Net Section Failure

[slickdeals]

Folks,
More often than not, a bolted tension member is controlled by tensile rupture of the net section. Shear lag in addition to bolt holes play an important role.

Are there connection tricks and tips (bolt layout) etc. to mitigate this failure mode? I have read that developed filler plates can be used to increase the net section and staggering web - flange bolts.

Can anyone explain to me this statement below?

While I think that Larry Muir's article in the July 2007 issue, 5 Common Myths of Steel Design Debunked, raises excellent points, I must take issue with the net section discussion listed under Myth #1. Many members can be connected with an effective net section much greater than 75%, even when heavy truss chord splices are made with 4 rows of bolts (double gage) across the flanges, by simply using one or two "lead-in" rows of bolts in the flanges before the web bolts are started

 

[paddingtongreen]

First the Quotation, I presume you are quoting somebody else. The 75% is an AISC requirement. Before that was introduced, we calculated the net areas and sometimes had more than 75% of the gross area, sometimes less.

You can weld additional steel to a member, it then becomes part of the gross area and the total is reduced to 75%.

It used to be, before the 75% rule, that in a large splice, we would use lead bolts or rivets. Suppose you had a web splice with 5 lines of bolts. Start with one bolt, net section equals gross area less one bolt hole, one bolt is developed. Next row, place two bolts, net section is gross less two bolt holes but the load has been reduced by the first bolt, now three bolts have been developed. Next row, use three bolts, net section is the gross less three bolt holes but the load is reduced by the three bolts already developed....and so on....but you can't do that now because of the 75% rule.

Selection by the 75% rule is good, the labor cost of welding additional material in place at each end is much greater than the cost of the material saved. Back when I was learning the business, the material cost more than the labor, now it is the reverse.

Timing has a lot to do with the outcome of a rain dance.

 

[slickdeals]

Yes, I was quoting Steven Hofmeister (TT Engineers) from a letter to the editor in Structure Magazine

 

[slickdeals]

"You can weld additional steel to a member, it then becomes part of the gross area and the total is reduced to 75%."

Can you provide some more information?

I am assuming that the area of reinforcing steel member provided will be equal to the area of material lost due to the bolt holes. This filler plate will carry a certain portion of the total member load based on ratio of areas of filler material to the area of primary member.

The force that this member carries will have to be developed using welds/bolts. The length of the weld will be provided to carry the force in the filler plate.

Am I understanding this correctly?

 

[kelowna]

Welding steel plate fillers increases the section, it also changes the class in fatigue, which could actually reduce the overall capacity of the section.

Unless I am missing something this is a simple issue of comparing the ratios of An/Ag with Fy/Fu to see if net or gross area govern and how the bolt holes change that. For higher strength steels, correct me if I am wrong but Fy/Fu decreases, making the net area strength more critical.

Regarding the statement you reproduced, if you put a first bolt line with only one or two 'lead' bolts, the applied load on the flange can be reduced by the contribution of those bolts. The next line of bolts and its An capacity are then calculated with a reduced load that makes the decrease on net load not to impair the capacity of the section.

 

[paddingtongreen]

Before I go on, I am not recommending this, I much prefer to go to a heavier member. The cost of the added labor for this reinforcement will probably exceed the cost of the heavier member, and it looks like a repair to a bad design. Generally, all you pay extra for a heavier member is the mill cost and shipping, there is no extra labor cost because there is no extra work to be done.

The added plate must have enough weld to develop its share of the load, proportional to the gross areas, before it gets to the bolt hole. The hole should be punched after the piece is welded in plate to assure simultaneous bearing on the bolt.

Timing has a lot to do with the outcome of a rain dance.