force or not?
2. What will be the load path for this horz. force in the absence of concrete slab. Will the tie rod hold the columns from moving out and no horz. force need to to be trasferred to piles and then to ground ?

Thanks

 

[shin25]

Question,

You may need to seperate the horizontal forces that are generated from the gravity loads (they will create tension in the tie rods) and from the lateral loads. The reactions from the lateral loads in the foundations will tend to work in the same direction for all foundations. For this situation, the tie rod may not be effective. Since, you do not have any floor slab nor grade beams, I am afraid that the piles need to be designed to handle these loads at service level.

I am wondering, what is the size of the building or wind speed that is causing such a high lateral reaction?

 

[msquared48]

I assume you are talking about the horizontal kick from the mainframes.

There are three normal ways to take this kick:

1. Hairpins in the slab, or
2. Lateral earth thrust, or
3. A direct tension tie bvetween the legs of the mainframes.

What you describe is condition 3.

The answer to your question #1 is no, not for the 50 Kip load you mention which is due to vertical dead and live load forces on the mainframes. The screw piles never see this load. However, they will see the overall building lateral load due to either wind or seismic forces. You may need batter piles if the lateral capacity is not enough.

Regarding question #2, yes, the load path is the tie rod to resist the tension forces. Think of this tie rod as the string on an archers bow. It serves to regulate the distance between the ends of the bow through tension.

Hope this helps.

Mike McCann
McCann Engineering

 

[LOKSTR]

shin25/msquared48,

Thanks for reply.

The 50kip factored load is from the pre-engg supplier calculation sheets. This load is the worst load out of different combinations of DL,LL,WIND etc.

Do you mean to say that I should look at only the "purely wind/siesmic load cases" to get pile horz. load and design my tie rod for cases other than wind/siesmic load?

Thanks

 

[msquared48]

No, but that is what I expected the max lateral kick would be from. Use the max force the combinations give you and factor them accordingly to design the proper tie beam steel.

The tie beam should link the pilasters for the mainframes, and be placed under the ground encased in concrete. The tie beam steel will have to deadhead within the pilaster steel, and the pilaster reinforced to take the lateral shear too.

This approach has worked for me many times. Good hunting.

Mike McCann
McCann Engineering

 

[shin25]

LOKSTR,

If you design your tie rod to take the horizontal tension forces from the gravity loading, then the system is in static equilibrium for these loadings.

When the wind load will be acting, the unbalanced horizontal loadings will be transferred directly into the piles, because, the tie rod may buckle trying to handle these loads.

 

[msquared48]

LOKSTR:

If the tension tie beam must be able to resist moment too, then what Shin25 says could happen if the beam is not properly constrained by the soil matrix or transverse grade beams...

The moment could be taken out with an additional pile and grade beam system inside the building too. Depends how you want to do it.

Mike McCann
McCann Engineering

 

[hokie66]

Don't know where you are located, but in high wind areas, wind parallel to the ridge can produce net uplift forces in excess of the net downward forces. This would put the "tension tie" in compression, so is another case for checking. If the tie can't resist much in compression, the piles may have to deal with the closing force in conjunction with the longitudinal shear.